Page 250 - Modern Analytical Chemistry

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Chapter 8 Gravimetric Methods of Analysis 233

8A Overview of Gravimetry

Before we look more closely at specific gravimetric methods and their applications,
let’s take a moment to develop a broad survey of gravimetry. Later, as you read gravimetry
through the sections of this chapter discussing different gravimetric methods, this Any method in which the signal is a mass
survey will help you focus on their similarities. It is usually easier to understand a or change in mass.
new method of analysis when you can see its relationship to other similar methods.

8A.1 Using Mass as a Signal
At the beginning of this chapter we indicated that in gravimetry we measure mass
or a change in mass. This suggests that there are at least two ways to use mass as an
analytical signal. We can, of course, measure an analyte’s mass directly by placing it
on a balance and recording its mass. For example, suppose you are to determine the
total suspended solids in water released from a sewage-treatment facility. Sus-
pended solids are just that; solid matter that has yet to settle out of its solution ma-
trix. The analysis is easy. You collect a sample and pass it through a preweighed fil-
ter that retains the suspended solids. After drying to remove any residual moisture,
you weigh the filter. The difference between the filter’s original mass and final mass
gives the mass of suspended solids. We call this a direct analysis because the analyte
itself is the object being weighed.
2+
What if the analyte is an aqueous ion, such as Pb ? In this case we cannot iso-
late the analyte by filtration because the Pb 2+ is dissolved in the solution’s matrix.
We can still measure the analyte’s mass, however, by chemically converting it to a
solid form. If we suspend a pair of Pt electrodes in our solution and apply a suffi-
ciently positive potential between them for a long enough time, we can force the
reaction
2+
+
Pb (aq)+4H 2 O(l) t PbO 2 (s)+H 2 (g)+2H 3 O (aq)
to go to completion. The Pb 2+ ion in solution oxidizes to PbO 2 and deposits on the
Pt electrode serving as the anode. If we weigh the Pt anode before and after applying
the potential, the difference in the two measurements gives the mass of PbO 2 and,
2+
from the reaction’s stoichiometry, the mass of Pb . This also is a direct analysis be-
cause the material being weighed contains the analyte.
Sometimes it is easier to remove the analyte and use a change in mass as the
analytical signal. Imagine how you would determine a food’s moisture content by
a direct analysis. One possibility is to heat a sample of the food to a temperature
at which the water in the sample vaporizes. If we capture the vapor in a
preweighed absorbent trap, then the change in the absorbent’s mass provides a di-
rect determination of the amount of water in the sample. An easier approach,
however, is to weigh the sample of food before and after heating, using the change
in its mass as an indication of the amount of water originally present. We call this
an indirect analysis since we determine the analyte by a signal representing its
disappearance.
The indirect determination of moisture content in foods is done by difference.
The sample’s initial mass includes the water, whereas the final mass is measured
after removing the water. We can also determine an analyte indirectly without its
ever being weighed. Again, as with the determination of Pb 2+ as PbO 2 (s), we take
3–
advantage of the analyte’s chemistry. For example, phosphite, PO 3 , reduces Hg 2+
2+
–
to Hg 2 . In the presence of Cl a solid precipitate of Hg 2 Cl 2 forms.
3–
–
+
3–
2HgCl 2 (aq)+PO 3 (aq)+3H 2 O(l) t Hg 2 Cl 2 (s)+2H 3 O (aq) + 2Cl (aq)+ PO 4 (aq)