Reagents

IF LABORATORY EQUIPMENT IS THE HAMMER and chisels of m e t - with "Practical" or "Technical" grades never employed for an-
a l s analysis, then reagents are the fine Carrara marble. Just alytical work if any higher grade is available.
as a sculptor chooses a starting piece for shape and grain and The first level of purity above ACS grade is variously termed
assesses surmountable versus fatal defects, all with an eye "Electronic," "Trace Metal," or "Instrument" grade. !t is usu-
toward his final purpose, so the analyst must plot a route to ally accompanied by an extensive lot analysis, which may,
his goal, selecting reagents for type and purity and weighing however, omit key elements that are present in measurable
compatibilities and reaction rates. quantities. The "ultra-purity" grades are sold at premium
To pick a number somewhat arbitrarily, about a thousand prices, often in special packaging, and are accompanied by
reagent chemicals may be involved in currently valid proce- an exhaustive lot analysis. If one is prepared to spend between
dures for the analysis of metals and alloys. Of these about a two and ten times the unit cost of ACS grade, there is no better
hundred are used with sufficient frequency to warrant men- choice for tramp and trace work where high or inconsistent
tion in this chapter; although experienced analysts who toil blank readings may otherwise constitute a problem. Some
in certain industries may have valid objections that their "fa- manufacturers also sell certain reagents in a "Primary Stan-
vorite reagent" was ignored. dard" grade. These can be used to prepare standard titrant
The plan is to cover purchased reagents first by categories solutions that do not themselves need to be standardized.
that either describe the compound or how it is commonly A completely separate argot of terms is being used for or-
used. One problem with this approach is that certain reagents ganic solvent grades. Once again, ACS grade is somewhat of
will fall into several categories. Thus ammonium hydroxide a baseline level. At or slightly below this level of purity are
is a base, a precipitant, and a complexing agent. In each case solvents sold for high-performance liquid chromatographic
of this sort, only the first occurrence will be listed (which, use ("HPLC" grade). "Pesticide" grades are low-residue sol-
hopefully, will be the most commonly held and familiar cat- vents, suitable where solvent evaporation concentration is
egory.) In the case of ammonium hydroxide, the category is needed. "Spectrophotometric" grades are very high purity, of-
"base." Later in the chapter, prepared reagents will be de- ten sold with accompanying ultraviolet absorption curves for
scribed. These are mostly standard titrant solutions and ele- the lot. The highest level organic solvents are prepared for use
mental standard calibration solutions that find use in a vari- in specific procedures, such as certain EPA methods. They
ety of different methods. can be five times the unit cost of ACS grade.
Gases are sold in purity level categories that are only a little
less bewildering. "Extra Dry" (or sometimes "Bone Dry") is a
PURITY LEVELS common appellation for fixed gases at around 99.6% purity.
"High Purity" usually means 99.99%, and "Zero Gas" refers
It would be gratifying to be able to say that reagent purity to a very low total hydrocarbon content (less than 0.5 ppm).
is no longer an issue in the laboratory and that any compound "Prepurified," "Ultra High Purity," and "Research" grades rep-
sold as an analytical reagent is going to be good enough. In resent the final ascent into five "9's" and beyond. Rarely does
fact, these days the analyst needs to be as vigilant as ever to the metals analyst need to climb those heights.
ensure that a week's worth of work will not be ruined by se-
lecting a contaminated reagent.
"ACS" on a reagent label is a claim by the manufacturer
that the compound meets the specifications of the Committee WATER
on Analytical Reagents of the American Chemical Society (as
set forth in Reagent Chemicals, American Chemical Society There is little need to remind a classical chemist that the
Specifications). It is not the final word in purity or a nihil ob- quality of the water available for his use can have a profound
stat for all analytical work. For example, ACS grade nitric acid effect on the analytical results he obtains. And as instruments
is often sold contaminated with titanium since that element have opened up the trace and ultratrace realms, analytical
is not specified. Reagent manufacturers chart a course water quality has become even more of an issue. High-purity
through specifications and known needs, offering a dozen or water is used almost everywhere and for almost everything--
more grades o f purity, sometimes with precipitous price labware is rinsed with it, reagents and samples are diluted
boundaries. In this hierarchy ACS grade has become the base- with it, sometimes instruments are "zeroed" with it, and de-
line with "CP" (Chemically Pure) grade just a step below and tection limits are frequently based on measurement noise
29
30 CHEMICAL ANALYSIS OF METALS

TABLE 3-I--ASTM Standard Specifications for Reagent Water GASES

, (Designation D 1193).
Type I TypeII Type III Type IV These days one thinks less frequently of gases as reagents,
Total matter, mg/L 0.1 0.1 0.1 2.0 yet they still qualify even if only occasionally u s e d as such.
Electrical resistivity, 16.67 1.0 1.0 0.2 Here we will briefly review all the gases i m p o r t a n t to metals
minimum megaohm analysis, including those t h a t do not function as reactants•
• cm at 25°C
pH at 25°C minimum NMa, 60 NM a, 60 6.2-7•5, 10 5.0-8•0, 10
color retention of Reactant Gases
KMNO4, in minutes
Max, soluble SiO2 NDb NDb 10 ppb NL~ Acetylene (C2H2) is the s t a n d a r d flame a t o m i c a b s o r p t i o n
aNot measured. fuel gas with b o n d energy to burn. It is sold dissolved in ac-
bNot detectable.
CNolimit. etone or a s i m i l a r solvent a n d should never be d i s c h a r g e d at
Preparation: outlet p r e s s u r e s that exceed 15 psig (104 kPa). S o m e glass-
Type I: The distillation of feedwater having a minimum resistivity of 0.05 blowers use an oxyacetylene t o r c h to w o r k Vycor a n d fused
rnegohm-cm at 25°C, followedby polishing with a mixed bed of ion exchange silica• The characteristic garlic-like o d o r w a r n s of a leak. Ace-
materials and a 0.2-~tmmembrane filter. tylene's f l a m m a b l e range with air is 2.5 to 82%, the b r o a d e s t
Type H: Distillation using a still designed to produce a distillate having a of a n y gas. S o m e analytical p r o c e d u r e s use air s p a r g e d
resistivity of more than 1 megohm-cm at 25°C. (Ion exchange, distillation, or
reverse osmosis may be required as a preliminary treatment prior to distilla- t h r o u g h solutions to a u g m e n t the a t m o s p h e r i c o x i d a t i o n of
tion.) s o m e analyte species; ferrous i r o n a n d s t a n n o u s tin are two
Type Ill: Distillation, ion exchange, reverse osmosis, or a combination r e a d y examples• Air is a m a j o r oxidant gas in flame AA work,
thereof, followedby filtration with a 0.45-/zmfilter. a n d "zero air" is the o x i d a n t gas for gas c h r o m a t o g r a p h i c
Type/V: Distillation,ion exchange,reverse osmosis, electrodialysis,or a com- flame i o n i z a t i o n detectors•
bination thereof. A m m o n i a (NH3) is s o m e t i m e s used to p r e p a r e silica-free
a m m o n i u m h y d r o x i d e by b u b b l i n g the gas t h r o u g h w a t e r in
from it. It is essential n o t only t h a t w a t e r of a d e q u a t e quality a hood. This practice is s o m e t i m e s necessary b e c a u s e com-
be available b u t also that the quality r e m a i n consistent• A mercial a m m o n i u m h y d r o x i d e (except u l t r a - p u r i t y grade) is
"surge" of, say, c a l c i u m c o n t a m i n a t i o n d u r i n g a t w o - d a y cal- always sold in glass bottles a n d is heavily c o n t a m i n a t e d with
c i u m d e t e r m i n a t i o n w o u l d create a p r o b l e m from w h i c h silicon. A n h y d r o u s a m m o n i a should be t r e a t e d with respect;
there w o u l d be no recovery except to start over after the prob- it is a noxious, d a n g e r o u s gas that will cause severe burns.
lem was corrected. Nearly 90 parts (by f o r m u l a weight) will dissolve in 100 p a r t s
The A m e r i c a n Society for Testing a n d Materials (ASTM) of water• Its f l a m m a b i l i t y range with air is 16 to 25%.
has p u b l i s h e d ASTM Specification for Reagent W a t e r (D Chlorine (C12) is u s e d to isolate oxide inclusions from steel
1193), w h i c h covers four types, s h o w n in Table 3-1. Types I b y s u b l i m i n g a w a y with h e a t the FeC13 f o r m e d f r o m the ma-
t h r o u g h III are analytically useful a n d should be p r a c t i c a l l y trix iron. Chlorine is a yellowish-green, highly toxic, a n d cor-
attainable in m o s t laboratories• C o m p a r e Table 3-1 with Ta- rosive oxidizer that causes severe burns. Chlorine should be
ble 1-1, w h i c h was b a s e d on actual l a b o r a t o r y experience. u s e d only in closed systems in an efficient h o o d with the ef-
The color r e t e n t i o n test listed in Table 3-1 is a m e a s u r e of fluent b u b b l e d t h r o u g h one o r m o r e traps of 50% (w/v) so-
oxidizable organic substances. To a mixture of 500 m L of re- d i u m hydroxide•
agent w a t e r c o m b i n e d with 1 m L of c o n c e n t r a t e d sulfuric Hydrogen (H2) has b e e n u s e d in pyrolysis m e t h o d s for in-
acid, 0.2 m L of p o t a s s i u m p e r m a n g a n a t e solution (0.316 g/L) terstitial nitrogen in steel a n d to e s t i m a t e certain readily re-
is added. The solution is m i x e d in a s t o p p e r e d borosilicate d u c e d oxides on steel surfaces• In the f o r m e r case NH3 is m e a -
glass bottle a n d allowed to stand. The w a t e r should not dis- sured, in the latter H20 is measured• H y d r o g e n is also used
charge the p i n k color w i t h i n the i n d i c a t e d t i m e intervals. occasionally in flame AA w o r k with a special a r g o n - e n t r a i n e d
Since w a t e r in contact with a i r will a b s o r b a t m o s p h e r i c car- a i r - h y d r o g e n burner. Such a flame is nearly invisible, thus
b o n dioxide a n d b e c o m e slightly acidic with c a r b o n i c acid, suitable care m u s t be t a k e n with its use. More frequently,
boiled, cooled, a n d inert gas-sparged distilled w a t e r is always h y d r o g e n is used as an argon additive in g r a p h i t e furnace AA
used for the m o s t a c c u r a t e p H a n d conductivity m e a s u r e - work. A low total h y d r o c a r b o n grade is usually used for GC
ments• flame i o n i z a t i o n detectors• S o m e t i m e s h y d r o g e n is used as a
L a b o r a t o r i e s with solution-based m u l t i - e l e m e n t spectro- GC c a r r i e r gas. Vycor a n d fused silica glasses are c o m m o n l y
m e t r i c capabilities, like ICP-OES, should r o u t i n e l y m o n i t o r w o r k e d w i t h an o x y h y d r o g e n torch. H y d r o g e n gas has a b r o a d
t h e i r distilled w a t e r taps, p e r h a p s even plotting elemental f l a m m a b l e range of 4 to 75% with air•
t r e n d charts. If one is alert to likely c o n t a m i n a n t s , a simple H y d r o g e n / a i r mixtures have the highest m a x i m u m flame
check b y flame AA m a y be sufficient. Silicon, calcium, mag- velocity of a n y fuel/air m i x t u r e (1.7 times that of acetylene/
nesium, copper, a n d iron c o m m o n l y show u p in certain p r o b - air). To prevent potentially c a t a s t r o p h i c detonations, espe-
lem situations, although others are possible d e p e n d i n g on the cially with " h o m e - m a d e " o r i m p r o v i s e d a p p a r a t u s , great
lab's system configuration• S o m e t i m e s the cleanest water, in- pains m u s t be t a k e n to ensure that air has been t h o r o u g h l y
organically speaking, is c o n t a m i n a t e d with organics from ion p u r g e d from the system with a heavy gas like argon before
exchange columns• And so, if the l a b o r a t o r y does m u c h or- a n y h y d r o g e n is introduced• Effluent h y d r o g e n should be
ganic analysis, p a r t i c u l a r l y trace organic analysis, special wa- " b u r n e d off" at a constricted exit t u b e to prevent b u i l d u p of
ter supplies m a y have to be provided• High-purity w a t e r in a h y d r o g e n in the room. W h e n the e x p e r i m e n t is complete, the
variety of grades can be p u r c h a s e d for prices ranging from h y d r o g e n should be i m m e d i a t e l y a n d t h o r o u g h l y p u r g e d f r o m
$15 to $70 or m o r e p e r liter• the system with argon.
 CHAPTER 3 REAGENTS 31

