Gravimetry
• Definition: a precipitation or
Gravimetry volatilization method based on the
determination of weight of a
substance of known composition that
Chem. 3320 is chemically related to the analyte
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T.W.Richards Gravimetry
• 1914 Nobel Prize to T.W.Richards for the atomic weights • The quantitative measurement of an
of Ag, Cl, and N
analyte by weighing a pure, solid form of
• Richards and his group determined atomic weights of 55 the analyte. Since gravimetric analysis is
of the 92 known elements using gravimetry an absolute measurement, it is the
• “Every substance must be assumed to be impure, every
principal method for analyzing and
reaction must be assumed to be incomplete, every preparing primary standards.
method of measurement must be assumed to contain
some constant error, until proof to the contrary can be • The quantitative determination of a
obtained. As little as possible must be taken for granted.” substance by precipitation followed by
isolation and weighing of the precipitate.
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Basic Method of Gravimetric
Gravimetric Analysis
Analysis
• A weighed sample is dissolved after which • Reaction: aA + rR -----> AaRr ppt where:
an excess of a precipitating agent is – a is # of moles of analyte A
added. – r is # of moles of reagent R
• The precipitate which forms is filtered, – AaRr is a pure, insoluble precipitate which we
dried or ignited and weighed. can dry and weigh or ignite to convert to
something we can weigh
• From the mass and known composition of
– ppt=precipitate
the precipitate, the amount of the original
ion can be determined.
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1
Gravimetric Analysis
Gravimetric Analysis
• Four steps:
• Identify insoluble form (i.e., choose suitable
precipitate)
weigh
sample Add precipitating
• Separate analyte from interfering constituents (i.e.,
agent compensate for lack of precipitant selectivity)
Heat
Filter and dry
• Wash precipitate (i.e., remove impurities and
coprecipitants)
Calculations
• Convert precipitate to reliable weighing form (if
Weigh
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precipitate
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needed) JA Negrón
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A typical experimental procedure to determine an
unknown concentration of an analyte in solution is
as follows: Gravimetry: Seven Steps
• quantitatively precipitate the analyte from • Dry and weigh sample
solution
• collect the precipitate by filtering and wash it to
• Dissolve sample
remove impurities • Add precipitating reagent in excess
• dry the solid in an oven to remove solvent • Coagulate precipitate usually by heating
• weigh the solid on an analytical balance
• Filtration-separate ppt from mother liquor
• calculate the analyte concentration in the
original solution based on the weight of the • Wash precipitate (peptization)
precipitate • Dry and weigh to constant weight
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Examples of Gravimetric Lab Precipitation process
Procedures
• Gravimetric Determination of Iron • Ideally, we’d like a precipitate that forms
– Determine constant weight of the crucibles quickly. This implies:
– Oxidation of iron sample – Large, pure crystals
– Precipitation of iron hydroxide – Low solubility
– Ignition of iron hydroxide to iron oxide – Easily filtered
– Determine constant weight of the crucibles – Easily washed
plus iron oxide
– Calculation of iron in the sample
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How do we get results?
Analyte Weighted sample
• % of analyte, % A
• In in In2O3
• HgO in Hg5(IO6)2
• %A = weight of analyte x 100 weight • I in Hg5(IO6)2
of sample • K3PO4 in K2PtCl6
• K3PO4 in Mg2PO7
• weight of ppt directly obtained →A
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A certain barium halide exists as the hydrated salt BaX2.2H2O, where X is the
halogen. The barium content of the salt can be determined by gravimetric
Calculate the % m/m Cu in a sample of methods. A sample of the halide (0.2650 g) was dissolved in water (200 cm3)
and excess sulphamic acid added. The mixture was then heated and held at
0.297 g CuSCN. boiling for 45 minutes. The precipitate (barium sulphate) was filtered off,
washed and dried. Mass of precipitate obtained = 0.2533 g. Determine the
identity of X.
Number of moles BaSO4 in the precipitate
0.2533 g BaSO4 (1mol BaSO4 /233.40g BaSO4) = 1.09 x 10-3 moles BaSO4
= 1.09 x 10-3 moles Ba
Since 1mol Ba = 1 mol halide (BaX2.2H2O) and since we have the
grams of the halide, the molecular mass (MM) of the halide can be
calculated.
RMM = (mass halide /moles halide)= 0.2650 g / 1.09 x 10-3 moles
= 244.18 g.mol-1
Atomic Mass of X = (244.18 – 173.34)/2 = 35.4g.mol-1
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A 0.5962 g sample of iron ore is dissolved in perchloric acid (HClO4). All iron present When an sample of impure potassium chloride (0.4500g)
is oxidized to Fe3+. The solution is filtered to remove solid matrix materials and made
basic with addition of ammonium hydroxide. The iron precipitates as the Fe(OH)3
was dissolved in water and treated with an excess of silver
.xH2O gel. The precipitate is collected in a cistern crucible and ignited to produce nitrate, 0.8402 g of silver chloride was precipitated.
Fe2O3. What is the wt. % of iron in the sample if the analysis produced 0.3210 g Calculate the percentage KCl in the original sample.
Fe2O3?
Iron ore (0.5962g)
• KCl + AgNO3 AgCl + KNO3
0.4500g “impure” 0.8402 g
? 0.3210g
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