Potassium ferricyanide, K₃[Fe(CN)₆]

Potassium ferricyanide, K₃[Fe(CN)₆], known commercially as red prussiate of potash, was discovered by Gmelin in 1822, and is obtained by the chlorination of potassium ferrocyanide: -

2K₄[Fe(CN)₆] + Cl₂ ⇔ 2K₃[Fe(CN)₆] + 2KCl,

and subsequent re-crystallisation, whereby the more soluble potassium chloride remains in solution. It yields dark red monoclinic prisms of density 1.8, which dissolve in water to a yellow solution. The solubility is as follows: -

Temperature, °C.	4.4	10	15.6	37.8	100
Grams K3[Fe(CN)6] in 100 grams H2O	33.0	36.6	39.7	58.8	77.6

The above data are only approximately correct, and further research on the subject would be welcome. According to Grube, one litre of saturated solution at 25° C. contains 385.5 grams of K₃Fe(CN)₆. The solution on exposure to light yields a blue precipitate, ferrocyanide remaining in solution. It is reduced by sodium amalgam to ferrocyanide, and hydrogen peroxide has the same effect: -

4K₃[Fe^•••(CN)₆] + 2H₂O₂ = 3K₄[Fe^••(CN)₆] + H₄[Fe^••(CN)₆] + 2O₂.

When oxidised, di-potassium per ferricyanide, K₂Fe(CN)₆, is stated to be produced, but the salt is probably a pentacyanoferrate, K₂Fe^•••(CN)₅. With solutions of ferrous salts potassium ferricyanide yields a deep blue precipitate, originally called Turnbull's blue, but полу believed to be identical with Prussian blue.

In neutral solution potassium ferricyanide undergoes hydrolysis to a small extent, ferric hydroxide being precipitated. Thus

K₃Fe(CN)₆ + 3HOH = Fe(OH)₃ + 3KCN + 3HCN.

The hydrolysis is accelerated by light. Hydrolysis also appears to take place to a slight extent according to the equation -

K₃Fe(CN)₆ + H₂O ⇔ K₂Fe(CN)₅H₂O + KCN,

aquo penta-cyanoferrate resulting. The solution is reduced by hydrogen sulphide, slowly at the ordinary temperature, but rapidly on warming. Thus: -

6K₃Fe(CN)₆ + 3H₂S = 4K₄Fe(CN)₆ + K₂Fe.Fe(CN)₆ + 6HCN + 3S.

Potassium ferricyanide is reduced in alkaline solution to ferrocyanide - in other words, the salt under these conditions is able to function as an oxidiser, thus: -

2K₃Fe(CN)₆ + 2KOH ⇔ 2K₄Fe(CN)₆ + H₂O + O.

The reaction velocity has been studied at 90° C., with interesting results. The velocity at first decreases slightly, then increases rapidly to a maximum, after attaining which it falls again. The reason for the increased velocity lies in the autocatalytic action of the potassium ferrocyanide produced during the reaction.

Below 60° C. and in the dark, the alkaline solution of potassium ferrocyanide is stable.

An aqueous solution of potassium ferricyanide is readily reduced by hydrochloric acid into ferrocyanide. Thus: -

2H₃Fe(CN)₆ + 2HCl ⇔ 2H₄Fe(CN)₆ + Cl₂,

the reaction proceeding to completion if the chlorine is removed as, for example, by addition of reduced silver. Ferric chloride acts similarly - not because it dissociates on solution into ferrous chloride and chlorine, but because it undergoes hydrolysis, the liberated hydrochloric acid acting as indicated above.

Hydrobromic acid, ferric bromide, and the chlorides and bromides of zinc and aluminium all react in an analogous manner. With potassium iodide in the presence of a zinc salt the reaction is quantitative, and may be used in the volumetric estimation of ferricyanides: -

2K₃Fe(CN)₆ + 2KI ⇔ 2K₄Fe(CN)₆ + I₂.

Potassium ferricyanide is reduced to ferrocyanide by an alkaline solution of ferrous sulphate.

Potassium ferricyanide is quantitatively reduced by an alkaline solution of hydrazine sulphate, the reaction proceeding as follows: -

4K₃Fe(CN)₆ + 4KOH = 4K₄Fe(CN)₆ + 2H₂O + O₂,
N₂H₄ + O₂ = N₂ + 2H₂O.

By effecting the reduction in a nitrometer and measuring the nitrogen evolved, the amount of hydrazine or potassium ferricyanide originally present can readily be calculated.

Hydroxylamine similarly reduces the salt, yielding nitrous oxide and nitrogen. Thus: -

4NH₂.OH + O₂ = 2N₂ + 6H₂O,
6NH₂.OH + O₂ = 2N₂O + 2NH₃ + 6H₂O.

Indigo is readily bleached by the salt.

When distilled with potassium cyanide, potassium ferrocyanide, hydrogen cyanide, and ammonium carbonate are produced: -

2K₃Fe(CN)₆ + 2KCN + 2H₂O = 2K₄Fe(CN)₆ + HCN + NH₃ + CO₂.

Continued passage of carbon dioxide through a boiling solution of potassium ferricyanide results in the precipitation of ferric hydroxide and the formation of potassium carbonate and hydrogen cyanide or its decomposition products, ammonia and formic acid.