Analytical Chem istry - DePauw University

390 Analytical Chemistry 2.0Alternatively, you can determine that thefinal product of the decomposition isMgO (see Practice Exercise 8.6) and usea conservation of mass for Mg to arrive atthe same answer.100.0 mg MgC OiHO2 4 2525 . mg lost×= 760 . mg MgC OiHO2 4 269.08 mg lostThe %w/w MgC 2 O 4 •H 2 O in the sample is760 . mg MgC OiHO2 4 2× 100 = 29.%0 w/wMgC O iH O2 4 226.23 mg sample8C.3 Evaluating Volatilization GravimetryThe scale of operation, accuracy, and precision of a gravimetric volatilizationmethod is similar to that described in Section 8B.4 for precipitationgravimetry. The sensitivity of a direct analysis is fixed by the analyte’schemical form following combustion or volatilization. We can improve thesensitivity of an indirect analysis by choosing conditions that give the largestpossible change in mass. For example, the thermogram in Figure 8.10shows us that an indirect analysis for CaC 2 O 4 •H 2 O is more sensitive if wemeasure the change in mass following ignition at 1000 o C than if we ignitethe sample at 300 o C.Selectivity is not a problem for a direct analysis if we trap the analyteusing a selective absorbent trap. A direct analysis based on the residue’sweight following combustion or volatilization is possible if the residue onlycontains the analyte of interest. As noted earlier, an indirect analysis is onlyfeasible when the change in mass results from the loss of a single volatileproduct containing the analyte.Volatilization gravimetric methods are time and labor intensive. Equipmentneeds are few, except when combustion gases must be trapped, or fora thermogravimetric analysis, when specialized instrumentation is needed.A particulate is any tiny portion of matter,whether it is a speck of dust, a globuleof fat, or a molecule of ammonia. Forparticulate gravimetry we simply need amethod for collecting the particles and abalance for measuring their mass.8DParticulate GravimetryPrecipitation and volatilization gravimetric methods require that the analyte,or some other species in the sample, participate in a chemical reaction.In a direct precipitation gravimetric analysis, for example, we convert asoluble analyte into an insoluble form that precipitates from solution. Insome situations, however, the analyte is already present as in a particulateform that is easy to separate from its liquid, gas, or solid matrix. When sucha separation is possible, we can determine the analyte’s mass without relyingon a chemical reaction.8D.1 Theory and PracticeThere are two methods for separating a particulate analyte from its matrix.The most common method is filtration, in which we separate solid par-