Cuprous ferrocyanide

Ammonium cuproferrocyanide, (NH4)2Cu2Fe(CN)6, results1 when cuprous cyanide is boiled with a solution of ammonium ferrocyanide containing ammonium sulphite, and hydrogen passed through the mixture. It may also be obtained by double decomposition of the sodium salt with ammonium nitrate. It crystallises in small, colourless six-sided prisms, which readily decompose both under water and on mere exposure to air or in a vacuum. 

Lithium cuproferrocyanide,2 Li2Cu2Fe(CN)6, is obtained by boiling cuprous cyanide with a solution of lithium ferrocyanide containing lithium sulphite, hydrogen gas being simultaneously bubbled through the solution. It crystallises in colourless, hexagonal prisms. 

Potassium cuproferrocyanide, K2Cu2Fe(CN)6, is prepared 1 by boiling cuprous cyanide with a solution of potassium ferrocyanide containing a little potassium sulphite or by boiling cuprous chloride or potassium cuprous cyanide with potassium ferrocyanide solution. When rapidly cooled, the solution yields colourless cubes, but the crystals are liable to undergo partial oxidation, turning yellow or brown in colour. 

Sodium cuproferrocyanide, Na2Cu2Fe(CN)6, results 1 on adding a cold, saturated solution of sodium copper cyanide to a hot solution of sodium ferrocyanide or by boiling a solution of sodium ferrocyanide with cuprous cyanide in the presence of a little sodium sulphite, and cooling m an atmosphere of hydrogen. It crystallises m minute, colourless hexagonal prisms, which are conveniently dried over sulphuric acid in a vacuum. 

If cuprous chloride is substituted for cupric sulphate, then different cuprous potassium ferrocyanides are obtained according to circumstances, namely K2Cu 2[Fe(CN)6] KCuCu 3[Fe(CN)6]2 and KCu a[Fe(CN)6]. The last of these is obtained, when cuprous chloride is in great excess, as a white precipitate. 

Solutions of potassium ferrocyanide are catalytically decomposed when boiled with cuprous chloride in the presence of hydrochloric acid.4 The action appears to consist in the alternate formation of cuprous cyanide and regeneration of cuprous chloride, hydrogen cyanide being evolved. By collecting the evolved acid in alkali, and afterwards titrating excess of the latter, the amount of ferrocyanide originally present may be conveniently estimated. 

Spot tests for hydrazine can be based on this reaction if carried out in the presence of anions which form cuprous salts that are insoluble in ammonia. Ferricyanide and ferrocyanide ions, whose cupric salts are easily soluble in ammonia, are suitable. 

The test for uranium with potassium ferrocyanide can also be carried out in the presence of ferric and cupric salts, if these metals are converted, before the addition of the ferrocyanide, into the nonreacting cuprous and ferrous forms. Reduction with iodide ions in acid solution serves this purpose ... 

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