QUALITATIVE INORGANIC ANALYSIS. Before a systematic qualitative analysis of a solid sub stance can be undertaken, the substance must be obtained in solution. Sometimes substances sub mitted for analysis are found to be directly sol uble in water. In most cases, however, substances cannot be dissolved unless transformed chemi cally. Since most chlorides and most inorganic acids are soluble in water, the desired transfor mation can usually be effected by treating the finely powdered substances with aqueous hydro chloric acid, which converts the metals or metal lic oxides present into chlorides, while the acids originally combined in the substance are set free. In case metals (such as silver) are present, which form insoluble chlorides, or in case non-metals (such as sulphur or arsenic) are present, or in case hydrochloric acid does not attack the sub stance, nitric acid is used. By this the metallic compounds present in the substance are trans formed into nitrates, and all normal nitrates are soluble in water; on the other hand, the non metals present are mostly changed into the cor responding oxygen acids, which are likewise soluble in water—sulphur, for instance, being transformed into sulphuric acid. Many impor tant and familiar substances, however, resist the action of both of these acids. A few, as gold and platinum, will dissolve, forming soluble com pounds in a mixture of hydrochloric and nitric acids, the so-called aqua regia, which, on warm ing, gives off free chlorine. But other substances, such as glass, porcelain, and many natural silicates, resist the action of acids almost en tirely. Such substances are usually broken up by melting them with carbonates of the alkali metals and potassium nitrate, or by treatment with hydrofluoric acid. Subsequent treatment with water and hydrochloric acid then usually yields the required solutions.
Let us suppose that we have obtained a clear solution in nitric acid, which may contain all the more familiar metals and is free from or ganic matter. To this solution we add hydro chloric acid; if we obtain a white solid substance, which does not dissolve in a moderate excess of acid, we know we must have present some or all of the three metals, lead, silver, or mercury in the univalent form, since, of all the more famil iar metals, only these three form insoluble, or nearly insoluble, chlorides. The solid precipitate
is separated from the liquid by filtration, and we have then on the filter a solid which may con sist of any or all of the chlorides of lead. silver, and univalent mercury. A study of the proper ties of these chlorides shows that lead chloride is freely soluble in hot water, while the other two are not. Therefore, if the mass is treated with hot water, the lead chloride, if present, will dissolve, and can be filtered off while the other two remain behind. The liquid is then examined for lead, which is easily done, since all metals which could interfere with the test have been separated. Further, since silver chloride is i known to be easily soluble in aqueous ammonia, while mercurous chloride is converted into a black, insoluble mass containing free mercury, one might assume that treatment of the two chlorides with ammonia solution would affect, an easy separation of silver chloride from mer curous chloride. This case, however, well illus trates one of the difficulties of analytical work. If the amount of mercurous chloride is large in proportion to the amount of silver chloride, the metallic mercury set free by the action of am monia causes the formation of metallic silver, which is practically insoluble in ammonia. If, therefore, ammonia has failed to extract any thing from the precipitate in question. we can not conclude that silver is absent. We must, then, treat the black mass with a mixture of nitric and hydrochloric acids, which dissolves the black substance containing mercury; while the silver, if at all present in the original substance, remains behind, again in the form of silver chlo ride, hut this time unmixed with anything else. Such eases frequently occur. So often is the be havior of a substance toward a reagent modified by the presence of other substances, that no scheme of analysis worked out at the writing table possesses any value until thoroughly tested in the laboratory.