Hydrogen Chloride (HC1) is still sometimes used from a cyl- ture, total hydrocarbons, and other contaminants. The liquid
inder to volatilize chromium out of a fuming perchloric acid gas is also sold and, as described in Chapter 1, is one of the
solution as chromyl chloride (CrO2C12). It remains one of the most economical and convenient ways to provide low-oxygen
best techniques because it does not significantly cool the so- argon for ICP and other optical emission needs. Smaller vol-
lution as dropwise hydrochloric acid does; it is less tricky to umes of argon are used for graphite furnace AA work and
use than a boiling azeotrope generator; and it adds no sodium (rarely) as a sheath-gas for an air-hydrogen burner in FAA.
as solid sodium chloride does. As one might expect, gaseous Occasionally argon is used as a GC carder gas for thermal
hydrogen chloride is highly toxic and corrosive. It requires conductivity detection despite generally poor sensitivity. An
the same type of corrosion-resistant regulator as does chlo- example of this use would be to measure the oxygen content
rine gas; these are usually constructed of stainless steel or of an argon sample since most molecular sieve columns do
nickel alloys with seats and seals of fluorocarbon polymers. not resolve oxygen and argon. Argon is a good purge gas since
Hydrogen sulfide (H2S), a dangerous toxic gas with the smell it is heavy and will readily displace air and most other gases.
of rotten eggs, was formerly used for many sulfide separa- But therein lies the danger of asphyxiation if argon gas is not
tions. Today it is frequently replaced with thioacetamide (un- properly vented. Once in the lungs argon is more difficult to
fortunately, a known carcinogen), which releases H2S in situ expel with air or oxygen than other gases.
in acid or basic solution. Those labs that still use hydrogen Carbon dioxide (CO2) is sold as a liquid and is best dis-
sulfide cylinders should install monitors and alarms since ol- pensed with a freeze-free regulator. It is sometimes used to
factory fatigue makes its pungent stench an unreliable indi- provide an oxygen-free inert blanket over tin or iron solu-
cation of concentration levels in room air. This is especially tions. It is considered low toxicity, but can paralyze the res-
important because of its highly toxic nature (a threshold limit piratory response at high concentrations.
value of 10 ppm). Hydrogen sulfide will burn in air over a Helium (He) is used for the inert gas fusion determinations
concentration range of 4.3 to 46%. of oxygen, nitrogen, and argon. It is the most common carder
Nitrous Oxide (N20) is used in flame atomic absorption as gas for gas chromatography and is occasionally used as an
a higher temperature oxidant. A stoichiometric mixture of ni- optics purge gas in optical emission and X-ray fluorescence.
trous oxide and acetylene achieves a temperature of 2800°C, Although helium is an asphyxiant, the danger at low flow
while air and acetylene reach only 2400°C. Nitrous oxide is rates is small because it is lighter than air and will usually
an anesthetic gas with a sweet odor. Like carbon dioxide, it find its way out of a room or building before displacing a
has a high enough critical temperature to be shipped as a significant amount of air.
liquid, but the work of expansion through regulators draws Nitrogen (N2) is arguably not an inert gas since it can be
enough heat away to freeze the mechanism. For this rea- made to undergo a host of reactions. The metals analyst, how-
son, specially designed regulators are often used for both ever, always employs it as a blanket, sparge, or purge gas to
gases. prevent the absorption of atmospheric carbon dioxide by a
Oxygen (02) is used most commonly for carbon and sulfur boiled aqueous solution, to control the atmospheric oxygen
determinations by the so-called "combustion" techniques. wave in polarography, or to eliminate the "water bucket" due
Sometimes an oxygen atmosphere is introduced into a muffle to atmospheric water vapor in infrared absorption spectro-
furnace to facilitate ignitions, especially when it is critical to photometry. Sometimes it is used as a carrier gas in GC work.
remove all traces of carbon and sulfur. The "pre-burning" of Flow rates are usually low, so there is little danger that room
combustion crucibles for trace carbon and sulfur analysis is air will be significantly displaced.
an example. Oxygen is the prime oxidant used in glassblowing Table 3-2 summarizes key information about the gases just
work. Organic microsamples are prepared for inorganic con- discussed. CGA numbers refer to the standard cylinder outlets
stituent analysis by Sch6niger flask or bomb calorimeter and connections specified by the Compressed Gas Associa-
combustions. In the former technique, a special glass flask is tion. Figure 3-1 depicts some frequently used CGA outlets
filled with oxygen at atmospheric pressure; in the latter, a and connections. Probably the only routinely purchased gas
stainless steel cylinder is pressurized with oxygen. At one time mixture will be "P-10" gas, which is 10% methane in argon.
oxygen was widely used for oxyacetylene flame emission It is used in flow proportional counters in X-ray fluorescence
work, but that is rare today. spectrometers. There are also special mixtures for GC elec-
Sulfur dioxide (SO2) can be used to economically prepare tron capture detectors and for other types of radiation and
sulfurous acid by bubbling the gas through a dilute sulfuric ionization detectors. If much gas analysis work is performed,
acid solution in a hood. The gas itself has been employed to especially by GC, a collection of custom-blended calibration
reduce vanadium, where sulfurous acid is ineffective due to and validation gas standard mixtures will be needed unless
contamination by other oxidizable sulfur compounds. Sulfur the analyst can satisfy his or her needs in the catalogs of
dioxide is highly toxic and nonflammable. It requires the the specialty gas companies or NIST. Such custom blends
same type of corrosion-resistant regulator as anhydrous am- are available by arrangement with many specialty gas com-
monia and hydrogen sulfide, made of stainless steel and flu- panies.
orocarbon polymers. Although sulfur dioxide has an irritating
and pungent odor, many claim they can taste it before they
smell it. ACIDS

Inert G a s e s Even the simplest solution-based techniques in metals anal-

ysis require acids in quantity and variety. Virtually all metals,
Argon (Ar), like most gases in this category, is sold in a alloys, and industrially useful minerals, even the most resis-
variety of grades with guaranteed low levels of oxygen, mois- tant, are attacked to some degree by some combination of
32 CHEMICAL ANALYSIS OF METALS

TABLE 3-2--Miscellaneous gas data.

Molecular Specific Commonly Major CGAd Outlet and
Gas Weight Gravity~, @21°C Shipped as TLVb, ppm Hazards¢ Connection
Acetylene, C2H2 26.04 0.9092 Gas in Solvent ... F 510
Air (28.96) (1) Gas P 346
Ammonia, NH3 17.03 0.5914 Liquid 25' I,C 660
Argon, Ar 39.95 1.378 Gas P,A 580
Carbon dioxide, CO2 44.01 1.519 Liquid 5000 P,I 320
Chlorine, Cl2 70.91 2.475 Liquid 1 I,C 660
Helium, He 4.003 0.1382 Gas ... P,A 580
Hydrogen, H2 2.016 0.0696 Gas F,P 350
Hydrogen chloride, HC1 36.46 1.226 Liquid 5~ I,C 330
Hydrogen sulfide, H2S 34.08 1.193 Liquid 10 I,F 330
Nitrogen, N2 28.01 0.9685 Gas ... P,A 580
Nitrous oxide, N20 44.01 1.536 Liquid ... S 326
Oxygen, 02 32.00 1.105 Gas . P,S 540
Sulfur dioxide, SO2 64.06 2.265 Liquid 5" I 660
aAir is defined as 1.000.
bThreshold limit value: airborne concentration to which it is believed that nearly all workers may be repeatedly exposed without harm.
CHazard Code: A--asphyxiation; C--bodily contact; F--fire; I--inhalation; P--high pressure; S--supports combustion.
dSee Fig. 3-1.
eTLV Ceiling: the airborne concentration that should never be exceeded, even instantaneously.

acids. And so acids p l a y a role in a l m o s t all dissolutions. Acids vides the "oxidizing" a n d "nonoxidizing" acids. S o m e t i m e s
are also used to p r e p a r e r e a g e n t solutions, a n d they serve as the t e r m "mineral acids" is used to distinguish i n o r g a n i c
s t a n d a r d titrants in v o l u m e t r i c w o r k a n d as pickling agents acids.
to r e m o v e scale a n d c o r r o s i o n f r o m the surface of solid m e t a l Hydrochloric acid (HCI) is p r o b a b l y the m o s t widely used
specimens. And, of course, they are u s e d to a d j u s t p H for m i n e r a l acid. R e a g e n t g r a d e is a 37% (w/w) solution of the
r e a c t i o n chemistry. While the m o d e m definition of an a c i d gas in water. It will dissolve a surprisingly large n u m b e r of
(by Lewis) is a n electron p a i r r e c e p t o r a n d the o l d e r definition metals a n d alloys given e n o u g h time, although it frequently
(by Lowry-Br6nsted) is a p r o t o n donor, it is sufficient for o u r leaves i m p o r t a n t oxide inclusions untouched. Technical
p u r p o s e s here to define an a c i d as a n electrolyte t h a t furnishes grade is c o m m o n l y referred to as " m u r i a t i c acid" a n d is often
h y d r o g e n ions in a q u e o u s solution. In this sense of the term, c o n t a m i n a t e d with a significant a m o u n t of iron a n d o t h e r
acids are classed as either "strong" o r "weak" b a s e d on t h e i r metals. The ACS reagent grade is usually g o o d e n o u g h for
degree of dissociation in the p r e s e n c e of water. These t e r m s m o s t d e t e r m i n a t i o n s d o w n to analyte c o n c e n t r a t i o n s of a b o u t
do n o t reflect, however, o n the reactivity o r the a s s o c i a t e d 0.01%. Below t h a t level the analyst should b e c o m e cautious,
h a z a r d . A m o r e useful classification for metals analysis di- a n d at 0.001% the "electronic" g r a d e is usually called for. At

320 326 330

346 350 510

660
idtr
540 580
FIG. 3 - 1 " - C G A outlets and connections.
 CHAPTER 3 - - R E A G E N T S 33

tramp levels, caution is again advised, and for certain deter- lutions are heated, a series of constant boiling azeotropes
minations an ultra-purity grade, prepared by sub-boiling, forms until all but 1.2% of the water has been expelled. This
may be necessary. "anhydrous" acid boils at 335°C. "Fuming sulfuric acid" or
Hydrochloric is a nonoxidizing acid, but its use frequently "oleum" (a saturate of SO3) is sold but finds no use today in
results in soluble chloride complexes that become important metals analysis. Concentrated sulfuric acid is noted for its
in subsequent manipulations, including ion exchange work. dehydrating power and its extremely high heat of hydration.
Some elements, like silver, copper (I), mercury (I), and bis- Thus it is the most hazardous acid to dilute.
muth, form insoluble compounds. Other elements like phos- As with all acids, good laboratory practice prescribes al-
phorus, arsenic, tin, antimony, selenium, tellurium, germa- ways adding acid to water, never the reverse, while stirring
nium, and boron form low boiling volatile chlorides that may cautiously and vigorously. With sulfuric acid a cooling bath
be lost by heating. Like all gaseous acids, when a water so- is necessary since the heat may be generated rapidly enough
lution of hydrochloric acid is boiled at atmospheric pressure to boil the solution or even crack a borosilicate vessel with
the vapor will eventually reach a constant boiling azeotrope disastrous results. Since dilutions of sulfuric acid and water
of HCI and water. This becomes important for methods where when cooled to room temperature occupy less volume than
the analyte is distilled away from the matrix in a closed sys- the sum of the original volumes of the components, it is sim-
tem. Inhalation and bodily contact are the primary hazards plest to specify concentrations as "parts" to be added, such as
associated with work with hydrochloric acid. Gloves and a "1 " 1 H 2 S O 4 " H 2 0 . " Some metals dissolve in the concentrated
hood are called for down to a concentration of at least 6 mo- acid, but many more require dilute sulfuric acid. And some
lar. metals are only partially attacked or untouched by any con-
Nitric acid (HNO3) is a strong oxidant. Reagent grade is centration.
70% (w/w). Alone or in dilute solution it readily dissolves cer- Sulfuric acid is more broadly effective as a dissolution
tain metals and passivates others. The passivation results agent in combination with other acids. By bringing the mixed
from the formation of an oxide on the surface of the metal acid solution to fumes of sulfuric acid, it is often possible to
particles and often is so refractory that subsequent treatment expel the (potentially interfering) anions of the other acids.
with other acids is ineffective. The oxidizing power of nitric Mercury, rhenium, and selenium can be lost as volatiles by
acid is particularly evident with organic materials, and cau- such a process, however. And lead, calcium, strontium, and
tion is always advised in these instances. A red fuming grade barium will precipitate in the presence of sulfuric acid.
at 90% (w/w) is available but unneeded for metals work. Ni- Concentrated sulfuric acid has a low coefficient of friction
tric acid is most effective for dissolutions in combinations (and has even been used as a lubricant in special limited ap-
with other acids such as hydrochloric or hydrofluoric, where plications). This can cause a contaminated vessel to slip out
volatile or unstable species are formed in solution. of a gloved hand, so special caution is advised. Skin contact
Those few elements that form volatile oxides, like osmium with sulfuric acid requires an immediate flush with a large
and ruthenium, will be partially or completely lost from a excess of cold water since a small amount of water may result
boiling nitric acid solution. Many other elements at high con- in a more serious b u m from the heat of hydration. All dilu-
centrations will precipitate as hydrous oxides, in particular tions of the concentrated acid and all fuming must be con-
the so-called "earth acids"-- titanium, zirconium, hafnium, ducted in a hood. Gloves, a face shield, and a rubber apron
niobium, tantalum, molybdenum, and tungsten. Always use are always a good idea.
ultra-purity grades for titanium determinations at levels be- Hydrofluoric acid (HF) is a unique, nonoxidizing, highly re-
low 0.1%. active complexing acid. Because of covalent bonding in the
Nitric acid is routinely used alone and in combination with molecule, hydrofluoric acid is incompletely ionized in aque-
perchloric and/or sulfuric acids to destroy organic matter. ous solution (a 0.1 molar solution is 15% dissociated). It is
The process is fraught with hazards and always requires a also one of the greatest hazards in the metals analysis labo-
good knowledge of both safe laboratory practices and the ex- ratory. ACS reagent grade is 49% (w/w) and is usually ade-
act nature of the sample material. Even with inorganic sam- quate for all but certain trace level work. "Electronic" or "in-
ples it is always best to apply nitric acid only where the ex- strument" grade is about twice as expensive, and ultra-purity
pected reaction and its rate are known since explosions with grade may be ten times as expensive at about the same con-
inorganic materials have also been reported. In general it is centration.
unwise to allow nitric acid to sit in contact with undissolved Alone, hydrofluoric acid will dissolve many metals and si-
metals for days at a time. liceous materials, but in combination with other mineral ac-
Nitric acid attacks the skin, forming a characteristic brown ids, it can be used to dissolve a great number of metals, alloys,
stain, thus gloves must be worn at all times. Gloves should ores, and other materials. Many metal ions, including those
also be rinsed and examined after use because nitric acid at- that precipitate as hydrous oxides in other acids, form soluble
tacks many glove materials. All work should be in an efficient fluoride complexes even at low concentrations of hydrofluoric
fume hood not only because of the acid vapor but also be- acid. Thus one or two drops are sometimes sufficient to hold
cause nitric acid reactions often produce highly toxic gaseous a great deal of tungsten, niobium, or tantalum in solution.
compounds. Discolored nitric acid has been contaminated The rare earths will precipitate as fluorides, and use of that
and should be disposed of by a safe standard procedure. reaction is readily applied to their determination. Certain el-
Sulfuric acid (H2SO4) is generally regarded as a nonoxidiz- ements like silicon, boron, arsenic, and antimony will be lost
ing acid, but at its fuming temperature (when heavy white as volatiles from heated solutions. In aqueous solution the
vapors of its anhydride, SO3 , begin to evolve) it becomes anionic species HF2 results from the association of free flu-
slightly oxidizing. Reagent grade is 96% (w/w). As dilute so- oride ion with undissociated HF. It is this species that is re-
34 CHEMICAL A N A L Y S I S OF METALS

