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 through the different gravimetric methods, this survey will help you focus on their similarities. It is usually easier to understand a new method of analysis when you can see its relationship to other similar methods.
We already 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.
What if the analyte is an aqueous ion, such as Pb2+? In this case we cannot iso- late the analyte by filtration because the Pb2+ 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
to go to completion. The Pb2+ ion in solution oxidizes to PbO2 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 PbO2 and, from the reaction’s stoichiometry, the mass of Pb2+. 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 Pb2+ as PbO2(s), we take advantage of the analyte’s chemistry. For example, phosphite, PO33–, reduces Hg2+ to Hg22+. In the presence of Cl– a solid precipitate of Hg2Cl2 forms.
If HgCl2 is added in excess, each mole of PO33– produces one mole of Hg2Cl2. The precipitate’s mass, therefore, provides an indirect measurement of the mass of PO33– 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.
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 method precipitation gravimetry. The indirect determination of PO 3– by precipi- tating Hg2Cl2 is a representative example, as is the direct determination of Cl– by precipitating AgCl.
In electrogravimetry the analyte is deposited as a solid film on one electrode in an electrochemical cell. The oxidation of Pb2+, and its deposition as PbO2 on a Pt anode is one example of electrogravimetry. Reduction also may be used in electro- gravimetry. The electrodeposition of Cu on a Pt cathode, for example, provides a direct analysis for Cu2+.
When thermal or chemical energy is used to remove a volatile species, we call the method volatilization gravimetry. In determining the moisture content of food, thermal energy vaporizes the H2O. The amount of carbon in an organic com- pound may be determined by using the chemical energy of combustion to convert C to CO2.
Finally, in particulate gravimetry the analyte is determined following its re- moval from the sample matrix by filtration or extraction. The determination of sus- pended solids is one example of particulate gravimetry.
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 Pb2+ and PO 3– described earlier, we can write the following conservation equations
Moles Pb2+ = moles PbO2
Moles PO33– = moles Hg2Cl2
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 H2O
Specific details, including worked examples.
Except for particulate gravimetry, which is the most trivial form of gravimetry, it is entirely possible that you will never use gravimetry after you are finished with this course. Why, then, is familiarity with gravimetry still important? The answer is that gravimetry is one of only a small number of techniques whose measurements re- quire only base SI units, such as mass and moles, and defined constants, such as Avogadro’s number and the mass of 12C.* The result of an analysis must ultimately be traceable to methods, such as gravimetry, that can be related to fundamental physical properties.1 Most analysts never use gravimetry to validate their methods. Verifying a method by analyzing a standard reference material, however, is com- mon. Estimating the composition of these materials often involves a gravimetric analysis.