Ferro-cyanic acid, H₄[Fe(CN)₆]

Hydrogen ferrocyanide, Ferro-cyanic acid, H₄[Fe(CN)₆], is conveniently prepared by adding concentrated aqueous hydrogen chloride solution to a saturated solution of potassium ferrocyanide, in the cold: -

K₄[Fe(CN)₆] + 4HCl = 4KCl + H₄[Fe(CN)₆].

The precipitate, dried in the absence of air, is dissolved in alcohol and re-precipitated as a white powder on addition of ether, but apparently combined with two molecules of ether, which, however, evaporate away upon exposure to air, leaving the pure acid as a residue.

Ferrocyanic acid is also obtained when a solution of insoluble Prussian blue in hot, concentrated hydrochloric acid is allowed to stand.

Hydrogen ferrocyanide is soluble in water and possesses a strong acid reaction. The solution decomposes on boiling, evolving gaseous hydrogen cyanide, and yielding a white precipitate of ferrous cyanide. Thus: -

H₄[Fe(CN)₆] = 4HCN + Fe(CN)₂.

Upon exposure to air hydrogen ferrocyanide absorbs oxygen, yielding hydrogen cyanide and a ferric salt of the original acid. Thus: -

7H₄[Fe(CN)₆] + O₂ = 24HCN + 2H₂O + Fe₄[Fe(CN)₆]₃.

Heated in the absence of air to 300° C., a pale yellow powder (ferrous cyanide) is obtained, hydrogen cyanide being evolved.

The heat of neutralisation of ferrocyanic acid with 4 molecules of potassium hydroxide is

H₄Fe(CN)₆ + 4KOH = K₄Fe(CN)₆ + 4H₂O + 57,900 calories

at 12° C. The heat of formation of ferrocyanic acid from its elements is -122,000 calories.