The Elements of Qualitative Chemical Analysis, vol. 1, parts 1 and 2. / With Special Consideration of the Application of the Laws of Equilibrium and of the Modern Theories of Solution.

by Julius Stieglitz

On the other hand, the separation of the arsenic group (arsenic, antimony, tin, gold and platinum) from the copper group, with which it is precipitated by hydrogen sulphide from acid solutions, depends, essentially, on a chemical difference between the groups. The oxides, especially the higher oxides, of the arsenic group, are preëminently acid-forming; the higher oxides form such acids as arsenic acid, H₃AsO₄, antimonic acid, H₃SbO₄, stannic acid, H₂SnO₃, platinic acid, H₂PtO₃, and auric acid, HAuO₂. These [p243] hydroxides are, however, all more or less weakly basic in character as well. The hydroxides of the lower oxides of the metals are, as one must expect, much more strongly basic, but most of them—arsenious, antimonous and stannous hydroxides—still show sufficient acid character to be distinctly amphoteric in behavior. But, with the exception of arsenious acid, the basic ionization of the hydroxides of the lower oxides is more pronounced than their acid ionization.

The basic ionization of the hydroxides of their lower and higher oxides brings these elements into the plan of analysis for the metal or positive ions in systematic analysis. In the presence of hydrochloric acid they form chlorides, which yield positive ions in sufficient quantity[493] to allow their extremely insoluble sulphides to be precipitated by hydrogen sulphide in acid solution, together with the, likewise, very insoluble sulphides of the copper group.

The acid-forming properties of the oxides of the arsenic group are maintained in their sulphides. Again, this is especially evident in the higher sulphides. The element sulphur is substituted for the closely related element oxygen without any profound change in the chemical behavior of the compounds. Advantage is taken of this acid-forming power to separate the sulphides of the arsenic group from the sulphides of the copper group, which either are not acid-forming at all, or exhibit this property only to a very slight degree.[494]

Sulpho-Salts.

CO₂ + 2 KOH ⇄ K₂CO₃ + H₂O.

(1)

CS₂ + 2 KSH ⇄ K₂CS₃ + H₂S.

(2)

CS₂ + K₂S ⇄ K₂CS₃.

(3)