The sulphides of the metal ions of the zinc group are readily precipitated by ammonium or sodium sulphide, but hydrogen sulphide, in the presence of a small excess of a strong acid, such as hydrochloric acid, does not precipitate any of these sulphides (or any of the sulphides of the aluminium, the alkaline earth and the alkali groups). Under the same conditions ‹the sulphides of the metal ions of the silver group›, Ag^{+}, Hg^{+}, Pb^{2+}, of ‹the copper group›, Hg^{2+}, Pb^{2+}, Bi^{3+}, Cu^{2+}, Cd^{2+}, and also of ‹the arsenic group›, As^{3+}, As^{5+}, Sb^{3+}, Sb^{5+}, Sn^{2+}, Sn^{4+}, Pt^{2+}, Pt^{4+}, Au^{+}, and Au^{3+}, are precipitated. Advantage is taken of these relations in the following way, in systematic analysis: after the separation of the silver group, by precipitation of the difficultly soluble chlorides, hydrogen sulphide, in the presence of an excess of acid, is used to precipitate the sulphides of the ions of the copper and the arsenic groups, the two groups being precipitated together. Hydrogen sulphide, under these conditions, does not precipitate any sulphides of the zinc group or those of any of the remaining groups. Hydrogen sulphide is used, in this way, as one of the most valuable reagents in analytical work, enabling the analyst to separate whole groups of metal ions from other groups. There is also no other agent, equally important, which is more likely to be used in a wrong way and to lead to error. «The Ionization of Hydrogen Sulphide.»—Before taking up the theory of the separation of these groups by precipitation with hydrogen sulphide in the presence of a strong acid, a discussion of some of the characteristics of the reagent will be in place. Hydrogen sulphide, like carbonic acid and other dibasic acids, ionizes in two stages; it produces first hydrogen-ion and hydrosulphide-ion, HS^{−}, and this ion in turn dissociates, producing hydrogen-ion and sulphide-ion, S^{2−}. [p200] For the first dissociation, HSH ⇄ H^{+} + HS^{−}, we have [H^{+}] × [HS^{−}] / [H_{2}S] = K_{1} (I) The value of the constant[395] for this primary ionization of hydrogen sulphide is 0.91E−7. It is apparent that hydrogen sulphide, even in the primary ionization, is a very weak acid and produces a very small concentration of hydrosulphide-ion. In a solution, saturated at 25°, the total concentration of hydrogen sulphide is approximately 0.1 molar, and the concentration of hydrosulphide-ion, therefore, in the absence of any foreign acid, at most[396] 0.95E−4. The concentration of the dissolved, nonionized hydrogen sulphide, [H_{2}S], is practically a constant, if solutions saturated with hydrogen sulphide under a given pressure, say under atmospheric pressure, are considered. For such solutions, then, we may put more simply[396] [H^{+}] × [HS^{−}] = ‹k› = (0.95E−4)^2 = 0.9E−8. (II)