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262 Modern Analytical Chemistry

EXAMPLE 8.7
A 26.23-mg sample of MgC 2 O 4 •H 2 O and inert materials is heated to constant
weight at 1200 °C, leaving a residue weighing 20.98 mg. A sample of pure
MgC 2 O 4 •H 2 O, when treated in the same fashion, undergoes a 69.08% change
in its mass. Determine the %w/w MgC 2 O 4 •H 2 O in the sample.
SOLUTION
The change in mass when analyzing the mixture is 5.25 mg, thus the grams of
MgC 2 O 4 •H 2 O in the sample is

100 mg MgC O 4 · H O
2
2
.
525 mg lost ´ = 7 60 mg MgC O 4 · H O
.
2
2
69.08 mg lost
The %w/w MgC 2 O 4 •H 2 O, therefore, is
.
2
2
mg MgC O 4 · H O ´100 = 760 mg ´100 =29 0.% w/w MgC O 4 · H O
2
2
mg sample 26.23 mg
8C. 3 Evaluating Volatilization Gravimetry
The scale of operation, accuracy, and precision of gravimetric volatilization
methods are similar to that described in Section 8B.4 for precipitation gravime-
try. The sensitivity for a direct analysis is fixed by the analyte’s chemical form
following combustion or volatilization. For an indirect analysis, however, sensi-
tivity can be improved by carefully choosing the conditions for combustion or
volatilization so that the change in mass is as large as possible. For example, the
H
thermogram in Figure 8.9 shows that an indirect analysis for CaC 2 O 4×2 O
based on the weight of the residue following ignition at 1000 °C will be more
sensitive than if the ignition was done 300 °C. Selectivity does not present a
problem for direct volatilization gravimetric methods in which the analyte is a
gaseous product retained in an absorbent trap. A direct analysis based on the
residue’s weight following combustion or volatilization is possible when the
residue only contains the analyte of interest. As noted earlier, indirect analyses
are only feasible when the residue’s change in mass results from the loss of a sin-
gle volatile product containing the analyte.
Volatilization gravimetric methods are time- and labor-intensive. Equipment
needs are few except when combustion gases must be trapped or for a thermogravi-
metric analysis, which requires specialized equipment.

8D Particulate Gravimetry

Gravimetric methods based on precipitation or volatilization reactions require that
the analyte, or some other species in the sample, participate in a chemical reaction
producing a change in physical state. For example, in direct precipitation gravime-
try, a soluble analyte is converted to an insoluble form that precipitates from solu-
tion. In some situations, however, the analyte is already present in a form that may
be readily separated from its liquid, gas, or solid matrix. When such a separation is
possible, the analyte’s mass can be directly determined with an appropriate balance.
In this section the application of particulate gravimetry is briefly considered.