Ferric ammonium alum, (NH₄)₂SO₄.Fe₂(SO₄)₃

Ferric ammonium alum or Iron ammonium alum, (NH₄)₂SO₄.Fe₂(SO₄)₃.24H₂O, has been obtained by electrolysing a solution of ferrous sulphate and ammonium sulphate. The cathode consists of platinum wire immersed in acidified ferrous sulphate solution contained in a porous pot. An acidulated solution of ferrous and ammonium sulphates surrounds the pot, and is contained in a platinum dish which serves as anode. A current of 0.04 ampere is passed through the cell, and after several hours crystals of the alum are deposited round the anode.

Aqueous solutions of the alum rapidly become turbid when dilute, although concentrated solutions remain clear for several months. Addition of ammonium sulphate increases the hydrolysis, whilst sulphuric acid exerts a marked retarding influence. The precipitate consists of a basic sulphate of iron, 2Fe₂O₃.SO₃, and the hydrolysis is believed to follow the course indicated by the equations: -

2Fe₂(SO₄)₃ + 2(NH₄)₂SO₄ + 5H₂O ⇔ 2Fe₂O₃.SO₃ + 5H₂SO₄ + 2(NH₄)₂SO₄;
or, expressing the change ionically:
4Fe^••• + 4NH₄^• + 8SO₄'' + 5H₂O ⇔ 2Fe₂O₃.SO₃ + 10H^• + 4NH₄^• + 7SO₄
or
4Fe^••• + SO₄'' + 5H₂O ⇔ 2Fe₂O₃. SO₃ + 10H^•.

The fact that ferric hydroxide is not one of the products of hydrolysis explains why ammonium sulphate does not retard the reaction.

On addition of concentrated sulphuric acid to a solution of the alum a white precipitate is obtained which contains ferric and ammonium sulphates in some uncertain state of combination. The precipitate when dry is stable in air, slowly soluble in cold water, but readily soluble in hot hydrochloric acid, and is possibly an ammonium salt of ferri-sulphuric acid.

The densities of various concentrations of iron-ammonium alum solutions in water at 32.5° C. are as follow: -

Grams (NH4)2SO4.Fe2(SO4)3.24H2O in 100 Grams H2O.	Density
20.00	1.07960
15.00	1.05907
10.00	1.03801
5.02	1.01670
2.01	1.00376
0.80	0.99849