=Potassium Ferrocyanide=, K₄Fe(CN)₆.3H₂O = 422.—The potassium iron cyanogen compounds are made in special works, particularly in the neighbourhood of large towns, by melting potashes with nitrogenous organic substances and iron, washing out the mass and purifying the salt so obtained by recrystallisation. Potassium ferrocyanide (yellow prussiate of potash) forms large transparent crystals of a peculiar soft nature, which dissolve readily in water. It often contains considerable quantities of potassium sulphate, up to 5 per cent., and it is to be noted that the impurity is much the cheaper of the two salts. When barium chloride is added to a solution of the salt, a white precipitate forms if sulphate be present. The behaviour of yellow prussiate towards iron salts is noteworthy. With ferrous salts, for example green vitriol (copperas), it gives a white precipitate which gradually turns blue in the air; with ferric salts, for example ferric chloride (“nitrate of iron”), it at once gives a blue precipitate. =Potassium Ferricyanide= (Red Prussiate of Potash), K₃Fe(CN)₆ = 329, is obtained by passing chlorine through a solution of yellow prussiate until the liquid smells strongly of chlorine and no longer gives a precipitate with a solution of a ferric salt. The solution then contains potassium ferricyanide and chloride. The former is obtained by evaporating and allowing to crystallise. Pure potassium ferricyanide forms beautiful dark red crystals, which readily dissolve in water. The solution gives a blue precipitate with ferrous salts, but only a brown colouration and no precipitate with ferric salts. Both yellow and red prussiate are used in the preparation of several much used colours, for Prussian and Chinese blues, and several others. All cyanogen compounds, with the exception of yellow prussiate, are extremely poisonous. The following table gives the solubility of potassium ferricyanide at different temperatures:— +-----------------------+------------+----------------+ | 100 Parts of Water |Temperature.|Specific Gravity| |dissolve Parts of Salt.+------------+ of Solution. | | | °C. | | +-----------------------+------------+----------------+ | 33 | 4.44 | 1.151 | | 36 | 10.00 | 1.164 | | 40.8 | 15.50 | 1.178 | | 58.8 | 37.80 | 1.225 | | 77.5 | 100.00 | 1.250 | | 82.6 | 104.40 | 1.265 | +-----------------------+------------+----------------+ =Sodium Salts.=—In chemical properties the sodium salts are very similar to the potassium salts, and, being cheaper, they are generally used in place of the latter.