sponsible for m a n y of the important reactions of this valuable fusion with a solution that was fumed with perchloric acid.
reagent. But it can also be used to advantage, as in the gravimetric
The only trouble with hydrofluoric acid is that it is very determination of potassium (where the even lower solubility
dangerous to work with. Any bodily contact with the reagent of KC104 in butyl alcohol is sometimes utilized).
or its vapors requires immediate flushing with copious Ammonium perchlorate is more soluble in water than the
amounts of water because hydrofluoric acid penetrates the potassium salt, but still of limited solubility. This makes it
skin very readily, producing burns deep in tissue. Such burns, impossible, for example, to fume the filtrate of an ammonia
especially from dilute solutions of the reagent, may not be separation in perchloric acid--a fortunate impediment since
painful until many hours after contact. First aid treatment the a m m o n i u m perchlorate would almost certainly explode.
after flushing with copious water consists of soaking all af- The only elements volatilized from fuming perchloric acid
fected skin areas with iced water solutions of the quaternary alone are osmium, ruthenium, and rhenium, which are lost
amine compounds HYAMINE 1622 benzethonium chloride as OsO4, RuO4, and ReO7. Numerous elements are, of course,
(1 part to 500 parts of water) or ZEPHIRAN benzalkonium volatilized from perchloric acid mixtures with other acids.
chloride (1 part to 750 parts of water). Fuming with perchloric acid will cause high concentrations
All burns require the immediate attention of a physician of the "earth acids" and tin and antimony to precipitate as
who is familiar with hydrofluoric acid burn treatment. But hydrous oxides. Perchloric acid must be heated only in a hood
the soaks with these solutions should not stop during any re- designed for its use (see Chapter 1). An approved safety shield
quired transportation or delay. These solutions should not be must be used when working with incompletely characterized
used for burns of the eye, which must be flooded with water. samples. Never heat a material with perchloric acid near the
In this case a physician, preferably an eye specialist familiar fuming point unless a large excess of nitric acid has also been
with proper procedures, is immediately required. For detailed present. For more information, safety tips, and references, see
information about hydrofluoric acid burns, write for infor- the monograph Perchloric Acid and Perchlorates by A. A. Schilt
mation to the major manufacturers (such as Allied Chemical, (published by GFS Chemical Company, 867 McKinley Ave.,
P.O. Box 1053R, Morristown, NJ 07960, Attn: Marketing Columbus, OH 43223). Perchloric acid is an invaluable re-
Manager--Hydrofluoric Acid). agent that has been used in quantity in metals analysis labs
Perchloric acid (HC104) is another unique acid. Its aqueous for over 60 years, almost since the pioneering efforts of G.
solutions are nonoxidizing at room temperature, but its hot, Frederick Smith, himself. Treated with the respect it de-
concentrated solutions are powerfully oxidizing. It is com- serves, it can well serve the needs of today's and tomorrow's
pletely ionized in concentrated sulfuric acid solution, which laboratories.
indicates that it is the stronger acid of the two and which puts Phosphoric acid (HaPO4) is a nonoxidizing acid used chiefly
it among the strongest of the acids. Perchloric acid is essen- for its complexing characteristics. It is sold as a 86% (w/w)
tially noncomplexing, a property that makes it valuable for solution that boils at 213°C. Anhydrous acid is a solid that
certain ion exchange work and in numerous situations where melts at 42.3°C; it is sometimes referred to as "ortho-phos-
complexes are to be left undisturbed by competing ligands. phoric acid." In combination with perchloric or sulfuric ac-
In hot, concentrated solution it is also a powerful dehydrating ids, phosphoric acid has good solvent properties and allows
agent, which makes it valuable in gravimetric silicon deter- many elements to remain in solution that would otherwise
minations, where it converts the soluble polymeric silicates precipitate as hydrous oxides. Zirconium, hafnium, and tita-
to insoluble silicic acid. The combination of the oxidizing and nium will precipitate as insoluble phosphates, although the
dehydrating power of the hot, concentrated solution makes precipitation of titanium can be avoided by the addition of
perchloric acid invaluable for the destruction of organic mat- hydrogen peroxide. Phosphoric acid is often added to solu-
ter. This "wet ashing" of organics must always be conducted tions for potentiometric titrations or color endpoint redox ti-
in the presence of a large excess of nitric acid and in carefully trations. Here it serves to adjust the formal endpoint potential
controlled sample sizes to avoid a violent explosion. High- for ease of detection. One problem that occurs on strong fum-
boiling organics are particularly dangerous to destroy in this ing with phosphoric acid mixtures is the formation of a poly-
manner, so special pretreatment approaches are sometimes meric form of the acid that is nearly insoluble.
necessary. Silver, bismuth, and antimony perchlorate salts Acetic acid (CHaCOOH) is the most commonly used car-
are shock-sensitive explosives, and many other inorganic ex- boxylic acid. It is sold at close to 100% purity in a form known
plosions have also been reported. The analyst should regard as "glacial" acetic acid. It is available in an array of grades
any metal perchlorate salt as a potential hazard. that reflects both the inorganic and organic applications of
The acid is sold commercially at two different concentra- the reagent--ACS grade, instrument grade, ultra-purity
tions: 60% (w/w) and 70% (w/w), the latter being close to the grade, HPLC grade, aldehyde-free grade, and others. Glacial
72.4% (w/w) azeotrope with water, which boils safely at acetic acid fumes slightly in air, has an irritating vinegar odor,
203°C. The anhydrous acid distills out of sulfuric acid/per- and should only be used in a hood. Even though only partially
chloric acid mixtures, but the distillate from such mixtures dissociated in dilute solution, the concentrated acid will
must never be collected since it will detonate spontaneously. cause serious burns and should be handled with gloves. In the
"Instrument" and ultra-purity grades are available at 70% metals analysis laboratory it is used primarily in the prepa-
(w/w). ration of organic reagent solutions, although it also finds use
Most perchlorates are very water soluble, but potassium in certain types of group separation schemes. And its prop-
(and rubidium and cesium) perchlorates are only slightly sol- erties as a "weak" acid are sometimes used to selectively leach
uble. This causes problems, for example, when there is a need readily soluble components from a solid matrix and for the
to combine the leach solution from a potassium pyrosulfate preparation of pH buffer solutions.
 CHAPTER 3 - - R E A G E N T S 35

TABLE 3-3mConcentration data for reagent grade acids and grades are available, the latter sometimes at a lower
ammonium hydroxide. concentration. The solubility limit of ammonia gas in water
Formula Specific at room temperature is about 48% (w/w).
Acid Weight % (w/w) Molarity Normality Gravity
Ammonium hydroxide is a weak base, Its degree of disso-
Acetic glacial 60.053 99.8 17.4 17.4 1.06 ciation increases rapidly with dilution. Like all weak acids
Formic 46.026 90.0 23.6 23.6 1.20
Hydrobromic 80.91 48.0 8.87 8.87 1.50 and bases, it obeys Ostwald's dilution law, which simplifies
Hydrochloric 36.461 37.2 12.1 12.1 1.18 to: x 2 -- KdV, where x is the fraction dissociated, Kd is the
Hydrofluoric 20.006 49.0 28.9 28.9 1.15 dissociation constant, and Vis the solution volume. Thus, am-
Nitric 63.013 70.4 15.9 15.9 1.14 monium hydroxide can be used to prepare buffer solutions
Perchloric 100.459 70.5 11.7 11.7 1.66
Phosphoric 97.995 85.5 14.8 44.4 1.69 that resist changes in pH.
Sulfuric 98.079 96.0 18.0 36.0 1.84 It is also used as a precipitant for the "R203" group of ele-
Ammonium hydroxide 35.046 56.6 14.5 14.5 0.90 ments (erroneously named for certain oxide forms that only
some of them attain at ignition). The group includes iron,
aluminum, titanium, the rare earths (including scandium and
yttrium), beryllium, gallium, indium, zirconium, hafnium, ni-
Other acids that may be used regularly in some metals anal- obium, tantalum, thorium, uranium, and chromium (III).
ysis labs can be briefly mentioned. Formic acid (HCOOH) at Chromium (VI) does not precipitate, but it and many other
around 90% (w/w) is used dropwise to destroy the last traces elemental species can be occluded in the "R203" group.
of nitric acid from solutions that must exclude it. Hydrobro- Ammonium hydroxide also serves as a complexing agent,
mic acid (HBr) at 48% (w/w) is used alone or in combination forming stable, soluble complexes with nickel, cobalt, copper,
with bromine to dissolve lead alloys, including lead-based sol- zinc, silver, cadmium, and other metals. Ammonium hydrox-
ders. It quickly dissolves copper alloys, unlike hydrochloric ide is infrequently employed as a dissolution medium, al-
acid, which attacks them slowly. It is used alone or in com- though molybdenum trioxide dissolves readily in it, and cop-
bination with hydrochloric acid to volatilize tin away from per and molybdenum metal will dissolve in a mixture of
other elements of interest or to distill and collect it for anal- ammonium hydroxide and hydrogen peroxide. Like the
ysis. Fluoboric acid (HBF4) is sold at a concentration of 49% strong bases, ammonium hydroxide feels slippery because it
(w/w). It finds application in special dissolution schemes. It readily decomposes the skin; thus, such contact must be
dissolves silicate minerals but not quartz, and thus it can be avoided. The vapors are very pungent and will burn skin and
used to determine quartz. It has also been used to dissolve eyes, so bottles should be opened and all work conducted in
tin-based alloys. a hood.
Some laboratories have found it worthwhile to prepare Sodium hydroxide (NaOH) is a solid at room temperature
their own ultra-purity acids with a sub-boiling still. This is (m.p. 328°C). ACS grade is sold in the form of pellets, lower
usually only practical for the gaseous acids (HC1, HNO3, HF, grades as pellets, flakes, or sticks. Look for labels that say "low
etc.). Mattinson (see references to this chapter) described a in carbonate," for others will not usually produce clear solu-
very simple device consisting of two Teflon FEP bottles of tions upon dissolution in water and must be filtered through
1000-mL capacity screwed into a Teflon TFE connector at a hardened filter paper. For convenience, solutions at various
right angle to one another. The bottle containing the "feed" concentrations are also sold, including standard solutions for
acid is heated by a 300-W heat lamp, and the collection bottle alkalimetric titrations.
is cooled in a running water bath. Over 500 mL of distillate Sodium hydroxide is a strong base with many uses. It dis-
are collected per week. Little and Brooks (see references to solves aluminum and its alloys with the formation of sodium
this chapter) later showed that the design can produce a prod- aluminate (NaA102). The reaction is generally quite vigorous
uct reduced by two to ten times in trace metals. Table 3-3 and exothermic (2Al°+ 2NaOH + 2H20 ~ 2NaAlO2 + 3H2).
lists concentration data for the common commercial reagent Frequently the solid is valuable as a flux in molten salt fu-
grade acids plus ammonium hydroxide. sions. Solid sodium hydroxide is delinquescent and cannot be
weighed accurately in air. It also absorbs carbon dioxide, and
this property is used to protect solutions from atmospheric
BASES CO2 by using a sodium hydroxide trap. It is also used to grav-
imetrically determine carbon dioxide.
The "caustics" or alkali compounds release hydroxyl ion in For these applications, the compound is adsorbed onto the
aqueous solution. According to our simplified definition, surface of an inert substrate (formerly asbestos, now a clay
these are the bases. As with acids, the extent of dissociation mineral) that does not clog as a trap of the pure compound
determines if they are "strong" or "weak." Bases are used as would do. Sodium hydroxide solutions will precipitate many
dissolution agents in molten salt fusions; they are even used metals, including iron, nickel, cobalt, copper, manganese, sil-
in aqueous solution to dissolve certain amphoteric metals like ver, cadmium, indium, thallium, magnesium, titanium, zir-
aluminum. They are used as standard solutions in acid-base conium, hafnium, scandium, yttrium, lanthanum, and the
titrimetry, and they aid in the preparation of some reagent lanthanides, thorium, and uranium. Many metals are incom-
solutions. Bases also serve as reagents themselves in precip- pletely precipitated, such as niobium, tantalum, lead, bis-
itation and complexation reactions. muth, mercury, ruthenium, rubidium, osmium, and others.
Ammonium hydroxide (NH4OH) is a water solution of am- Thus sodium hydroxide separations are best applied after at
monia gas sold at a concentration of 29% (w/w) NH3 [equiv- least one separation by other means.
alent to 59.7% (w/w) NH4OH]. "Instrument" and ultra-purity The dissolution of sodium hydroxide in water is exother-
36 CHEMICAL ANALYSIS OF METALS

