2 AlCl_{ 3} + 3 (NH_{ 4})_{ 2}S + 6 H_{ 2}O → 2 Al(OH)_{ 3} ↓ + 6 NH_{ 4}Cl + 3 H_{ 2}S ↑
or 2 Al^{ 3+} + 3 S^{ 2−} + 6 HOH → 2 Al(OH)_{ 3} ↓ + 3 H_{ 2}S ↑.
A similar result is obtained when the solution of a chromium salt is treated with a solution of ammonium sulphide. Only ferric hydroxide is capable of forming a sulphide, ferric sulphide, Fe_{ 2}S_{ 3}, which is precipitated when solutions of ferric salts are treated with ammonium sulphide.[382]
Ammonium sulphide will, consequently, precipitate aluminium and chromium ‹hydroxides› and ferric, ferrous, nickel, cobalt, manganese and zinc ‹sulphides›, from a solution of the chlorides of the metals.
Now, both the sulphides and the hydroxides of the alkaline earths and alkalies are sufficiently soluble not to be precipitated by ammonium sulphide, or by a mixture of it with ammonium hydroxide, if ammonium chloride be added to the mixture to prevent the precipitation of magnesium hydroxide (see p. 168), which is the least soluble of the hydroxides of the alkaline earth group. [p191] ‹A mixture of ammonium sulphide, ammonium hydroxide and ammonium chloride will, therefore, precipitate the aluminium and zinc groups together, separating them from the alkaline earth and alkali groups.›[383]
«Separation of the Aluminium Group from the Zinc Group by Means of Ammonium Chloride and Ammonium Hydroxide.»—The precipitation of the two groups together makes their subsequent separation necessary. Some analysts attempt to avoid the extra operations involved, by making use of the fact that the hydroxides of the bivalent group, although difficultly soluble, are, like magnesium hydroxide, still sufficiently soluble not to be precipitated by ammonium hydroxide in the presence of sufficient ammonium chloride, while the hydroxides of the trivalent metals of this group are so insoluble that they may be precipitated quantitatively by such a mixture (p. 170). The trivalent hydroxides may be first precipitated by ammonium hydroxide, in the presence of ammonium chloride, and, subsequently, the sulphides of the bivalent metals may be precipitated by ammonium sulphide, the two precipitates being collected separately. The method has the disadvantage that it is not always accurate. The acid character of aluminium and chromium hydroxides (and even of ferric hydroxide, see p. 195), as well as of zinc hydroxide, leads, to a certain extent, to the precipitation, from such ‹alkaline› solutions, of ‹salts› of these amphoteric hydroxides with the basic hydroxides of the bivalent group; the latter are thus liable to be ‹lost› in the analysis. It will be recalled, that the equilibrium conditions in alkaline solutions ‹favor› the ‹ionization› of amphoteric substances in the acid form (Part III), and ‹alkaline› solutions would favor the precipitation of aluminates, chromites, etc., of the ions of the zinc group. Methods have, therefore, been devised to separate the two groups in neutral, or very slightly acid, media, and they give quantitative separations and are preferable to the method just described. The separation by means of suspended ‹barium carbonate›, in which carbonic acid is liberated and the solution is practically neutral, will be discussed below on page 193. A second method, frequently used in quantitative analysis, is based on the [p192] ‹decomposition› of the ‹acetates› of the aluminium group by boiling water, acetic acid being liberated.[384]