This behavior of zinc acetate[412]—and zinc salts of other ‹weak acids› show, of course, the same behavior—represents one of the pitfalls, into which the unwary analytical chemist is liable to fall, when he uses hydrogen sulphide. The separation of groups by hydrogen sulphide depends, as stated, on the fact, that, in the presence of a certain concentration of hydrogen-ion, hydrogen sulphide will not precipitate zinc sulphide and the remaining sulphides of the zinc group. To secure this concentration of hydrogen-ion, some hydrochloric acid is added to solutions, from which hydrogen sulphide is expected to precipitate none but sulphides of the copper and arsenic groups—and, as a rule, the purpose is accomplished, as desired. It is evident, however, that if a solution contains an acetate, say sodium acetate, or the salt of any other weak acid, ‹e.g.› a borate or a phosphate, the addition of hydrochloric acid will result, at least at first, in the ‹liberation› of the ‹weaker acid› and will not produce the excess of hydrogen-ion, required for the analysis. Zinc sulphide, and possibly nickel and cobalt sulphides,[412] may, under such conditions, be precipitated with the sulphides of the groups mentioned. Unless provision is made, therefore, ‹to insure a certain excess of hydrogen-ion› (p. 213), or unless we are on our guard and look for zinc, nickel and cobalt in [p209] the analysis of the precipitate formed by hydrogen sulphide,[413] serious errors obviously could result. To add an inordinately large excess of hydrochloric acid to mixtures, in order to avoid this pitfall, will, as we shall presently see, only throw us more certainly into still another error, to which we are exposed in the use of this important reagent, hydrogen sulphide. «III. Precipitation of Cadmium Sulphide.»—Now, if a 0.1 molar solution of cadmium sulphate (22.6 grams of the salt,[414] CdSO_{4}, H_{2}O, ‹per› liter) is prepared, equivalent, in concentration, to the solutions of ferrous and zinc sulphates used previously, we may consider that the concentration of cadmium-ion is, approximately, the same as the concentrations of the ferrous-ion and zinc-ion in the solutions of their sulphates. If the solution of cadmium sulphate is added to the acidified solution of zinc sulphate, which was saturated with hydrogen sulphide but from which no zinc sulphide could be precipitated (‹exp.›, p. 207), a heavy precipitate of cadmium sulphide is at once obtained (‹exp.›). Or, if 2 c.c. of hexamolar hydrochloric acid is added to 50 c.c. of the cadmium sulphate solution and the mixture is saturated with hydrogen sulphide,[415] cadmium sulphide is precipitated readily (‹exp.›) and, in fact, quantitatively. We would decide, on the basis of the principle of the solubility-product, that, in this mixture, precipitation results because ([Cd^{2+}] × [S^{2−}] / ‹x›) > K_{CdS}.