Ferric Alums

Ferric sulphate unites with sulphates of the alkali metals and ammonium to yield well-defined crystalline salts of general formula: -

M₂SO₄.Fe₂(SO₄)₃.24H₂O.

The iron alums are isomorphous, not merely with one another, but with the wide series of salts known as alums of generic formula: -

M₂SO₄.R₂(SO₄)₃.24H₂O,

in which M may, as before, be ammonium or an alkali metal, whilst R may stand not merely for Fe, but also for Al, Cr, Co, Ga, In, Mn, Rh, Ti, or V. All possible combinations of these have not as yet been prepared, but such as are known crystallise in the cubic system as regular octahedra or as combinations of the octahedron and cube.

The internal structure of crystals of various alums has been studied by Bragg's X-ray method, and the conclusion arrived at is that the water of crystallisation is not to be regarded as in any way distinct from the other constituents of the molecule, since its removal necessitates the destruction of the crystalline structure. Werner's generic formula for the alums is in complete harmony with this, namely: -

[R[(H₂O)₂]₆](SO₄)₂M

Sometimes the formula for an alum is written as MR(SO₄)₂.12H₂O,

indicative of an associated sulphate anion. The trivalent metal, however, figures among the cations along with the alkali metal, and does not enter the anion. Thus, for example, in the case of the iron alums, the iron is one of the positive radicles; it does not constitute part of the negative radicle, as in the case of the ferrocyanides. Hence a solution of an iron alum gives all the reactions of a ferric salt. Raoult has shown that the molecular depression of the freezing-point of water by - to quote a specific example - ferric potassium alum, namely 85.0° C., is practically identical with the sum of the molecular depressions produced separately by potassium sulphate and ferric sulphate, namely 82.1° C. In aqueous solution, therefore, the alum appears to be dissociated into its constituents.

The refractive indices for the D-line and densities of the iron alums are as follow: -

Alum.	Refractive Index.	Density.
Ammonium	1.48482	1.713
Potassium	1.48169	1.806
Rubidium	1.48234	1.916
Caesium	1.48378	2.061
Thallium	1.52365	2.385

The refractive index is seen to increase with the molecular weight, except in the case of the iron ammonium alum.

The solubilities of the alums in water at 25° C. are as follow: -

Alum	Gram-molecules dissolved per Litre of Water.	Grams of (Anhydrous) Alum per Litre of Water.
Ammonium	1.659
Rubidium	0.293	97.4
Caesium	0.045	17.1
Thallium	0.799

The relatively low solubility of the caesium salt is remarkable in view of the great solubility of ferrous caesium sulphate.