mic, accompanied by the generation of an aerosol, and is best Boric acid is also useful to eliminate the interference of flu-
conducted in a cooling bath in a hood. Excessive cooling, oride ion in many determinations.
however, will cause the solid to stick to the bottom of the The borates include sodium tetraborate or borax (NaEB407),
vessel, so vigorous stirring is called for. Sodium hydroxide lithium tetraborate (Li2B407), and lithium metaborate (LiBO2).
solutions readily leach silicon, aluminum, and boron from They are all available in anhydrous form and find use in the
borosilicate glass vessels, so solutions that are to be used for dissolution of high-alumina materials by molten salt tech-
the determination of these (and other elements that are con- niques. Combinations of carbonates and borates are some-
stituents of this type of labware) should be prepared, stored, times recommended as "universal" fluxes for handling a va-
and used in plastic vessels. In general, it is wise to store all riety of silica/alumina sample materials.
sodium hydroxide solutions in plastic and certainly any con- Potassium pyrosulfate (K2S207) is perhaps the m6st impor-
centrations above 1 mol. Always wear gloves since the slip- tant of the acidic fluxes. Manufacturers are reluctant to assign
pery feel of sodium hydroxide solutions is your own skin dis- the above formula to their product since the commercial re-
solving. Flush any contact with a great deal of cold water. agent is a mixture with potassium bisulfate, from which it is
Other bases include potassium hydroxide (KOH), which is formed (2KHSO4 ~ K 2 S 2 0 7 -b H20). A more correct way to
also sold as pellets and as solutions of various concentrations. refer to it is potassium bisulfate, fused. This flux will dissolve
This reagent finds more applications in organic analysis, al- most forms of alumina (including corundum, ruby, and sap-
though it is sometimes used as a molten salt flux. The same phire) and most metals and alloys. Potassium pyrosulfate is
is true of lithium hydroxide (LiOH), which is sometimes spec- also used to clean platinum crucibles. Since its attack on sil-
ified for molten salt fusions. The anhydrous form of this re- ica is limited, fusions can be conducted in Vycor, fused silica,
agent may be difficult to find, although it is more suitable for and even porcelain crucibles.
fusions than the monohydrate. Sodium bisulfate, fused (NaHSO4) is valuable when per-
chloric acid is to be used since sodium perchlorate is much
more soluble than potassium perchlorate. Thus, fusions in
this medium will not cause salting problems when leached in
M O L T E N SALT F L U X E S a perchloric acid solution. This flux tends to spatter more
than potassium pyrosulfate fusions, but this effect can be
We have already discussed hydroxides as molten salt fluxes ameliorated by the addition of a few drops of concentrated
for metals, ores, and other materials, but there are many sulfuric acid.
more compounds that find application for treating samples Sodium peroxide (Na202) will dissolve some of the most re-
that are refractory to acids. Here we will briefly review some fractory materials by molten salt fusion in a zirconium cru-
of the commonly used fluxes. cible (chromite ore, for example). For the metals analyst it
Sodium carbonate (Na2CO3) is one of the most widely ap- comes as close as anything to the universal solvent, but it is
plied compounds in this category. As an anhydrous powder usually relied upon only when necessary because of the haz-
or anhydrous granules, it will fuse most siliceous materials. ard and difficulties associated with its use. It is also used as
Since its melting point (851°C) is quite high, sodium carbon- a reagent to provide oxidation in caustic separations. When
ate fusions are performed in a platinum crucible with a lid, used as a flux, extreme caution is necessary since sodium per-
either over a burner or in a muffle furnace. Without a lid to oxide may react with pyrotechnic violence with certain sam-
contain the heat, the melting point will not be attained with ples. An approved shield is always advised. The cooled melt
a Meker burner. The ACS grade is good enough for most ap- reacts vigorously with water to produce a strong sodium hy-
plications, although an ultra-purity grade is available at about droxide solution. The oxidizing power of sodium peroxide
twice the cost. Sodium carbonate fusions have the advantage can be moderated by adding anhydrous sodium carbonate to
of not adding anions to the sample since carbonate is lost as the crucible before applying heat. Flush spilled reagent with
gaseous CO2 when the cooled, leached melt is acidified. a great deal of water and never use cloth, paper, or anything
Potassium carbonate (K2CO3) is specified for certain fu- organic to clean up a spill until the area has been flooded with
sions. Its solvent properties as a molten salt flux are fre- water.
quently enhanced by mixture with sodium carbonate. Alone,
it melts at 891°C. In a 50-50 (w/w) mixture with sodium car-
bonate, the eutectic melts at 715°C. OXIDIZING AGENTS
Lithium carbonate (Li2CO3) is the lowest melting of this se-
ries (618°C). It is also the most corrosive to the platinum cru- Reagents are frequently used for their oxidizing properties
cibles used for carbonate fusions. Because of the strong at- in solution chemistry. Sometimes these effects are selective
tack on the vessel materials, porcelain, Vycor, and fused silica and even surprising. Here we will summarize some facts
crucibles are unsuitable for any carbonate fusion. about the more commonly used oxidizing reagents.
Boric acid (HaBO3) and boron trioxide (B203) are an acid Potassium permanganate (KMnO4) is sold as dark purple or
and its anhydride (2HaBO3 --~ B2Oa + 3H20). Both are avail- black crystals that dissolve in water. When used for volumet-
able, although the acid is more accessible in high-purity form. ric work, the solution is allowed to stand in the dark for two
At 300°C the acid decomposes to the trioxide, which melts at weeks and then filtered. It must then be standardized before
460°C. Since the loss of water does not cause spattering, the use by titration of a weighed portion of either oxalic acid or
acid can be used effectively as a molten salt flux. These are sodium oxalate, the latter being preferred since it is not as-
acidic flux materials, suitable for a wide range of dissolutions. sociated with waters of hydration. Since the reaction is slow,
 CHAPTER 3--REAGENTS 37

the solution must be i~e/~ted near the endpoint. Once stan- moisture is ever suspected, place the crystals in an ungreased
dardized, potassium permanganate solutions can be used to desiccator for several days (ungreased because even trace or-
titrate the reduced form of many elements, most notably iron ganic contamination must be avoided). In aqueous solution,
(II). Today potassium permanganate has fallen from favor ex- the reacting species is actually always triiodide ion (Is).
cept where it is irreplaceable (as in the Lingane-Karplus ti- While I2 is volatile and has very low solubility in water, Is
tration for manganese) because of its need for daily stan- tends to be less volatile and is very soluble in water. Thus a
dardization. Solutions of potassium permanganate are also 0.1 N iodine standard solution is prepared by weighing 12.7
used for their oxidizing power in other types of methods and g of iodine into a beaker, then weighing 40 g of potassium
to test reagents for the presence of reducing substances. iodide into the same beaker. About 10 mL of water are added,
Potassium dichromate (K2Cr207), unlike the above, is a pri- and the solution is alternately swirled and allowed to stand
mary standard when purchased in a sufficiently pure grade. until all is dissolved (swirling rather than stirring lessens vo-
Primary standard grade potassium dichromate is prepared by latile loss of iodine). The solution is transferred to a 1-L vol-
repeated crystallizations from aqueous solutions and may be umetric flask, diluted to the mark, and mixed. It is then trans-
obtained from reagent chemical manufacturers or standard- ferred to an amber or low-actinic glass bottle with a glass
izing bodies such as NIST. These products can usually be stopper for storage in a cool, dark location. The analyst must
weighed without drying because they are not hygroscopic. always remain aware that iodine has significant volatility;
The weighed crystals dissolve readily in water, and the resul- thus, solutions should be removed from storage vessels by
tant solutions require no further standardization. Protected pipet, not poured, and storage bottles must be kept stoppered.
from evaporative losses, potassium dichromate solutions Iodine standard solutions are almost always used with a sol-
keep a stable titer for many weeks or even months. They are uble starch indicator (best prepared by dissolving a water
employed in the redox titration of iron, tin, tellurium, and paste of potato starch in boiling water). Many elements are
tungsten, and as back-titrants for chromium and other anal- determined iodimetrically, notably tin, arsenic, and anti-
ytes reduced by iron (II). Unlike potassium permanganate, mony. A disadvantage with iodine titrations is that most re-
which is self-indicating in all but the most dilute concentra- action rates are very sensitive to pH and temperature.
tions, potassium dichromate requires an indicator (fre- Potassium iodate (KIOs) provides a more convenient way
quently one of the diphenylamines) for visual endpoint work. of performing iodimetric work. It is a primary standard that
Ceric sulfate (Ce(SO4)2) should not be purchased as such will generate triiodide in the presence of iodide ion and acid
when it is to be employed as a redox titrant. The commercial (IO3 + 8I- + 6H ÷ -* 313 + 3H20). The potassium iodate
product is usually impure and difficult to dissolve. The analyst should be dried at about 170°C for 1 h. To prepare a 0.1 N
should obtain primary standard grade ceric a m m o n i u m ni- solution, combine 3.5667 g of potassium iodate with 10 g of
trate ((NH4)ECe(NO3)6), dry it at 85°C, and, after cooling in a potassium iodide and 1 g of sodium hydroxide, dissolve in
desiccator, weigh an appropriate amount into a large beaker water, and dilute to 1 L. The reactant species (I~ is instantly
[54.83 g for a 0.1 N Ce(SO4)2 solution]. Concentrated sulfuric generated upon addition to the acid sample solution. Potas-
acid is then added (56 mL for a solution that is to be 1M sium iodate is also sometimes used to precipitate thorium,
H2SO4), and the crystals are stirred for a time. Then (in vio- especially to remove it from the rare earths.
lation of the acid-water rule) very small increments of water Potassium bromate (KBrO3) is also available in a primary
are very cautiously added, while the solution is continuously standard grade. It should be dried at 150°C. It is sometimes
stirred until the heat of reaction has dissolved all of the solid used to standardize sodium thiosulfate solutions. Usually in
reagent. This process takes some time. When cool, the solu- this procedure an exact volume of 0.1 N potassium bromate
tion is transferred to a 1-L volumetric flask, diluted almost to is allowed to react with potassium iodide in the presence of
the mark, mixed, cooled to room temperature again, diluted hydrochloric acid. After 5 rain the liberated iodine is titrated
to the mark and mixed. Standardization is not needed for a with sodium thiosulfate solution (near 0.1 N). Arsenic (III),
solution prepared in this way, and the titer is stable for antimony (III), and iron (II), among others, can be titrated
months. directly with sodium bromate. In these reactions bromate is
Cerium (IV) sulfate undergoes most of the same reactions reduced to bromide (BrO~ + 6H ÷ -~ Br- + 3H20), and at
as potassium permanganate. Its formal redox potentials the endpoint the slight excess of bromate generates bromine
are higher, however, and show marked variation in differ- (BrO3 + Br- + 6H ÷ --~ Br2 + 3H20), which is easy to detect
ent acid media. The reason for this may be that instead with the proper indicator. A mixture of potassium bromate
of the simple Ce4+/Ces÷ couple, complex ions like and potassium bromide is sometimes useful for generating
Ce(SO4)2-/(Ce3÷)(SO42-)s may be involved. As with potassium free bromine in solution. Such a mixture can also be used in
dichromate, endpoints are determined either potentiometri- the precipitation of bismuth oxybromide (BiOBr).
caUy, or visually with an internal indicator (often ferroin, Bromine (Br2) is a useful oxidizing agent in many proce-
which is the ferrous complex of o-phenanthroline), and some- dures. The pure halogen is a noxious, corrosive, dark-brown
times an osmium tetroxide catalyst is employed. liquid (b.p. 58.76°C) in equilibrium with the heavy, reddish-
Iodine (I2) is a very useful oxidizing titrant, but its solutions brown fumes of its vapor phase. Except for bromine-metha-
must be standardized by means of a primary standard ma- nol or bromine-methyl acetate inclusion isolations and the
terial such as arsenious oxide. The best grade of iodine avail- bromine-hydrobromic acid dissolution of lead alloys, most
able is sublimed and recrystallized. Never put iodine in a applications of bromine utilize a saturated water solution
drying oven; it is a low-temperature, high-vapor pressure ma- ("bromine water"). Elemental bromine and its vapors are ex-
terial that would quickly corrode the inside of an oven. If tremely hazardous, and all work must be conducted with
38 CHEMICAL ANALYSIS OF METALS

