Analytical Chem istry - DePauw University

386 Analytical Chemistry 2.03. Why is H 2 SO 4 added with the HF?Many samples containing silicon also contain aluminum and iron,which, when dehydrating the sample, form Al 2 O 3 and Fe 2 O 3 . Theseoxides are potential interferents because they also form volatile fluorides.In the presence of H 2 SO 4 , however, aluminum and iron preferentiallyform non-volatile sulfates. These sulfates eventually decomposeback to their respective oxides when we heat the residue to1200 o C. As a result, the change in weight after treating with HF andH 2 SO 4 is due only to the loss of SiF 4 .8C.2 Quantitative ApplicationsUnlike precipitation gravimetry, which is rarely used as a standard methodof analysis, volatilization gravimetric methods continue to play an importantrole in chemical analysis. Several important examples are discussedbelow.In o r g a n i c An a l y s i sDetermining the inorganic ash content of an organic material, such as apolymer, is an example of a direct volatilization gravimetric analysis. Afterweighing the sample, it is placed in an appropriate crucible and the organicmaterial carefully removed by combustion, leaving behind the inorganicash. The crucible containing the residue is heated to a constant weight usingeither a burner or an oven before determining the mass of the inorganicash.Another example of volatilization gravimetry is the determination ofdissolved solids in natural waters and wastewaters. In this method, a sampleof water is transferred to a weighing dish and dried to a constant weight ateither 103–105 o C or at 180 o C. Samples dried at the lower temperatureretain some occluded water and lose some carbonate as CO 2 . The loss of organicmaterial, however, is minimal. At the higher temperature, the residueis free from occluded water, but the loss of carbonate is greater. In addition,some chloride, nitrate, and organic material is lost through thermal decomposition.In either case, the residue that remains after drying to a constantweight at 500 o C is the amount of fixed solids in the sample, and the loss inmass provides an indirect measure of the sample’s volatile solids.Indirect analyses based on the weight of residue remaining after volatilizationare commonly used in determining moisture in a variety of products,and in determining silica in waters, wastewaters, and rocks. Moistureis determined by drying a preweighed sample with an infrared lamp or alow temperature oven. The difference between the original weight and theweight after drying equals the mass of water lost.