FeCl2 + Cl = FeCl3.
When the pure salt is heated with water it is partly hydrolyzed:
FeCl3 + 3 H2O <--> Fe(OH)3 + 3HCl.
This is a reversible reaction, however, and hydrolysis can therefore be prevented by first adding a considerable amount of the soluble product of the reaction, namely, hydrochloric acid.
Ferric sulphate (Fe2(SO4)3). This compound can be made by treating an acid solution of green vitriol with an oxidizing agent. It is difficult to crystallize and hard to obtain in pure condition. When an alkali sulphate in proper quantity is added to ferric sulphate in solution an iron alum is formed, and is easily obtained in large crystals. The best known iron alums have the formulas KFe(SO4)2·12H2O and NH4Fe(SO4)2·12H2O. They are commonly used when a pure ferric salt is required.
Ferric hydroxide (Fe(OH)3). When solutions of ferric salts are treated with ammonium hydroxide, ferric hydroxide is formed as a rusty-red precipitate, insoluble in water.
Iron cyanides. A large number of complex cyanides containing iron are known, the most important being potassium ferrocyanide, or yellow prussiate of potash (K4FeC6N6), and potassium ferricyanide, or red prussiate of potash (K3FeC6N6). These compounds are the potassium salts of the complex acids of the formulas H4FeC6N6 and H3FeC6N6.
Oxidation of ferrous salts. It has just been seen that when a ferrous salt is treated with an oxidizing agent in the presence of a free acid a ferric salt is formed:
2FeSO4 + H2SO4 + O = Fe2(SO4)3 + H2O.
In this reaction oxygen is used up, and the valence of the iron is changed from 2 to 3. The same equation may be written