gloves in an efficient hood. One advantage to oxidation with but the variation is much less dramatic than for ferrous sulfate
bromine water is that excess reagent can be readily removed (FeSO4 • 7H20). The reducing power of ferrous sulfate is best
by boiling the solution. Bromine will oxidize sulfur to sulfate utilized when it is added to a solution as a weighed salt. Fer-
and is often employed for this purpose in gravimetric sulfur rous ammonium sulfate solutions, on the other hand, are eas-
procedures. Bromine water is also employed to prevent any ily prepared and reasonably stable. It is important, however,
reduction of molybdenum in its precipitation with alpha-ben- that ferrous salts not be placed in a drying oven before weigh-
zoinoxime. ing, as they will oxidize. Ferrous ammonium sulfate is used
Hydrogen peroxide (H202) serves as both an oxidizing and in the redox titration of chromium and vanadium and in nu-
a reducing agent. Adding it during the acid dissolution of met- merous methods as a back-titrant for excess additions of per-
als creates oxidizing conditions, but does not passivate metal manganate, dichromate, or cerium (IV).
surfaces (as nitric acid often does). A brief boil converts it to Sodium oxalate (Na2C204) and oxalic acid (H2C204 • 2H20 )
oxygen and water, so it is easily removed from the solution. can both be used to standardize potassium permanganate
Hydrogen peroxide is most useful as the 30% (w/w) ACS grade since both can be primary reductimetric standards. Sodium
reagent that is stabilized with sodium stannate (NaESnO3); an oxalate is the material of choice, however, since it is not a
ultra-purity grade at about the same concentration is also hydrate (many hydrates, including oxalic acid, show some
available. It readily oxidizes ferrous iron, but, interestingly, it variation in the degree of hydration). Even so, sodium oxalate
reduces permanganate and dichromate. These reactions take must be dried at 100°C before use. The titration of either re-
different paths; thus, iron (II) is oxidized as follows: 2Fe 2÷ + agent with permanganate involves an inherently slow reac-
H202 + 2H + ~ 2Fe 3+ + 2H20, and permanganate is reduced tion, so it is important to warm the solution a few millilitres
by this means: 2MnO4 + 5H202 + 6H + --~ 2Mn 2+ + 502 + short of the final endpoint.
8H20. Arsenious oxide (As203) and sodium meta-arsenite (NaAsO2)
With dichromate, a deep blue flash of color when hydrogen are even better reductimetric standards than the above, al-
peroxide is first added shows that a brief excursion to a very though both are highly toxic. Arsenious oxide is generally
high, unstable oxidation state (perchromate ion) precedes re- available in a purer form than the water-soluble sodium ar-
duction to Cr 3+. Hydrogen peroxide at 30% concentration will senite, but it must be dissolved in dilute sodium hydroxide
produce white stinging burns on the skin that persist for 5 to solution and the solution neutralized with acid before dilu-
15 rain, even if flushed with water. Thus, gloves should be tion. These reagents are used in the redox titration of per-
worn when working with this reagent. Some labs extend the manganate, cerium (IV), bromine, and iodine, and frequently
shelf life of hydrogen peroxide by storing the reagent in a in the back-titration of excess additions of permanganate and
refrigerator. Discard any reagent that has become contami- cerium (IV). It is extremely important not to dry these com-
nated by flushing it down a drain with a large amount of pounds in a drying oven before weighing since toxic vapors
water. would be generated.
Ammonium persulfate ((NH4)25208), also known as am- Sodium thiosulfate (Na25203) is available in anhydrous
monium peroxydisulfate, is a very powerful oxidizing agent, form and as a 5-hydrate (photographic "hypo"). As with all
especially in the presence of a silver ion, which serves as a materials for standard solution preparation, the water-free
catalyst in the oxidation of Cr 3+ to dichromate, Mn 2+ to per- form, if available, is usually preferred because hydrates can
manganate, and VO2÷ to VO~ (in boiling dilute sulfuric acid). effloresce when exposed to moist air, producing a substance
Advantage is often taken of these reactions in the volumetric of indefinite composition. Sodium thiosulfate is not a primary
determination of chromium, manganese, and vanadium. The standard. Also, its solutions are subject to deterioration by
reagent is also sometimes used to oxidize chromium when the presence of a thiobaciUus microorganism. But, neverthe-
the use of perchloric acid is precluded. less, it is a valuable reductimetric titrant, especially for io-
Obviously, there are many other oxidizing agents used in dine. Various sterilizing additives have been suggested to pre-
the analysis of metals, some with narrow or specific uses-- vent the growth of sulfur bacteria in the solution, including
potassium metaperiodate (KIO4) or periodic acid (H5IO6) sodium carbonate, sodium borate, di-basic sodium phos-
to oxidize manganese to permanganate for its spectro- phate, and chloroform. At the very least the standard solution
photometric determination; potassium ferrocyanide should be prepared with freshly boiled and cooled distilled
(K3Fe(CN)6 • 3H20) to oxidize cobalt (II) to cobalt (III) in its water. Just prior to each use, the sodium thiosulfate solution
potentiometric titration. These are but two examples. should be standardized by titration against standard potas-
sium permanganate or standard iodine solutions. If the so-
lution becomes turbid or if sulfur coats the glass storage ves-
REDUCING AGENTS sel walls, the solution must be discarded and the vessel
sterilized by soaking in chromic acid before it is reused.
As with oxidizing agents, there is no clear line between re- Stannous chloride (SnC12 • 2H20) is an important reducing
ducing agents used as standard solutions in redox volumetric agent in solution chemistry. It dissolves in hydrochloric acid
work and reducing agents used in general reaction chemistry. and will remain a clear solution down to about 1 : 1 HC1 : H20.
Ferrous ammonium sulfate (Fe(NH4)2(SO4)2.6H20), also To prevent hydrolysis of stannous oxide at lower hydrochloric
known as ammonium iron (II) sulfate, hexahydrate, or Mohr's acid concentrations, a few tin shot pellets are usually added
salt, is a widely used reducing titrant. Properly classed as a to the solution. However, the solution must not be stored in
secondary standard, its exact normality can be established a tightly sealed container since pressure may develop from
easily by titration with standard potassium dichromate. The the formation of hydrogen gas. Stannous chloride is com-
normality or titer will change daily due to some air oxidation, monly used to reduce iron for its subsequent volumetric de-
 CHAPTER 3 - - R E A G E N T S 39

TABLE 3--4--Half-cell potentials for selected redox species.

Ion/Compound Half-CellReaction E°aox, V
Persulfate $202- + 2e- ---, 2SO2- 2.01
C o 3+ + e- --~ C o 2+ 1.82
Hydrogen Peroxide H202 + 2H + + 2e ---2H20 1.77
Ce (IV) + e- ~ Ce (III) (1 M HCIO4) 1.61
Permanganate MnO4 + 8H ÷ + 5e- ~ Mn 2÷ + 4H20 1.51
Dichromate Cr202- + 14H ÷ + 6e- --->Cr3+ + 7H20 1.33
VO~ + 2 H ÷ + e ~ V O 2÷ + H 2 0 1.00
Fe 3+ + e- ~ F e 2+ 0.771
Hydrogen Peroxide H202 --~ 02 + 2H + + 2e- 0.682
Iodine 13 + 2 e - ~ 3 I - 0.545
Ferrocyanide Fe(CN)a6- + e --~ Fe(CN)~- 0.36
Cu2+ + 2 e - ~ C u ° 0.337
Saturated Calomel Electrode Hg2C12 + 2e- ~ 2Hg° + 2C1- 0.244
Sulfurous Acid SO2- + 4H ÷ + 2e- ~ SO2(aq.) + 2H20 0.17
Sn '*+ + 2e- ---> S n 2+ 0.154
Thiosulfate $402- + 2e- --', 2 5 2 O ] - 0.08
2H + + 2e- --o H20 0.000
Hydrazinium N2 + 5H + + 4e- --->H2NNH~- -0.23
Ni2+ + 2e- --->Ni° -0.246
Oxalic Acid 2CO2 + 2H + + 2e- --> H2C204 -0.49
Hypophosphorous Acid H3P03 + 2H + + 2e- --->H3PO2 + H20 -0.50

t e r m i n a t i o n a n d to r e d u c e m o l y b d e n u m p r i o r to its spectro- phorous acid (H3PO2) is sold as a 50% (w/w) solution. It is the
p h o t o m e t r i c d e t e r m i n a t i o n . It will also reduce selenium, strongest r e d u c i n g agent in c o m m o n use a n d will reduce cer-
tellurium, a n d arsenic to the elemental state. tain metal ions to the elemental state. Titanous chloride
Sulfurous acid (H3SO3) is sold as an 8% (w/w) solution of (TiCl3), sold as a 20% (w/w) solution, c a n be used to titrate
SO2 gas in water. It reduces v a n a d i u m (V), iron (III), a n d i r o n (III) (when used as a 1% solution p r o t e c t e d b y a b l a n k e t
a n t i m o n y (V) to v a n a d i u m (IV), i r o n (II), a n d a n t i m o n y (III), of c a r b o n dioxide). Sodium nitrite (NaNO2) is convenient (as
respectively. It reduces s e l e n i u m a n d t e l l u r i u m to elemental a fresh a q u e o u s solution) for destroying the p e r m a n g a n a t e
form. And it causes the "earth acids" to p r e c i p i t a t e as h y d r o u s color after its s p e c t r o p h o t o m e t r i c m e a s u r e m e n t (so t h a t the
oxides. It is s o m e t i m e s useful to redissolve MnO2, w h i c h m a y s a m p l e b a c k g r o u n d color can be m e a s u r e d ) . Sodium sulfite
p r e c i p i t a t e from solutions of h i g h - m a n g a n e s e alloys. It also (Na2SO3), sodium bisulfite (NaHSO3), a n d sodium dithionite
reduces c h r o m i u m to the trivalent state in the presence of (Na2S204) have all b e e n used for their r e d u c i n g p o w e r in the
h y d r o c h l o r i c acid. Sulfurous a c i d t h a t has b e e n s t o r e d for analysis of metals. Ascorbic acid (C6HsO6) can be used as a
s o m e t i m e loses strength due to outgassing a n d the f o r m a t i o n r e d u c t i m e t r i c titrant, b u t it is p r i n c i p a l l y u s e d as a r e d u c i n g
of sulfur c o m p o u n d s (which adversely affect its r e d u c i n g agent in p r o c e d u r e s like the TOPO/iodide/MIBK extraction of
power). The bottle should b e o p e n e d cautiously in a h o o d trace tin a n d o t h e r metals.
since p r e s s u r e m a y develop, a n d all o p e r a t i o n s with sulfurous A related a r e a m i g h t be m o r e p r o p e r l y t e r m e d "reducing
acid m u s t be similarly c o n d u c t e d in a hood. Sulfurous acid techniques." Thus nascent hydrogen f o r m e d in solution b y the
has the a d v a n t a g e that any excess m a y be c o m p l e t e l y expelled a c t i o n of acid on s o m e p u r e reactive m e t a l has b e e n used to
f r o m a n acidic s a m p l e solution with 5 o r 6 m i n of boiling. reduce metal ions. The Jones reductor a n d the silver reductor
Hydrazine sulfate (HENNH2 • H 2 5 0 4 ) is a powerful r e d u c i n g a r e r e d u c i n g systems confined in glass c o l u m n s t h r o u g h
agent, which, like h y d r a z i n e a n d its dihydrate, is very toxic, w h i c h a n acid solution of the s a m p l e is passed. The Jones
It is, however, m u c h m o r e stable t h a n those two c o m p o u n d s . r e d u c t o r is filled with a z i n c - m e r c u r y a m a l g a m (1 to 5% mer-
H y d r a z i n e sulfate is a solid t h a t dissolves in water. S u c h so- cury) of 20 to 30 m e s h (850 to 600/zm) a n d will reduce iron,
lutions s h o u l d always be p r e p a r e d fresh on the d a y of use. c h r o m i u m , v a n a d i u m , m o l y b d e n u m , t i t a n i u m , a n d o t h e r el-
One i m p o r t a n t a p p l i c a t i o n is in the r e d u c t i o n of heteropoly- e m e n t s i n t r o d u c e d in dilute h y d r o c h l o r i c or sulfuric acid so-
m o l y b d a t e complexes to their intensely colored forms, as in lution. The silver r e d u c t o r is filled with metallic silver (pre-
the s p e c t r o p h o t o m e t r i c d e t e r m i n a t i o n of p h o s p h o r u s . The cipitated by ionic d i s p l a c e m e n t with p u r e c o p p e r from a silver
h y d r a z i n i u m ion, in turn, d e c o m p o s e s into h y d r o g e n ion a n d nitrate solution). Iron, m o l y b d e n u m , v a n a d i u m , a n d o t h e r el-
N2 (which leaves the solution). e m e n t s are r e d u c e d b y the silver reductor, b u t t i t a n i u m is not.
Hydroxylamine hydrochloride (NHEOH • HC1) is a c o m p a r - Table 3-4 shows s o m e half-cell reactions a n d potentials for
atively m i l d r e d u c i n g agent t h a t works b e s t in very dilute acid selected redox reactants. Table 3-5 illustrates the p r e p a r a t i o n
solution. Thus it finds use wherever large a m o u n t s of a c i d are of certain redox s t a n d a r d solutions.
not r e q u i r e d to keep alloy c o m p o n e n t s in solution. It has b e e n
u s e d to r e d u c e i r o n p r i o r to the gravimetric d e t e r m i n a t i o n of
b a r i u m in b a r i u m ferrite m a g n e t m a t e r i a l s a n d to r e d u c e
m a n g a n e s e a n d c e r i u m in m a g n e s i u m alloy solutions. It is PRECIPITANTS
a d d e d either as a freshly p r e p a r e d solution o r directly as solid
crystals. The r e a c t i o n p r o d u c t s are N2, H +, a n d H20. We have a l r e a d y discussed several i m p o r t a n t p r e c i p i t a n t s
A few o t h e r r e d u c i n g agents b e a r brief mention. Hypophos- in previous c a t e g o r i e s - - a m m o n i u m a n d s o d i u m hydroxides,
40 CHEMICAL ANALYSIS OF METALS

TABLE 3-5--Preparation for selected redox standard solutions (0.1N). (In all cases dilute to 1 L.)
Compound Weight, g Procedure Standardization Shelf Life
Ceric sulfate, 54.83 (ceric ammonium Add 56 mL conc. H2SO4, stir, then Primary standard Stable
Ce(SO4)2 nitrate, dried at 8 5 ° C ) cautiously add H20 in small
increments with stirring until
dissolved, then dilute
Potassium 3.2 Add 1 L H20 and stir. Dry primary standard grade sodium Standardize for
permanganate, Heat below 90°C for 1 h. Cool, oxalate and accurately weight 0.3 each use.
KMn04 store in dark two weeks, then g into 600 mL beaker. Add 250 (Store in
filter. mL H2504 (5 + 95), which has dark.)
been boiled and cooled. Stir to
dissolve. Add 40 mL KMnO4 by
buret and stir until color is gone.
Heat to 60°C and carefully finish
titration to first permanent pink.
Potassium 4.903 (dried at 100°C) Dissolve in H20 and dilute. Primary standard Stable
dichromate,
K~Cr207
Iodine, I2 12.7 Add 40 g KI to the weighed I2 in a Accurately weigh 0.3 g primary Standardize for
50 mL beaker. Add 20 mL H20 standard grade As2Oa into a 250 each use.
and swirl gently until dissolved, mL Erlenmeyer flask; add 10 mL (Store in
then dilute. 1 N NaOH and stir to dissolve. dark.)
Add 15 mL 1 N H2SO4, stir, then
add 50 mL NaHCO3 solution
(40g/L). Add 5mL 0.5% starch
solution and titrate with I2 to first
blue color.
Sodium 25. (5-hydrate) Dissolve with 0.1 g Na2CO3 in 1 L Add I0 g KI to each of two 250 mL Standardize for
thiosulfate, of freshly boiled, cooled H20. Erlenmeyer flasks. Add 100 mL each use.
Na25203 boiled and cooled H20 and 2 mL (Discard if
HC1 to each. To one flask add 40 turbidity or
mL standard KMnO4 by buret. deposits
Stopper both and let stand 10 appear.)
min in the dark. Add 2 mL 0.5%
starch solution to both flasks and
titrate both with thiosulfate to
first blue color. Subtract the
blank.

