«The Separation of the Copper and Arsenic Groups from the Zinc Group.»—Ferrous-ion and zinc-ion may be taken as typical representatives of the ions of the ‹zinc group, whose sulphides are not precipitated by hydrogen sulphide in the presence of a definite concentration of hydrogen-ion›, cadmium-ion as a representative of the ‹copper›, ‹silver› and ‹arsenic› groups, ‹whose sulphides are precipitated under the same conditions›. The separation of the groups depends, therefore, on the different solubilities of the sulphides of these [p211] groups, the sulphides of the zinc group being the most soluble. Since even these sulphides are precipitated quantitatively by ammonium sulphide and are very difficultly soluble, the separation is a kind of fractional precipitation of difficultly soluble salts, in which the fractionation is made possible and convenient by the use of an agent, hydrogen sulphide, the concentration of whose active precipitating component, the sulphide-ion, S^{2−}, is easily regulated and readily made sufficiently small, not to precipitate even the difficultly soluble sulphides of the iron group. Solubilities vary from salt to salt, and we have already found that, in the zinc group, zinc sulphide is less soluble than the sulphide of a second member of the group, ferrous sulphide, and that the difference is revealed in a somewhat different behavior of their salts toward hydrogen sulphide, when the action is studied in some detail. Similar differences must be expected to exist among the sulphides of the groups that hydrogen sulphide precipitates even in the presence of an excess of hydrochloric acid. As these differences are the sources of some of the most common and most serious errors which analysts are liable to commit, the detailed study of the action of hydrogen sulphide must be continued a little further. «The Effect of a Large Excess of Acid.»—The precipitation of cadmium sulphide depends on the relation of the product of the concentrations of the cadmium-ion and the sulphide-ion to the solubility-product constant for cadmium sulphide (see the equation, p. 209). Now, it is clear that if a ‹larger› and ‹larger excess of hydrogen-ion› is introduced by the addition of more concentrated hydrochloric acid to the cadmium sulphate solution, the concentration of sulphide-ion is correspondingly ‹reduced›.[416] The point might be reached, where the sulphide-ion factor becomes so small, that the product of the ion concentrations remains smaller than the value K_{CdS}, required for precipitation of cadmium sulphide. In fact, if a large excess[417] of hydrochloric acid is added to 50 c.c. [p212] of the 0.1 molar solution of cadmium sulphate, hydrogen sulphide fails to precipitate any of the sulphide (‹exp.›).