~Ferric sulphate~ (Fe_{2}(SO_{4})_{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(SO_{4})_{2}.12H_{2}O and NH_{4}Fe(SO_{4})_{2}.12H_{2}O. 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 (K_{4}FeC_{6}N_{6}), and potassium ferricyanide, or red prussiate of potash (K_{3}FeC_{6}N_{6}). These compounds are the potassium salts of the complex acids of the formulas H_{4}FeC_{6}N_{6} and H_{3}FeC_{6}N_{6}. ~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: 2FeSO_{4} + H_{2}SO_{4} + O = Fe_{2}(SO_{4})_{3} + H_{2}O. 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 2Fe^{++}, 2SO_{4}^{--} + 2H^{+}, SO_{4}^{--} + O = 2Fe^{+++}, 3SO_{4}^{--} + H_{2}O. Hydrogen ions have been oxidized to water, while the charge of each iron ion has been increased from 2 to 3. In a similar way the conversion of ferrous chloride into ferric chloride may be written Fe^{++}, 2Cl^{-} + Cl = Fe^{+++}, + 3Cl^{-}. Here again the valence of the iron and the charge on the iron ion has been increased from 2 to 3, though no oxygen has entered into the reaction. As a rule, however, changes of this kind are brought about by the use of an oxidizing agent, and are called oxidations. The term "oxidation" is applied to all reactions in which the valence of the metal of a compound is increased, or, in other words, to all reactions in which the charge of a cation is increased. ~Reduction of ferric salts.~ The changes which take place when a ferric salt is converted into a ferrous salt are the reverse of the ones just described. This is seen in the equation FeCl_{3} + H = FeCl_{2} + HCl In this reaction the valence of the iron has been changed from 3 to 2. The same equation may be written Fe^{+++}, 3Cl_{-} + H = Fe^{++}, + H^{+} + 3Cl_{-}