h y d r o g e n sulfide, sulfur dioxide, s t a n n o u s chloride, a n d oth- t r e a t m e n t . Ferric i r o n forms a d a r k b r o w n precipitate, a n d

ers. Here we will e x a m i n e a few more. c o p p e r (II) forms a gray precipitate, b u t these elements are
Dimethylglyoxime (CH2C(:NOH)C(:NOH)CH2) is a white m o s t frequently r e m o v e d before c u p f e r r o n is used. The "earth
p o w d e r soluble in m e t h a n o l o r in s o d i u m h y d r o x i d e solution. acid" elements (groups IVB, VB, a n d VIB m i n u s Cr) all pre-
It is a specific p r e c i p i t a n t for nickel a n d p a l l a d i u m . Nickel cipitate. Significantly, a l u m i n u m , c h r o m i u m , cobalt, a n d
precipitates as a b r i g h t red v o l u m i n o u s c o m p o u n d f r o m am- nickel do not precipitate. Cupferron precipitates are either
m o n i a c a l solutions, while p a l l a d i u m comes d o w n as a yellow "wet ashed" o r ignited to oxides. The r e a g e n t is listed as a
c o m p o u n d from dilute h y d r o c h l o r i c acid solutions. These c a r c i n o g e n a n d should be h a n d l e d w i t h gloves in an efficient
precipitates are d r i e d a n d w e i g h e d as s t o i c h i o m e t r i c com- hood; drying, charting, a n d igniting the precipitates m u s t
pounds. likewise be c o n d u c t e d in a n efficient hood.
alpha-Benzoinoxime (C6HsCHOHC(:NOH)C6Hs) or benzoin 8-Hydroxyquinoline (HOC6H3N:CHCH:CH) or 8-quinolinol
anti-oxime is a m e t h a n o l - s o l u b l e white p o w d e r that is a spe- is a white p o w d e r w i t h a c h a r a c t e r i s t i c o d o r that dissolves in
cific p r e c i p i t a n t for m o l y b d e n u m . The p r e c i p i t a t e is n o t stoi- acetic acid solution. It is a nonspecific p r e c i p i t a n t that has
c h i o m e t r i c a n d m u s t be ignited at low t e m p e r a t u r e (<525°C) found use in the p r e c i p i t a t i o n of a l u m i n u m as a yellow com-
to MoO3. Tungsten is c o p r e c i p i t a t e d quantitatively in the p o u n d after t h a t e l e m e n t has b e e n isolated b y p r e l i m i n a r y
p r e s e n c e of excess m o l y b d e n u m , b u t its p r e c i p i t a t i o n is oth- separations. It will n o t p r e c i p i t a t e beryllium, w h i c h is other-
erwise incomplete. wise difficult to s e p a r a t e from a l u m i n u m . 8-hydroxyquinoline
Cupferron (C6H5N(NO)ONH4) is the ammonium salt of has also b e e n u s e d to p r e c i p i t a t e zinc from acetic a c i d solu-
N-nitroso-N-phenylhydroxylamine. It is a crystalline m a t e r i a l tion a n d m a g n e s i u m from a m m o n i a c a l solution (with the
usually sold with cloth p o u c h e s of a m m o n i u m c a r b o n a t e in r e a g e n t a d d e d as a n alcohol solution). S o m e t i m e s these pre-
the bottle to m a i n t a i n a slight a m m o n i a a t m o s p h e r e to pre- cipitates a r e dissolved in h y d r o c h l o r i c acid, a n d the released
vent d e c o m p o s i t i o n . Cupferron dissolves in water, b u t m u s t 8-hydroxyquinoline is r e a c t e d with excess b r o m i n e ( s t a n d a r d
be filtered before use. A 6% (w/v) solution is stable for a week b r o m a t e plus b r o m i d e ) a n d the excess b a c k - t i t r a t e d with stan-
u n d e r refrigeration. It is a nonspecific p r e c i p i t a n t that is sel- d a r d s o d i u m thiosulfate. More c o m m o n l y , they a r e d r i e d a n d
dom applied without some preliminary separation or other weighed or s o m e t i m e s ignited to the oxide a n d weighed.
 CHAPTER 3--REAGENTS 41

TABLE 3-6---Solubilityof selected metal compounds.

Br- CO2- C1- CrO~4 CN F OH- I- C202- PO43 SO]- S2- SO2- C4I-I4062- SCN
A1 S ... S ...... S A S A A S D ... A ...
Sb D ... S ...... S ... D ...... A A ... S ...
As D D S D I
Ba S "A" S "A' 'S A "S" S "A' "A" "i" O "S" "A" "S"
Be S S S ... S D S S S A ...
Bi O _ D _ "A" S "A" A A A O A _ 'A" iii
Cd S "A' S "A" S S A S A A S A "A' A
Ca S A S S S A S S A A A A A A "S'-
Cr B S B _ A A A B S A B D _ A
Co S A S "A" A S A S A A S A "A' A "S'-
Cu S ... S ... A A A A A A S A A A D
Au B B B B I I
Fe S "A' S 'A" A "A" A S 'B" "A' "B" A "A' "B" "S"
Pb S A S A A A A A A A A A A A A
Mg S A S S S A A S A A S D S A S
Mn S A S _ _ A A S A A S A ... A S
Hg B A B "A" "B" D A A A A A I _ I A
Ni S A S A A A A S A A S A "A" A ...
Pt A S ... I S A I ... S I . . . . . . . . .
RE S "A'- S I A S "A'- S D
Ag A g g "A" "A" S - I A "A' A A "A' "A" "i"
Sr S A S A S A "S" S A A A S A A S
Sn S S B S A S A A S A S
Zn S "A'- S A "A'- A A S A A S A "A'- g "S'-
NOTE:
S = Solublein water.
B = One form is solublein water and one is insolublein water.
D = Decomposesin water.
A = Insolublein water or slightlysolublein water, but solublein acids.
I = Insolublein water and insolublein acids.

para-Bromomandelic acid (BrC6H5CHOHCOOH) is the ganic Analysis describe the use of sodium acetate
most useful of a series of substitution products of m a n d e l i c (CH3COONa) i n the "basic acetate method" a n d sodium suc-
acid. It is a selective precipitant for z i r c o n i u m a n d h a f n i u m cinate (C4H404Na2 • 6H20) for separating iron, b u t these ap-
from hot hydrochloric acid solution. It works best w h e n proaches are little used today. Nitron (C20H16N4) will precip-
added as the solid crystals to the acidified sample, which is itate r h e n i u m as the perrhenate.
t h e n digested in a boiling water bath. This reagent is expen- A m o n g the i n o r g a n i c precipitants, hydrochloric acid is used
sive to use since it is only available as a "fine chemical" from for silver, m e r c u r y (I), a n d t h a l l i u m (I); sulfuric acid is used
specialty chemical companies. for b a r i u m , lead, a n d strontium. Dibasic ammonium phos-
Cinchonine (CI9H22N20) is a white powder soluble in 1 : 1 phate ((NH4)EHPO4) has b e e n applied to precipitate alumi-
hydrochloric acid: water. It is a specific precipitant for tung- n u m , bismuth, c a d m i u m , zinc, zirconium, a n d h a f n i u m . Po-
sten always used in c o m b i n a t i o n with conditions that favor tassium chromate (K2CrO4) is useful for b a r i u m , lead, a n d
the hydrolysis of t u n g s t e n trioxide. The resultant precipitate thallium. Potassium fluoride (KF), ammonium fluoride
is always c o n t a m i n a t e d , so the ignited, weighed residue is (NH4F), a n d hydrofluoric acid have been variously applied to
treated to dissolve the WO3, a n d the impurities are weighed precipitate calcium, t h o r i u m , scandium, yttrium, a n d the lan-
as a correction. thanides. Silver nitrate (AgNO3) has b e e n used for chlorine (as
Thioacetamide (CHaCSNH2) is the principal agent for per- in t i t a n i u m metal), a n d barium chloride (BaC12) has b e e n used
f o r m i n g sulfide separations by h o m o g e n o u s phase g e n e r a t i o n for sulfur (after oxidation to sulfate). Table 3-6 s u m m a r i z e s
of hydrogen sulfide. The reaction (CHaCSNH2 + H20 ---> the solubilities of a selection of metal c o m p o u n d s .
CH3CONH2 + HES) proceeds u n d e r mildly acidic, neutral, or
mildly basic conditions. The reagent is a k n o w n carcinogen,
a n d H2S is highly toxic, thus gloves a n d a hood are essential. COMPLEXING AGENTS
There is a long list of elements that precipitate as sulfides,
forming subgroups based o n the solution's hydrogen ion con- I n this category we will review some of the c o m m o n l y used
centration. Detailed separation schemes f o u n d e d principally substances that form stable, soluble complexes with metal
o n sulfide separations have b e e n devised, b u t are n o t m u c h ions in solution. These reagents are used in two m a j o r w a y s - -
used today. The technique is still valuable as a single-step as titrants in complexometric titrations a n d as m a s k i n g
separation, however. agents to prevent a precipitation or other reaction from oc-
There are, of course, m a n y other organic precipitants that curring.
find use from time to time. Ammonium oxalate EDTA (Ci0 H16N2Os) or (ethylenedinitrilo)tetraacetic acid is
((NH4)2C204. H20) is sometimes used for calcium, stron- certainly the most i m p o r t a n t substance in this category. Since
tium, a n d the rare earths. Hillebrand et al. i n Applied Inor- the acid form is difficult to dissolve even with heating, the
42 CHEMICAL ANALYSIS OF METALS

TABLE 3-7--Stable soluble complexes.

NH3 Br- CI- CN- F- I Acetate Citrate Oxalate Tartrate EDTA
A1 X X X
Ba X X X
Be X X X X
Bi X X X
Cd X X X X X X X X
Ca X X X
Cr X X
Co X X X X
Cu X X X X X X
Au X X X
Fe X X X X X X X
Pb X X X X X
Mg X X X
Mn X X X X
Hg X X X X X
Ni X X X X
Pt X X X X
RE X X X
Ag X X X X
Sr X X X
Sn X X X X
Ti X
Zn X X X X
Zr X X X

d i s o d i u m salt (a dihydrate) is m o s t frequently used. S o m e fine to c o m p l e x iron; fluoride, tartrate, a n d oxalate are u s e d to h o l d
c h e m i c a l houses m a r k e t an a m m o n i u m salt that finds appli- the easily h y d r o l y z a b l e e a r t h acids in solution. Peroxide holds
cation in GFAA work. EDTA is b y far the m o s t widely used t i t a n i u m a n d v a n a d i u m as peroxy complexes. Thiocyanate has
chelating agent; either direct o r indirect titration m e t h o d s b e e n u s e d for m o l y b d e n u m a n d copper. And ammonia c o m -
have been p u b l i s h e d for nearly every metallic element, Most plexes hold nickel, cobalt, silver, a n d c o p p e r in solution. E a c h
of this w o r k e m p l o y s visual e n d p o i n t s with a d d e d indicators, of these anions complexes with m a n y o t h e r cations, a n d the
each of which in s o m e w a y r e s p o n d s to an excess of EDTA. list of complexing anions could go on, b u t even t h e n we w o u l d
In the metals analysis laboratory, EDTA titrations are u s e d have only s c r a t c h e d the surface of this rich field of a q u e o u s
for cobalt, nickel, zinc a n d a l u m i n u m , a m o n g o t h e r elements. chemistry. See T a b l e 3-7 for a selection of stable, soluble
EDTA is also a d d e d to s a m p l e s in the ferrocyanide t i t r a t i o n complexes.
of cobalt to prevent the interference of diverse ions. In this
a p p l i c a t i o n cobalt u n d e r g o e s oxidation b y ferrocyanide ion
while held in the "grip" of the chelate. While there are m a n y
related chelating c o m p o u n d s of this class (generically, ami- SPECTROPHOTOMETRIC REAGENTS
nopolycarboxylic acids o r "complexanes"), surprisingly few
are used in analytical procedures. The inclusion of this category was d e b a t a b l e b e c a u s e of the
Potassium cyanide (KCN) has b e e n used as b o t h a t i t r a n t impossibility of d o i n g justice in a few p a r a g r a p h s to an a r e a
a n d a m a s k i n g agent for decades, b u t its use has fallen out of that m u l t i - v o l u m e tracts have b e e n written about. However,
favor b e c a u s e of disposal a n d o t h e r toxic h a z a r d p r o b l e m s . it was d e c i d e d to include a few w o r d s a b o u t s o m e c o m p o u n d s
Acidification generates d e a d l y h y d r o g e n cyanide gas a n d with a k n o w n t r a c k r e c o r d in metals analysis labs.
m u s t be avoided at all costs. Gloves a n d an efficient h o o d are Acetylacetone ((CHaCO)2CH2) or 2,4-pentanedione is b o t h a
absolutely essential for all w o r k with this c o m p o u n d . All solvent e x t r a c t a n t a n d a c h r o m o p h o r e . It is r e a s o n a b l y sen-
waste solutions m u s t be stored in closed c o n t a i n e r s for p r o p e r sitive for i r o n and, since the color is b l e a c h e d b y hydrofluoric
disposal. At one t i m e a p o t a s s i u m cyanide titration was one acid, has b e c o m e the basis of one of the p r o c e d u r e s for H F
of the p r i n c i p a l m e t h o d s for high levels of nickel. T o d a y it in pickling baths.
finds restricted use for its ability to m a s k certain reactions in Dithizone (C6HsNHNHCSN:NC6Hs) o r 1,5-diphenylthiocar-
gravimetry and spectrophotometry. bazone is soluble in c h l o r o f o r m a n d c a r b o n tetrachloride. It
Sodium pyrophosphate (NagP207 • 10H20) forms a wine- forms colored complexes with trace levels of lead, bismuth,
red, stable, soluble complex with m a n g a n e s e , w h i c h is u s e d a n d zinc.
as the basis of a clever a n d a c c u r a t e titration of that e l e m e n t Silver diethyldithiocarbamate ((C2Hs)2NCSSAg) dissolved in
with s t a n d a r d p e r m a n g a n a t e solution. In the Lingane-Kar- p y r i d i n e is used to collect evolved arsine gas (AsH3) from the
plus titration, b o t h s a m p l e m a n g a n e s e a n d t i t r a n t m a n g a n e s e sample. The colored c o m p l e x that forms is t h e n m e a s u r e d
end u p in the s a m e c o m p l e x e d state. spectrophotometrically.
S o m e o t h e r complexing substances are best d e s c r i b e d b y Nitroso R acid, disodium salt (CloHsNOsS2Na2) is an excel-
the anionic species involved since they are a p p l i e d in a variety lent color r e a g e n t for cobalt. W i t h steel s a m p l e s the iron ma-
of forms. Phosphate a n d citrate c o m p o u n d s are widely used trix is first r e m o v e d b y m e a n s of a zinc oxide separation.
 CHAPTER 3--REAGENTS 43

