Page 272 - Modern Analytical Chemistry

1400-CH08 9/9/99 2:18 PM Page 255

Chapter 8 Gravimetric Methods of Analysis 255

Consider, for example, the determination of Fe as Fe 2 O 3 . Using a conservation
of mass for Fe we write
2 ´moles Fe 2 O 3 = moles Fe

Converting moles to grams and rearranging yields an equation in the form of 8.13
1 FW Fe O 3
2
g Fe O 3 = ´ ´ g Fe
2
2 AW Fe
where k is equal to
1 FW Fe O 3
2
k = ´ 8.14
2 AW Fe
As can be seen from equation 8.14, we may improve a method’s sensitivity in two
ways. The most obvious way is to increase the ratio of the precipitate’s molar mass
to that of the analyte. In other words, it is desirable to form a precipitate with as
large a formula weight as possible. A less obvious way to improve the calibration
sensitivity is indicated by the term of 1/2 in equation 8.14, which accounts for the
stoichiometry between the analyte and precipitate. Sensitivity also may be improved
by forming precipitates containing fewer units of the analyte.

Selectivity Due to the chemical nature of the precipitation process, precipitants
are usually not selective for a single analyte. For example, silver is not a selective
precipitant for chloride because it also forms precipitates with bromide and iodide.
Consequently, interferents are often a serious problem that must be considered if
accurate results are to be obtained.

Time, Cost, and Equipment Precipitation gravimetric procedures are time-intensive
and rarely practical when analyzing a large number of samples. However, since much
of the time invested in precipitation gravimetry does not require an analyst’s immedi-
ate supervision, it may be a practical alternative when working with only a few sam-
ples. Equipment needs are few (beakers, filtering devices, ovens or burners, and bal-
ances), inexpensive, routinely available in most laboratories, and easy to maintain.

8C Volatilization Gravimetry
A second approach to gravimetry is to thermally or chemically decompose a solid
sample. The volatile products of the decomposition reaction may be trapped and
weighed to provide quantitative information. Alternatively, the residue remaining
when decomposition is complete may be weighed. In thermogravimetry, which is thermogravimetry
one form of volatilization gravimetry, the sample’s mass is continuously monitored A form of volatilization gravimetry in
while the applied temperature is slowly increased. which the change in a sample’s mass is
monitored while it is heated.

8C.1 Theory and Practice
Whether the analysis is direct or indirect, volatilization gravimetry requires that the
products of the decomposition reaction be known. This requirement is rarely a
problem for organic compounds for which volatilization is usually accomplished by
combustion and the products are gases such as CO 2 , H 2 O, and N 2 . For inorganic
compounds, however, the identity of the volatilization products may depend on the
temperature at which the decomposition is conducted.