Alkalinity,
electrometric
titration
Parameter and Code:
Alkalinity,
I-1030-85 (mg/L as CaCOJ: 00410
1. Application
This method is suitable for analyzing water
with any amount of alkalinity, but aliquots for
analysis should be taken to avoid a titration
volume of standard acid in excess of 50 mL.
2. Summary of method
2.1 Alkalinity is determined by titrating the
water sample with a standard solution of strong
acid. The end point of the titration is selected
as pH 4.5.
2.2 For waters that contain only small quantities of dissolved mineral matter, the alkalinity
determination is likely the largest single source
of error in the analysis. Alkalinity is very susceptible to change between time of collection
and analysis, with changes occurring more
rapidly after the sample bottle is opened. The
overall alkalinity value is probably somewhat
more stable than the relative values of the common alkalinity components. Unless a gross error
is made in the initial determination of alkalinity, it is seldom advisable to try to check the
results if several days have elapsed since the
bottle was first opened. The alkalinity of some
samples may change appreciably in a few hours.
The determination should be performed at the
time of sampling for highest accuracy.
2.3 Selection of pH 4.5 as the titration end
point for determining total alkalinity is arbitrary and corresponds to the true equivalence
point only under ideal conditions. The equivalence point of the bicarbonate-carbonic acid
titration varies with the concentration of bicarbonate present; the deviation from pH 4.5 is
particularly serious at low bicarbonate-ion concentrations. When greater accuracy in the determination is needed, the titration equivalence
point may be determined for each sample by
adding the titrant in small increments in the
vicinity of ph 4.5 and by recording the pH of
the solution after each measured addition. The
true end point is then determined from either
(1) a plot of pH versus total titrant volume,
where the end point is the pH corresponding to
a change in slope of the curve, or (2) a plot of
~PH
versus titrant volume
AmL t&rant
where the end point is that volume at which
there occurs a maximum rate of change of pH
per volume of t&rant added.
2.4 There are other methods for arriving at
a more reliable determination of total alkalinity. Barnes (1964) discusses the several factors
involved in the accurate measurement of alkalinity, particularly under field conditions. A
routine laboratory method for determining total
alkalinity with improved accuracy for widely
differing types of water has been described by
Larson and Henley (1956) and further evaluated
by Thomas and Lynch (1960).A rather complete
consideration of carbonate equilibria and their
analytic considerations may be found in a
publication by Stumm and Morgan (1970).
3. Interference8
3.1 Any ionized substance that reacts with
a strong acid can contribute to alkalinity if the
reaction occurs at a pH above that of the specified end point; examples are salts of weak
organic and inorganic acids.
3.2 Oils and greases, if present, may tend to
foul the pH-meter electrode and prevent its
proper operation.
56
56
TECHNIQUES
OF WATER-RESOURCES
INVESTIGATIONS
4. Apparatus
4.1 &ret, 50-mL capacity.
4.2 pH meter.
4.3 Stirrer, magnetic
where
6. Reagents
5.1 Sodium carbonate standard solution, 1.0
mL o 1.00 mg HCOg: Dry 1.0 g primary standard Na,COs at 150 to 160C for 2 h. Cool in a
desiccator and dissolve 0.8685 g in carbon dioxidefree water; dilute to 1,000 mL.
5.2 Sulfuric acid stundud solution, O.O1639N,
1.00 mL 0 1.00 mg HCO& CuutiousZy add 0.5
mL concentrated H,SO, (sp gr 1.84) to 950 mL
water. (The t&ant is stable for several months
if protected from ammonia fumes and is usually
prepared in larger quantities.) After the solution
has been thomughly mixed, standardize by tit&i
ing 25.00 mL N CO, standard solution (1.00mL
? ) to pH 4.5. Adjust the concen0 1.00 mg HCO$
t&ion Qf the sulfuric acid standard solution to
exactly 0.01639N by dilution with water or by
addition of dilute acid as indicated by the first titration Confirm the exact normality by m&andardization. Although the sulfuric acid standard
solution is reasonably stable, its normality should
be verified at least monthly (NOTE 1).
NOTE 1. Preparing standard sulfuric acid that
is not, exactly 0.01639N may be more convenient. Standard sulfuric acid that is approximate
ly 0.01639N (but the exact normality of which
is known) can be used if the appropriate factor
is applied in the calculations.
8. Report
Report alkalinity, total (00410), concentrations as follows: less than 1,000 mg/L, whole
numbers; 1,000 mg/L and above, three significant figures.
6. Procedure
6.1 Water samples for the determination of
alkalinity should not be filtered, diluted, concentrated, or altered in any way. The determination
should be performed without delay after the
sample bottle has been opened.
6.2 From a settled, unfiltered sample, pipet
a volume containing less than 40 mg alkalinity
as HCOi1(50.0 mL max) into a suitable beaker.
6.3 Titrate immediately with 0.01639N
H,S04 and record the titrant volume at pH 4.5.
7. Calculations
Total alkalinity
as CaCO, in mg/L=
1,000
X0.820?X(mLa to pH 4.5)
*s
mL, and mLQ=volumes of standard acid and
sample, respectively.
9. Precision
9.1 Precision for alkalinity for 36 samples
within the range of 3.6 to 316 n&L calcium carbonate may be expressed as follows:
+=0.034x
+ 1.93
where
ST= overall precision, milligrams per liter,
and
X= concentration of alkalinity as CaCO,,
milligrams per liter.
The correlation coefficient is 0.5308.
9.2 Precision for alkalinity for six of the 36
samples expressed in terms of the percent
relative standard deviation is as follows:
Number of
laboratorlas
Mean (mglL
as oaCO$
25
14
f:
3.6
56.7
107
119
Eli
iz
Relative standard
deviation
(percent)
25
11
13
3
6
References
Barnes, Ivan, 1964, Field ~tofalkaliityandpH:
U.S. Geological Survey Water-Supply Paper 1535-H.
17 p.
Larson, T. E., and Henley, Laurel, 1955, Determination of
low alkalinity or acidity in water: Analytical Chemistry,
v. 27, p. 851-2.
Stumm, Werner, and Morgan, J. J., 1970, Aquatic Chemistry: New York, John Wiley and Sons, 583 p.
Thomas, J. F. J., and Lynch, J. J., 1960, Determination of
carhonate dkaliulty in nafxral waters: American Wabr
Works Association Journal, v. 52, p. 259-68.