PyrogaUol (1,2,3-(HO)3C6H3) reacts with tantalum in the of aluminum. Similarly, dimethylglyoxime not only precipi-
presence of a m m o n i u m oxalate to produce a yellow complex. tates nickel and palladium, but also forms soluble colored
Hydroquinone (1,4-(HO)2C6H4) is a light-sensitive com- complexes with both elements.
pound that dissolves in concentrated sulfuric acid. It forms
colored complexes with tungsten, titanium, and niobium and
allows combinations of these analytes to be determined by ORGANIC SOLVENTS
measurement of the developed color at two different wave-
lengths. The metals analysis laboratory must keep a supply of sol-
Dianthrimide (C28H1504) or 1,1 "iminodianthraquinone was vents, both for sample preparation and for chemistry. Since
formerly used (in concentrated sulfuric acid solution) for the nearly all of these are either flammable or toxic, it is essential
direct determination of trace boron. It is one of the few spec- that they be properly stored, used, and disposed of. An ap-
trophotometric compounds for boron that does not require proved, grounded solvent cabinet should be the repository for
isolation of the analyte. most of these chemicals when not in use. Flammable solvent
Curcumin (C21H2206) is widely used for the spectrophoto- storage drums and cans must always be grounded. A few labs
metric determination of boron following its isolation by dis- have attempted to redistill their own solvents for reuse, but
tillation as the methyl borate ester. this practice is inherently hazardous since perchlorates and
Diantipyrylmethane (C23HE4N402 • H20 ) forms a colored other potentially explosive residues may contaminate the lab-
complex with titanium in aqueous solution that is used for oratory waste. Waste solvents should be stored in approved
the spectrophotometric measurement of that element. containers for disposal by approved techniques. Often it is
Brilliant green (CasHa404N2S) is the basis for a colorimetric appropriate to segregate polar and nonpolar waste solvents,
method for antimony. and in some cases each solvent compound should be sepa-
Neocuproine (C14H12N2) or 2,9-dimethyl-l,10-phenanthro- rately stored. The analyst should be aware that acid residues
line is the most widely used of a series of related reagents for that accompany many waste solvents may attack steel solvent
the spectrophotometric measurement of low levels of copper. storage cans.
It is used in a chloroform solution. Alcohols are essential in the metals analysis lab. A general
Trioctylphosphine oxide ((CH3(CH2)7)aPO) is not a spectro- solvent grade of methanol (CH3OH) should be available for
photometric reagent per se, but rather a valuable extractant degreasing and for removing organic residues from glass-
for lead, bismuth, tin thallium, and silver in trace amounts. ware. HPLC grade or higher is needed for methyl borate dis-
The extraction takes place in the presence of iodide with tillations. Methanol is a systemic poison and should be kept
methylisobutylketone as the solvent. Measurement is com- in bottles with appropriate warning labels. Both denatured
monly performed by flame atomic absorption. The same re- and absolute ethanol (C2HsOH) are usually necessary; the lat-
agent is used to extract zirconium into cyclohexane, in this ter is often stored with other precious commodities in the
case for spectrophotometric measurement with pyrocatechol laboratory safe. Both 1-propanol (CH3CH2CH2OH) (or N-pro-
violet (see below). panol) and 2-propanol (CH3CHOHCH3) (or isopropanol) are
Diphenylcarbazide (C6H5NHNHCONHNHC6Hs) or 1,5-di- often required. Similarly, 1-butanol (CH3CH2CH2CH2OH) (or
phenylcarbohydrazide is frequently used for the spectropho- N-butanol) and 2-methyl-l-propanol ((CH3)2CHCH2OH) (or
tometric determination of low levels of chromium. isobutyl alcohol) are frequently needed for solvent extraction
1, lO-Phenanthroline (C12HsN 2 • H20 ) or o-phenanthroline is work. Little else beyond this is usually necessary.
a sensitive spectrophotometric reagent for traces of iron. Ketones are even more restricted in use. Generally
Sodium thiocyanate (NaSCN) is a useful reagent in spectro- only acetone (CH3COCH3), methylisobutylketone
photometric methods for molybdenum in the presence of re- (CH3COCH2CH(CH3)2) (or MIBK) and perhaps methylethyl-
ducing conditions. The molybdenum color formation re- ketone (CHaCOCH2CH3) (or 2-butanone) are necessary.
quires the presence of iron. The colored complex tends Esters are required by some metals analysis work. Methyl
toward instability, and measures are often taken to ensure acetate (CH3COOCH3), used with bromine, is the basis for
reproducible readings. most ester-halogen steel inclusion isolations. Butyl acetate
Dithiol (C7H852) has been frequently employed for traces (CH3COO(CH2)a(CH3) is used in certain solvent extraction
of tungsten. Molybdenum interferes, but can be removed by procedures.
solvent extraction. Chlorinated hydrocarbons are a major category of impor-
Sodium molybdate (Na2MoO4 • 2H20) and ammonium mo- tant solvents. Methylene chloride (CHEC12) (or dichlorometh-
lybdate ((NH4)6Mo7024 • 4H20) are both used for spectropho- ane) is the simplest and least toxic. Despite recent hazard
tometric methods that produce the heteropolyacids--phos- warnings that implicate it with the other members of this cat-
phomolybdate or molybdivanadophosphate for phosphorus; egory, it remains a valuable general purpose solvent. It is
arsenomolybdate for arsenic; or molybdisilicate for silicon. flammable and has carcinogenic properties. It should be used
Pyrocatechol violet (C19H14075) has been used in the spec- in a hood (electric hotplate only) with gloves. Many labs use
trophotometric determination of zirconium and tin, among methylene chloride for sample degreasing. Chloroform
other metals. (CHC13) at one time served this purpose, but today, because
Already mentioned, hydrogen peroxide forms colored com- of the hazard associated with its use, it should be restricted
plexes with titanium, vanadium, and (in sulfuric acid) nio- to solvent extraction work conducted with gloves in a hood.
b i u m - - e a c h is used for spectrophotometric measurement. 8- Carbon tetrachloride (CC14) is so hazardous that some labs
hydroxyquinoline, as well as being an important precipitant, restrict its use, although there are methods where it is ana-
provides a very good spectrophotometric method for traces lytically a better choice than chloroform. Similar bans on the
44 CHEMICAL ANALYSIS OF METALS

use of trichloroethylene (C1CH:CC12)and 1,1,1,-trichloroethane sponges are easy to weigh but are often contaminated with
(CH3CC13), both of which are specified in some older meth- massive amounts of surface oxygen that does not show up on
ods, are in place in some organizations. accompanying certificates of purity. Wire or thin rod are usu-
Aromatics are even more of a dilemma. Benzene (C6H6) is ally better because of much lower surface area. Elemental
now outlawed in most labs as a known carcinogen. Toluene oxide powders can be ideal starting materials if they are stoi-
(C6HsCH3) and xylene (C6H4(CH3)2) are not far behind. It is chiometric and readily dissolvable, but some are neither.
best to forego methods that require these solvents. Some easily dissolved salts of acid-resistant metals are hy-
Of the aldehydes, only formaldehyde (HCHO) is used and groscopic (ZrOC12 • 8H20 and HfOC12 • 8H20 are examples),
that mostly in a rarely used method for trace sulfur. The para- and solutions prepared from them should always be stan-
rosaniline method is a sensitive means to quantify very low dardized using classical methods. It may be stylistically sat-
sulfur levels, but the hazard of formaldehyde suggests that isfying to prepare integral concentrations of all elemental
modern trace methods will prevail. standards, but a tremendous amount of time can be wasted
Unsubstituted hydrocarbons are rarely used, although they trying to subdivide a chromium pellet to produce, say, a
sometimes are indispensable for certain (especially organic 1.0000-mg Cr/mL solution, when a 1.0239-mg Cr/mL solution
analyte) extractions. Hexanes (C6H14)is a practical solvent in will work just as well.
this area for general use. The flammability hazard is, of The analyst or lab manager should keep an orderly file of
course, a major concern. all certificates of purity for elemental standard materials (as
Some labs forbid the use of ethers because of flammability well as for all primary standard compounds), adding new cer-
hazards. They are important reagents in metals analysis, but tificates for each new lot purchased. Prepared elemental stan-
should be purchased in small quantities that will be used up dard solutions (with few exceptions) are best stored in plastic
relatively quickly. This is to avoid the explosion hazard as- bottles. These should be prominently marked with the ele-
sociated with peroxide buildup in long storage. Only diethyl ment, its concentration, the solution medium, and the date
ether (C2HsOC2H5) is necessary for most work, although iso- and initials of the preparer. If the preparation involves sig-
propyl ether ((CH3)2CHOCH(CH3)2) is sometimes needed for nificant detail, a code on the bottle should reference the prep-
certain extractions. aration procedure in a central file. The bottle caps should be
kept tightly closed and the solutions thoroughly shaken be-
fore use. A valuable reference is ASTM Practice for Prepara-
ELEMENTAL STANDARDS tion of Calibration Solutions for Spectrophotometric and for
Spectroscopic Atomic Analysis (E 452) (Annual Book of ASTM
This last category represents one of the biggest challenges Standards, Vol. 03.06, American Society for Testing and Ma-
to the metals analyst. Until the advent of the ICP-OES tech- terials, Philadelphia, 1993).
nology, most chemists believed they knew how to prepare Table 3-8 lists some suggested elemental standard prepa-
standard solutions of pure elements. But the remorseless sta- rations; the tabulation is not meant to be either exhaustive or
bility of this new excitation source began to suggest that some definitive. The weights listed in the table are based on 1987
of the older procedures and measures of accuracy were not atomic weights. For the most accurate work with elements
good enough. A standard cobalt solution that yielded, say, (like boron and lithium) that have two or more stable, natu-
perfectly good results at the 0.5% level may not be quite ad- rally occurring isotopes, the analyst should assure himself
equate to calibrate an ICP-OES instrument at the 10% level. that the standard material has not been enriched or depleted
Perhaps there was some surface oxide on the cobalt metal. isotopically before sale. In these cases the vendor should cer-
Perhaps the solution was still a few degrees above ambient tify the atomic weight of the element of interest.
temperature when the volumetric flask was diluted to the In ICP-OES work it is often very desirable to calibrate with
mark. Maybe there was a clinging microdrop in a transfer the minimum number of mixed elemental standards. The so-
pipet. For purposes of discussion, let us assume that these lution matrix often limits compatibilities, however, and it is
effects amount to a cumulative error of 2.6% (relative). At the seldom possible to set up a large array of even two-point cal-
0.5% cobalt level the error is invisible for routine work (say ibrations without using five or more mixed element solutions.
we find 0.51% Co on a standard certified at 0.50% Co), but Table 3-9 illustrates some compatibilities and incompatibil-
the same error at the 10% cobalt level (say, 9.79% on a 10.05% ities. It should be noted that it is sometimes possible to escape
Co standard) begins to wave a red flag at us. While the clas- some of these solubility rules for sample solutions, especially
sical wet analyst is used to striving for relative standard de- at high dilutions and when measurements are made soon af-
viations of 0.1% or lower, until the advent of the ICP-OES it ter dissolution. However, it is always a mistake to violate
was seldom within the framework of pure element standards these rules in the preparation of standard calibration solu-
that work of this caliber was required. tions, which are expected to be used repeatedly and upon
Thus, extreme attention to detail and the best laboratory which the measurement process accuracy must stand or fall.
practice are the order of the day if modern labs are to get the Advantage can be taken of alternate preparation schemes,
best work out of modern instruments. Pipets and burets must however, when compatibility is an issue. Thus if one needs to
be cleaned regularly with concentrated sulfuric acid to re- calibrate for barium and titanium, he or she can combine
move residues that lead to drainage errors. Dilutions to the solutions only if the titanium is prepared in hydrochloric acid
mark and aliquot transfers should be performed on room (which ordinarily would be a less desirable medium for that
temperature solutions as close to the time of final measure- element).
ment as possible. Only the best water, acids, and elemental Today many laboratories purchase elemental standard so-
standard materials should be used. Fine metal powders and lutions rather than preparing their own. Because of a large
 CHAPTER 3--REAGENTS 45

TABLE 3 - 8 - - S u g g e s t e d preparation of elemental standard solutions (1000 ixg/mL).

