Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods387Or g a n i c An a l y s i sThe most important application of volatilization gravimetry is for the elementalanalysis of organic materials. During combustion with pure O 2 ,many elements, such as carbon and hydrogen, are released as gaseous combustionproducts, such as CO 2 (g) and H 2 O(g). Passing the combustionproducts through preweighed tubes containing selective absorbents andmeasuring the increases in mass provides a direct analysis for the mass ofcarbon and hydrogen in the organic material.Alkaline metals and earths in organic materials can be determined byadding H 2 SO 4 to the sample before combustion. After combustion is complete,the metal remains behind as a solid residue of metal sulfate. Silver,gold, and platinum can be determined by burning the organic sample,leaving a metallic residue of Ag, Au, or Pt. Other metals are determined byadding HNO 3 before combustion, leaving a residue of the metal oxide.Volatilization gravimetry is also used to determine biomass in watersand wastewaters. Biomass is a water quality index that provides an indicationof the total mass of organisms contained within a sample of water.A known volume of the sample is passed through a preweighed 0.45-mmmembrane filter or a glass-fiber filter, and dried at 105 o C for 24 h. Theresidue’s mass provides a direct measure of biomass. If samples are knownto contain a substantial amount of dissolved inorganic solids, the residuecan be ignited at 500 o C for one hour, volatilizing the biomass. The resultinginorganic residue is wetted with distilled water to rehydrate any clayminerals and dried to a constant weight at 105 o C. The difference in massbefore and after ignition provides an indirect measure of biomass.Instead of measuring mass, modern instrumentsfor completing an elementalanalysis use gas chromatography (Chapter12) or infrared spectroscopy (Chapter 10)to monitor the gaseous decompositionproducts.Qu a n t i t a t i ve Ca l c u l at i o n sFor some applications, such as determining the amount of inorganic ashin a polymer, a quantitative calculation is straightforward and does notrequire a balanced chemical reaction. For other applications, however, therelationship between the analyte and the analytical signal depends upon thestoichiometry of any relevant reactions. Once again, a conservation of massis useful when solving problems.Example 8.5A 101.3-mg sample of an organic compound containing chlorine is combustedin pure O 2 and the volatile gases collected in absorbent traps. Thetrap for CO 2 increases in mass by 167.6 mg and the trap for H 2 O showsa 13.7-mg increase. A second sample of 121.8 mg is treated with concentratedHNO 3 producing Cl 2 , which subsequently reacts with Ag + , forming262.7 mg of AgCl. Determine the compound’s composition, as well as itsempirical formula.