Copper ferrocyanide, Cu₂[Fe(CN)₆]

Copper (cupric) ferrocyanide, Cu₂[Fe(CN)₆]. + Aq., is obtained as a brown precipitate by mixing solutions of copper sulphate and potassium ferrocyanide, the former salt being present in excess. An interesting experiment consists in preparing in a gas jar a saturated solution of copper sulphate and diluting some 50 per cent. A drop of saturated potassium ferrocyanide solution is now introduced by means of a pipette and rapidly sinks to the bottom of the jar. It does not mix with the copper sulphate solution, for its surface which came into contact with the latter solution is now covered with a thin and invisible membrane of copper ferrocyanide. After lying at the bottom of the jar a little while, the drop will be seen to increase slightly in bulk, and then rise up through the copper sulphate solution to the top of the jar. This is due to water passing through the copper ferrocyanide membrane and reducing the density of the potassium ferrocyanide solution.

Evidence has been obtained suggestive of the possible existence of the trihydrate, Cu₂Fe(CN)₆.3H₂O.

The salt is largely used in experimental work dealing with osmosis. The pores in a copper ferrocyanide membrane are exceedingly fine, ranging from 8 to 60 μμ in diameter, the average being from 15 to 20 μμ. The osmotic action is not due to a selective mechanical filtration, but to selective adsorption on the surface of the membrane. Hence such membranes as have the smallest pores and expose the greatest superficial area are the most efficient semipermeable structures.

Copper ferrocyanide may be obtained in colloidal form of high chemical purity by prolonged dialysis of the solution obtained by mixing equivalent solutions of sodium ferrocyanide and copper chloride (or sulphate).

The molecular weight of colloidal copper ferrocyanide has been calculated as 700,000.

When boiled with concentrated nitric acid, a ferroferricyanide is obtained of the composition

Cu₇[Fe''Fe'''(CN)₂₂]₂.30H₂O.

When boiled with concentrated hydrochloric acid, the acid salt CuH₂Fe(CN)₆.4H₂O, is obtained as a yellow, insoluble substance. This, when digested with solutions of the chlorides of the alkali metals, liberates hydrochloric, acid, yielding alkali cupriferrocyanides. Thus, for example, with potassium chloride: -

CuH₂Fe(CN)₆ + 2KCl = CuK₂Fe(CN)₆ + 2HCl.

With sodium chloride solution in the cold only half the hydrogen is replaced, thus: -

CuH₂Fe(CN)₆ + NaCl = CuNaHFe(CN)₆ + HCl.

This latter salt, when digested with potassium or ammonium chloride, yields CuNaKFe(CN)₆ or CuNaNH₄Fe(CN)₆ respectively.

The acid salt CuH₂Fe(CN)₆ thus behaves as a dibasic acid, namely cupriferrocyanic acid, and gives rise to a definite series of salts known as cupriferrocyanides.