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254 Modern Analytical Chemistry

which can be solved for g Na 3 PO 3 and %w/w Na 3 PO 3 in the sample.
g Hg Cl 2 ´ FW Na PO 3 0 4320 g ´ 147.94 g /mol
.
3
2
g Na PO 3 = = = 0 1354 g
.
3
FW Hg Cl 2 472.09 g /mol
2
.
g Na PO 3 0 1354 g
3
´100 = ´100 =97 27 % w/w Na PO 3
.
3
.
g sample 0 1392 g
8B.3 Qualitative Applications
Precipitation gravimetry can also be applied to the identification of inorganic and
organic analytes, using precipitants such as those outlined in Tables 8.1, 8.2, 8.4,
and 8.5. Since this does not require quantitative measurements, the analytical signal
is simply the observation that a precipitate has formed. Although qualitative appli-
cations of precipitation gravimetry have been largely replaced by spectroscopic
methods of analysis, they continue to find application in spot testing for the pres-
ence of specific analytes. 8

8B. 4 Evaluating Precipitation Gravimetry

Scale of Operation The scale of operation for precipitation gravimetry is governed
by the sensitivity of the balance and the availability of sample. To achieve an accu-
racy of ±0.1% using an analytical balance with a sensitivity of ±0.1 mg, the precipi-
tate must weigh at least 100 mg. As a consequence, precipitation gravimetry is usu-
ally limited to major or minor analytes, and macro or meso samples (see Figure 3.6
in Chapter 3). The analysis of trace level analytes or micro samples usually requires
a microanalytical balance.

Accuracy For macro–major samples, relative errors of 0.1–0.2% are routinely
achieved. The principal limitations are solubility losses, impurities in the precipitate,
and the loss of precipitate during handling. When it is difficult to obtain a precipitate
free from impurities, an empirical relationship between the precipitate’s mass and
the mass of the analyte can be determined by an appropriate standardization.

Precision The relative precision of precipitation gravimetry depends on the
amount of sample and precipitate involved. For smaller amounts of sample or
precipitate, relative precisions of 1–2 ppt are routinely obtained. When working
with larger amounts of sample or precipitate, the relative precision can be ex-
tended to several parts per million. Few quantitative techniques can achieve this
level of precision.

Sensitivity For any precipitation gravimetric method, we can write the following
general equation relating the signal (grams of precipitate) to the absolute amount of
analyte in the sample
Grams precipitate = k ´grams of analyte 8.13
where k, the method’s sensitivity, is determined by the stoichiometry between the
precipitate and the analyte. Note that equation 8.13 assumes that a blank has been
used to correct the signal for the reagent’s contribution to the precipitate’s mass.