Ferric thiocyanate, Fe(CNS)₃

Ferric thiocyanate, Fe(CNS)₃.6H₂O, may be prepared by dissolving ferric hydroxide in aqueous thiocyanic acid, as also by metathetical decomposition of a ferrous salt and potassium thiocyanate. It crystallises in cubes, deep red in colour, which slowly dissolve in water, yielding an intensely red solution. The formation of this red colour constitutes a delicate method of detecting the presence of traces of ferric iron.

In very dilute solution the intensity of colour produced is not quite proportional to the amount of iron present - indeed, the more concentrated solution becomes decolorised on dilutiqn, as also by addition of oxalates, tartrates, etc. The decoloration on dilution is usually explained on the assumption that the water hydrolyses the red un-dissociated salt into yellow colloidal ferric hydroxide and free thio-cyanic acid: -

2Fe(CNS)₃ + 3H₂O ⇔ 2Fe(HO)₃ + 6HCNS.

The effect of the organic acids is attributed to their union with the iron ions of the dissociated ferric thiocyanate, thereby displacing the equilibrium

Fe^••• + 3CNS' ⇔ Fe(CNS)₃

in the direction of right to left - that is, reducing the concentration of the undissociated coloured salt.

A modification of this view lies in the theory that the coloured compound is the undissociated double salt Fe(CNS)₃.9KCNS, and not simply the simple undissociated molecule Fe(CNS)₃. This is apparently supported by the observation that a maximum coloration is produced, save in very dilute solutions, with twelve molecules of potassium thiocyanate to one of ferric chloride. Thus: -

FeCl₃ + 12KCNS = Fe(CNS)₃.9KCNS + 3KCl.

In dilute solution even more potassium thiocyanate is required, no doubt on account of dissociation, as explained above.

Tarugi, on the other hand, attributes the colour to the formation of ferrous peroxy thiocyanic acid, FeH(CNOS)₃, according to the reversible reaction: -

12FeCl₃ + 6KCNS + 6H₂O = 2FeH(CNOS)₃ + 6KCl + 10FeCl₂ + 10HCl.

The free acid, H₃(CNOS)₃, as well as its acid salts, have a red colour. The decolorising influence of oxalates, etc., is attributable to their converting the acid or its acid salts into colourless normal salts.

This theory, however, has not met with a wide acceptance.

Ferric thiocyanate solution gradually loses its intensity of colour when kept, a reduction of the iron from the ferric to the ferrous condition being observable.

The exact course of this reduction is uncertain, but the carbon of the acid radicle is oxidised to carbon dioxide and the sulphur to sulphuric acid. The equation might therefore be written as: -

8Fe(CNS)₃ + 6H₂O = 8Fe(CNS)₂ + 7HCNS + CO₂ + H₂SO₄ + NH₃.

This equation, however, postulates the production of more ammonia than is actually found, by about 50 per cent. The nitrogen therefore appears only partly as ammonia, and no definite information has been obtained as to the product or products into which the remaining portion of the element is converted.

The beautiful colour of the amethyst is thought to be due to ferric thiocyanate, and this is supported by the fact that, on heating, the colour of the stone becomes yellow, its absorption spectrum now closely resembling that of ferric oxide or a ferric compound. The calcined stone closely resembles the natural citrine, which latter may have been obtained in an analogous manner from the amethyst through natural agencies.

Fe(CNS)₃.9KCNS. 4H₂O is obtained by dissolving freshly precipitated ferric hydroxide in thiocyanic acid, adding the requisite quantity of potassium thiocyanate, and concentrating over sulphuric acid. The salt yields dark red rhombic prisms, with a greenish reflex. It is fairly stable, but slowly deliquesces upon exposure to moist air. Its solution in pure water is stable, but traces of electrolytes induce decomposition. The corresponding ammonium, sodium, lithium, and caesium salts have been prepared.