The present status of gravimetric analysis as a specialty within analytical chemistry is discussed at both the academic and industrial levels. This background is then related to the present American Society for Testing and Materials' standard methods for metals and alloys which use gravimetric procedures for 16 elements. Theoretical considerations for gravimetry include: (a) solubility of the precipitate, (b) purity and ease of filtration of the precipitate, and (c) stoichiometric composition of the dried or ignited precipitate.
Applied concepts mostly dealing with the prior separation of interferences must be considered before application can be made using theoretical principles. selected gravimetric procedures are discussed to illustrate these concepts for two of the more common elements (carbon and silicon), as well as two of the more difficult (rare earths and tungsten). In the carbon method using evolution gravimetry (CO2 absorption on ascarite) the principal items include: induction versus resistance heating, reagents for the removal of interferences, sample preparation, and lower scope limits for acceptable gravimetric results. The silicon procedure using volatilization as SiF4 treats: the choice of acid for coagulation and partial dehydration, the advantages and precautions with perchloric acid, and occlusion of possible interferences with the coagulated silica. The rare earth method encompasses preseparation of interferences, comparison of the oxalate and fluoride gravimetric finishes, and instrumental treatment of the gravimetric residues to determine individual rare earths. Tungsten is determined by separation as a hydrolytic oxide. The obtained oxide is usually impure and the method is evaluated on the basis of the analysis of occluded impurities—principally molybdenum.