Page 251 - Modern Analytical Chemistry

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234 Modern Analytical Chemistry

3–
If HgCl 2 is added in excess, each mole of PO 3 produces one mole of Hg 2 Cl 2 . The
precipitate’s mass, therefore, provides an indirect measurement of the mass of
3–
PO 3 present in the original sample.
Summarizing, we can determine an analyte gravimetrically by directly deter-
mining its mass, or the mass of a compound containing the analyte. Alternatively,
we can determine an analyte indirectly by measuring a change in mass due to its
loss, or the mass of a compound formed as the result of a reaction involving the
analyte.

8A.2 Types of Gravimetric Methods
In the previous section we used four examples to illustrate the different ways that
mass can serve as an analytical signal. These examples also illustrate the four gravi-
metric methods of analysis. When the signal is the mass of a precipitate, we call the
3–
precipitation gravimetry method precipitation gravimetry. The indirect determination of PO 3 by precipi-
A gravimetric method in which the tating Hg 2 Cl 2 is a representative example, as is the direct determination of Cl by
–
signal is the mass of a precipitate.
precipitating AgCl.
In electrogravimetry the analyte is deposited as a solid film on one electrode in
electrogravimetry an electrochemical cell. The oxidation of Pb , and its deposition as PbO 2 on a Pt
2+
A gravimetric method in which the
signal is the mass of an electrodeposit on anode is one example of electrogravimetry. Reduction also may be used in electro-
the cathode or anode in an gravimetry. The electrodeposition of Cu on a Pt cathode, for example, provides a
2+
electrochemical cell. direct analysis for Cu .
When thermal or chemical energy is used to remove a volatile species, we call
volatilization gravimetry the method volatilization gravimetry. In determining the moisture content of
A gravimetric method in which the loss food, thermal energy vaporizes the H 2 O. The amount of carbon in an organic com-
of a volatile species gives rise to the pound may be determined by using the chemical energy of combustion to convert
signal.
C to CO 2 .
Finally, in particulate gravimetry the analyte is determined following its re-
particulate gravimetry
A gravimetric method in which the mass moval from the sample matrix by filtration or extraction. The determination of sus-
of a particulate analyte is determined pended solids is one example of particulate gravimetry.
following its separation from its matrix.
8A. 3 Conservation of Mass
An accurate gravimetric analysis requires that the mass of analyte present in a sam-
ple be proportional to the mass or change in mass serving as the analytical signal.
For all gravimetric methods this proportionality involves a conservation of mass.
For gravimetric methods involving a chemical reaction, the analyte should partici-
pate in only one set of reactions, the stoichiometry of which indicates how the pre-
cipitate’s mass relates to the analyte’s mass. Thus, for the analysis of Pb 2+ and PO 3 3–
described earlier, we can write the following conservation equations
Moles Pb 2+ = moles PbO 2

3–
Moles PO 3 = moles Hg 2 Cl 2
Removing the analyte from its matrix by filtration or extraction must be complete.
When true, the analyte’s mass can always be found from the analytical signal; thus,
for the determination of suspended solids we know that
Filter’s final mass – filter’s initial mass = g suspended solid
whereas for the determination of the moisture content we have

Sample’s initial mass – sample’s final mass = g H 2 O