Analytical Chem istry - DePauw University

356 Analytical Chemistry 2.0Method 2540D in Standard Methods forthe Examination of Waters and Wastewaters,20th Edition (American Public HealthAssociation, 1998) provides an approvedmethod for determining total suspendedsolids. The method uses a glass-fiber filterto retain the suspended solids. After filteringthe sample, the filter is dried to aconstant weight at 103–105 o C.Method 925.10 in Official Methods ofAnalysis, 18th Edition (AOAC International,2007) provides an approvedmethod for determining the moisturecontent of flour. A preweighed sample isheated for one hour in a 130 o C oven andtransferred to a desiccator while it cools toroom temperature. The loss in mass givesthe amount of water in the sample.8AOverview of Gravimetric MethodsBefore we consider specific gravimetric methods, let’s take a moment todevelop a broad survey of gravimetry. Later, as you read through the descriptionsof specific gravimetric methods, this survey will help you focuson their similarities instead of their differences. You will find that it is easierto understand a new analytical method when you can see its relationship toother similar methods.8A.1 Using Mass as an Analytical SignalSuppose you are to determine the total suspended solids in the water releasedby a sewage-treatment facility. Suspended solids are just that—solidmatter that has yet to settle out of its solution matrix. The analysis is easy.After collecting a sample, you pass it through a preweighed filter that retainsthe suspended solids, and dry the filter and solids to remove any residualmoisture. The mass of suspended solids is the difference between the filter’sfinal mass and its original mass. We call this a direct analysis becausethe analyte—the suspended solids in this example—is the species that isweighed.What if our analyte is an aqueous ion, such as Pb 2+ ? Because the analyteis not a solid, we cannot isolate it by filtration. We can still measure theanalyte’s mass directly if we first convert it into a solid form. If we suspend apair of Pt electrodes in the sample and apply a sufficiently positive potentialbetween them for a long enough time, we can force the following reactionto completion.2+ +Pb ( aq) + 4HO() l PbO () s + H ( g) + 2HO( aq )2 2 23Oxidizing Pb 2+ deposits PbO 2 on the Pt anode. If we weigh the anode beforeand after applying the potential, the change in its mass gives the massof PbO 2 and, from the reaction’s stoichiometry, the amount of Pb 2+ in thesample. This is a direct analysis because PbO 2 contains the analyte.Sometimes it is easier to remove the analyte and let a change in massserve as the analytical signal. Suppose you need to determine a food’s moisturecontent. One approach is to heat a sample of the food to a temperaturethat vaporizes the water, capturing it in a preweighed absorbent trap.The change in the absorbent’s mass provides a direct determination of theamount of water in the sample. An easier approach is to weigh the sample offood before and after heating, using the change in its mass as an indicationof the amount of water originally present. We call this an indirect analysisbecause we determine the analyte using a signal that is proportional itsdisappearance.The indirect determination of a sample’s moisture content is done bydifference. The sample’s initial mass includes the water, but its final massdoes not. We can also determine an analyte indirectly without its ever being