Note: Correct weights for assay, if necessary. Dilute to 1 L with water and store in plastic bottles (unless indicated).
Element Preparation
Aluminum 1.0000 g Al wire + 50 mL HC1 (1 + 1) + one small drop of Hg.
Antimony (1) 1.0000 g Sb + 10 mL HNO3, then 5 mL HC1.
Antimony (2) 2.7427 g K(SbO)C4H406 • 1/2H20 (potassium a n t i m o n y tartrate hemihydrate) + H20.
Arsenic In a plastic vessel: 1.3203 g primary standard As203 + 2 g NaOH + 20 mL H20; dilute to 200 mL, neutralize with 20%
(v/v) H2SO4 (phenolphthalein).
Barium 1.4370 g BaCO3 + 300 mL H20, add 10 mL HC1 slowly with stirring.
Beryllium 1.0000 g Be + 25 mL HC1 (1 + 4) (Warning: Be & its solutions are very toxic!).
Bismuth 1.0000 g Bi + 10 mL H20 + 5 mL HNO3; boil to expel NOx.
Boron 5.7190 g NIST SRM 951 (HaBO3) or equiv.; dissolve in H20.
Bromine 1.2877 g NaBr; dissolve in H20.
Cadmium 1.0000 g Cd + 20 mL H20 + 5mL HC1.
Calcium 2.4973 g CaCO3 + 300 mL H20, add 10 m L HC1 slowly with stirring.
Cerium Dry (NH4)2Ce(NO3)6 at 85°C. Weigh 3.9127 g; dissolve in H20.
Cesium 1.2668 g CsC1; dissolve in H20.
Chlorine 1.6485 g NaC1; dissolve in H20.
C h r o m i u m (1) 1.0000 g Cr + 10mL HC1, then 5mL HNOa.
C h r o m i u m (2) 2.8289 g NIST SRM 136e (K2Cr2OT) or equiv.; dissolve in H20.
Cobalt 1.0000 g Co + 20 mL H20 + 10 mL HNO3; boil to expel NOx.
Copper 1.0000 g Cu + 20 mL H20 + 10 mL HNO3; boil to expel NOx.
Dysprosium 1.1477 g Dy203 + 20 mL H20 + 20 mL HC1.
Erbium 1.1436 g Er203 +20 mL H20 + 20 mL HC1.
Europium 1.1579 g Eu203 + 20 m L H20 + 20 mL HC1.
Fluorine 2.2101 g NaF; dissolve in H20 in a plastic vessel. Dilute in a plastic volumetric flask.
Gadolinium 1.1526 g Gd203 + 20 m L H20 + 20 mL HC1.
Gallium 1.0000 g Ga + 10 mL HC1 + 5 mL HNO3.
Germanium In a n 800-mL Teflon beaker: 1.0000 g Ge + 20 mL HF, add HNO3 dropwise until complete. Dilute in a plastic
volumetric flask.
Gold 1.0000 g Au + 10 mL HC1, add HNO3 dropwise while heating; boil to expel NOx. Store in dark.
Hafnium In a n 800-mL Teflon beaker: 1.0000 g Hf + 10 mL HF, add HNO3 dropwise until dissolved. Dilute in plastic volumetric
flask.
Holmium 1.1455 g Ho203 + 20 mL H20 + 20 mL HC1.
Indium 1.0000 g In wire + I0 mL HC1 + 5 mL HNO3. W a r m gently until dissolved.
Iodine 1.3081 g KI; dissolve in H20.
Iridium 2.3882 g (NH4)aIrC16; dissolve in 100 mL 1% (v/v) HC1; dilute with same.
Iron 1.0000 g Fe wire + 50 mL HC1 (1 + 1).
Lanthanum 1.1728 g La203 + 20 mL H20 + 20 mL HC1.
Lead 1.0000 g Pb + 10 mL HNO3.
Lithium 5.3228 g Li2CO3 + 300 mL H20; add 15 mL HC1 with stirring.
Lutetium 1.1372 g Lu203 + 20 mL H20 + 20 m L HC1.
Magnesium 1.0000 g Mg + 50 mL H20, then 5 mL HC1.
Manganese (1) 1.0000 g M n + 15 mL HC1 + 5 mL HNO3.
Manganese (2) 1.5825 g MnO2 + 15 mL HC1 + 5 mL HNO3.
Mercury 1.0000 g Hg + 10 mL H20 + 5 mL HNO3.
Molybdenum 1.5003 g MoO3 + 15 m L NH4OH + 15 rnL H20.
Neodymium 1.1664 g Nd2Oa + 20 m L H20 + 20 mL HC1.
Nickel 1.0000 g Ni + 20 mL H20 + 20 mL HNO3; boil to expel NOx.
Niobium In a n 800-mL Teflon beaker: 1.0000 g Nb + 15 mL HF; add HNOa dropwise. Dilute in a plastic volumetric flask.
Nitrogen 3.81890 g NH4C1; dissolve in H20.
Osmium Purhcase 0.01 M OsO4 solution (= 1.902 mg/mL). Transfer 25 mL by pipet to a 50-mL volumetric flask; dilute to m a r k
with 1% (v/v) H2SO4 a n d mix. This solution is 951/~g Os/mL. (Warning: Os in solution is very toxic a n d volatile!)
Store in glass.
Palladium 1.0000 g Pd wire + 10 mL HNO3; heat a n d add HC1 dropwise until dissolved.
Phosphorus 4.2635 g (NH4)2HPO4; dissolve in H20.
Platinum 1.0000 g Pt + 30 mL HC1 + 10 m L HNO3; evaporate to 1 mL; add 10 mL HC1.
Potassium 1.9068 g KC1; dissolve in H20.
Praeseodymium 1.1703 g Pr203 + 20 mL H20 + 20 mL HC1.
Rhenium 1.4406 g NHiReO4; dissolve in 200 mL H20. Dilute with 1% (v/v) H2SO 4.
Rhodium 3.7680 g (NHi)3RhC16 • H20; dissolve and dilute with HC1 (1 + 9).
Rubidium 1.4148 g RbC1; dissolve in H20.
Ruthenium 2.0523 g RuC13; dissolve a n d dilute with HC1 (1 + 4).
Samarium 1.1596 g Sm203 + 20 mL H20 + 20 mL HC1.
Scandium 1.5338 g Sc203 + 20 mL H20 + 20 mL HC1.
Selenium 1.0000 g Se + 5 mL HNO3.
Silicon Fuse 2.1393 g SiO2 with 5 g Na2CO3 in a p l a t i n u m crucible with a lid. Cool a n d leach in H20 (plastic beaker). Dilute in
a plastic volumetric flask.
Silver 1.5748 g AgNOa; dissolve in H20.
Sodium 2.5421 g NaC1; dissolve in H20.
Strontium 1.6849 g SrCO3 + 300 mL H20; add 10 mL HC1 slowly with stirring.
Sulfur 4.1209 g (NH4)2804; dissolve in H20.
46 CHEMICAL A N A L Y S I S OF METALS

TABLE 3-8--(continued)
Element Preparation
Tantalum In an 800-mL Teflon beaker: 1.0000 g Ta + 15 mL HF; add HNO3 dropwise. Dilute in a plastic volumetric flask.
Tellurium 1.0000 g Te + dropwise HNO3 to dissolve. Add 50 mL H2 O, stir, add HC1 to clear solution. Dilute with 1% (v/v) HC1.
Terbium 1.1762 g Tb407 + 20 mL H20 + 20 mL HC1.
Thallium 1.3034 g T1NO3; dissolve in H 2 0 .
Thorium 2.3794 g Th(NO3) 4 - 4H20; dissolve in 50 mL H20, add 5 mL HNO3.
Thulium 1.1421 g Tm2Oa + 20 mL H20 + 20 mL HC1.
Tin 1.0000 g Sn + 100 mL HC1.
Titanium (1) 1.0000 g Ti + 100 mL HC1 (1 + 1); dilute with same.
Titanium (2) In a large flask: 1.0000 g Ti + 100 mL H2SO4. Very cautiously add 300 mL H 2 0 in small increments, mixing well. Keep
warm until dissolved. Add three drops HNO3.
Tungsten (1) Fuse 1.2611 g WO3 with 5 g Na2CO3 in a platinum crucible with a lid. Leach in 100 mL H 2 0 ÷ 100 mL 10% (w/v)
NaOH.
Tungsten (2) 1.7942 g Na2WO4 • 2H20; dissolve in H20, add 100 mL 10% (w/v) NaOH.
Uranium 2.1095 g UO2(NO3) 2 • 6H20; dissolve in H20.
Vanadium 2.2963 g NH4VO3 + 100 mL H 2 0 ÷ 10 mL HNO3; heat, cool, add 10 mL additional HNO3. Heat and cool alternately
until dissolved.
Ytterbium 1.1387 g Yb203 + 20 mL H~O + 20 mL HC1.
Yttrium 1.2699 g 3(203 + 20 mL H 2 0 + 20 mL HC1.
Zinc 1.0000 g Zn + 10ml HC1.
Zirconium In an 800-mL Teflon beaker: 1.0000 g Zr wire + 20 mL H20 + 20 mL HF. Dilute in a plastic volumetric flask.

potential d e m a n d , especially f r o m e n v i r o n m e n t a l analysis n e e d to use in a career. E a c h of these reagents will have as-
laboratories, suppliers have proliferated. In fact, a n y o n e with sociated handling, storage, a n d waste disposal requirements,
an analytical b a l a n c e a n d a small h o o d m a y set themselves a n d each will have a n a s s o c i a t e d a r r a y of hazards.
u p as a s u p p l i e r of certified c a l i b r a t i o n solutions. The d a n g e r W h e n a reagent s h i p m e n t is received, a n y p a c k a g i n g dates
of abuse is obvious. Therefore, it behooves the analyst to crit- or e x p i r a t i o n dates should be noted, a n d the date of receipt
ically evaluate suppliers b y c o m p a r i s o n with b o t h in-house should be m a r k e d on each bottle. Even w i t h o u t an e x p i r a t i o n
p r e p a r e d solutions a n d with n a t i o n a l l y certified reference m a - date from the m a n u f a c t u r e r , the analyst m u s t be a w a r e of
terials before such solutions are used for r o u t i n e work. As those reagents with limited shelf life a n d of those that p r e s e n t
with elemental s t a n d a r d materials, all p u r c h a s e d c a l i b r a t i o n a h a z a r d w h e n old. Discolored, off-color, or c o n t a m i n a t e d re-
solution certificates should be filed b y e l e m e n t a n d lot n u m - agents should be p r o p e r l y d i s p o s e d of. Old c o n t a i n e r s of e t h e r
ber. Solutions t h a t have exceeded their expiration dates m a y have developed explosive peroxides. Chlorate a n d per-
should either be d i s c a r d e d o r d e a r l y m a r k e d , "For m a t r i x syn- chlorate salts b e c o m e explosive w h e n c o n t a m i n a t e d , as does
thesis o n l y - - n o t for calibration." s o d i u m azide if exposed to metals. Such reagents m u s t be
d i s p o s e d of b y an a p p r o v e d procedure.
M a n y m a t e r i a l s will b u i l d u p explosive p r e s s u r e in sealed
OTHER MATTERS c o n t a i n e r s if exposed to m o i s t u r e before s e a l i n g - - a l u m i n u m
chloride, alkali metals, c a l c i u m metal, s o d i u m b o r o h y d r i d e ,
I n this c h a p t e r we have discussed s o m e of the uses a n d s o d i u m peroxide, c a l c i u m carbide, a n d others fall into this
p r o p e r t i e s of a n u m b e r of reagents c o m m o n l y e m p l o y e d in group. C o n t a m i n a t e d 30% h y d r o g e n peroxide m a y d e c o m -
metals analysis laboratories, b u t we have b a r e l y s c r a t c h e d the pose with violence. S o m e m a t e r i a l s lose their usefulness u p o n
surface of the variety of chemicals that a classical analyst will r e p e a t e d exposure to r o o m a i r - - d e l i n q u e s c e n t salts t u r n to
slush, absolute alcohol loses its "absoluteness," a n d t i t a n o u s
chloride loses its r e d u c i n g strength. Even w a t e r b e c o m e s
slightly acid by a b s o r b i n g CO2 from the air.
TABLE 3-9--Elemental standard solution compatibilities.
Storage is a key issue for m a n y reagents. S o m e p r e p a r e d
(All other elements may be in the category listed unless
stated otherwise.) s o l u t i o n s - - b a s e s a n d basic solutions, fluoride a n d hydro-
Must Be Here,
fluoric a c i d solutions, a n d solutions of EDTA a n d o t h e r che-
Matrix If Needed Must NOT Be Here l a t e s - m u s t be stored in plastic bottles. The best storage
ACID vessels for m o s t elemental s t a n d a r d s are m a d e of Teflon PFA.
C1- + NO~ Hg, Ag If these are too expensive, high-density polyethylene will usu-
C1- only Sn, Ti Hg, Ag ally do. Despite the h a z a r d associated with possible breakage,
NO~ only Hg, Ag Hf, Mo, Nb, Ta, Ti, W, Zr borosilicate glass is still the best substitute for Teflon PFA for
F - + NO~ Hf, Mo, Nb, Ta, Ba, Ca, Cr, Cu, Fe, Pb, Mg, Ni,
Ti, W, Zr RE, Sr, Zn c h l o r i n a t e d h y d r o c a r b o n solutions. S o m e solutions, like sil-
SO42- Ti Sb, Ba, Ca, Pb, Hg, Ag, Sr ver, gold, o r m e r c u r y standards, h y d r o q u i n o n e , p o t a s s i u m
BASIC ferrocyanide, iodine, o r p o t a s s i u m p e r m a n g a n a t e m u s t be
NHiOH Mo A1, Bi, Cd, Cr, Au, Fe, Pb, Mg, kept in the d a r k to prevent p h o t o c h e m i c a l changes. Low ac-
Mn, RE, Sr, Sn, most others tinic o r a m b e r glass are usually called for, a l t h o u g h o p a q u e
NaOH Si, Mo, W Most others
plastics are s o m e t i m e s used. S t a n d a r d base in a u t o m a t i c pi-
 CHAPTER 3--REAGENTS 47

pets should be protected from C O 2 pickup with traps on air (RCRA). The Department of Transportation (DOT) controls
intakes. the transport of chemicals.
Since sodium hydroxide dispersed on a clay mineral is the With proper knowledge and attention to detail there is little
standard trapping material, careful packing is required to to fear in the metals analysis laboratory. Following approved
prevent clogging or contamination of the titrant. Similarly, safety practices, a chemical analyst will not jeopardize his life
titanous chloride must be protected from air oxidation with or his health. But it is always wise to r e m e m b e r that with
a CO2 or Ar blanket over the solution in a sealed bottle. Some reagents, as with all hazards, it is mostly what you don't know
prepared reagents, like cupferron solutions, need to be refrig- that can hurt you.
erated, and certain purchased reagents like hydrogen perox-
ide keep for longer periods if refrigerated. The practice of
refrigerating all organic reagents is unnecessary, however,
and m a y even lead to deterioration if cold bottles are opened REFERENCES
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As discussed previously, good safety practice makes it im- Analytical Methods for Atomic Absorption Spectrophotometry, Perkin-
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nocuous, but most reagents can hurt you in some way. As Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., and Hoffman, J. I.,
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a reagent are usually classed as acute or chronic. Among re- lytical Chemistry, Vol. 21, No. 1, 1988, pp. 1203A-1218A.
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are those that provide the least degree of warning. Thus, while 1910.1450, Occupational Safety and Health Administration, Wash-
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Johnson, Ed., Chemical Publishing Co., New York, 1955.
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reveal themselves. Between 1000 and 2000 c o m p o u n d s are National Academy Press, Washington, DC, 1983.
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w o m e n of child-hearing potential should be aware of embry- Committee on Hazardous Substances in the Laboratory, Assembly
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48 CHEMICAL A N A L Y S I S OF M E T A L S

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