Cadmium Sulphate, CdSO₄

A mixture of cadmium sulphide and sulphate is formed by the action of sulphur dioxide on heated cadmium. Rhombic prisms of anhydrous sulphate, CdSO₄, were obtained by evaporating a solution of cadmium sulphate in sulphuric acid and carefully heating off the excess of acid. Their density at 15° C. was 4.72. Anhydrous cadmium sulphate is obtained as a white powder by heating hydrated salts to expel their water. The density of the unmelted salt has been given as 4.447. To obtain pure CdSO₄, since it is difficult to free the anhydrous salt completely from moisture and it decomposes at high temperatures, the hydrated sulphate (usually 3CdSO₄.8H₂O) should be heated to 200° C. in vacuo, then to 700° C. in a current of air containing sulphur trioxide, and finally in pure dry air at 200° C.

It is reduced to the sulphide, with the simultaneous production of a little metal, by heating in a current of hydrogen. One part of water dissolves about 0.59 part of CdSO₄ at 23° C., and the solubility does not increase very much with the temperature.

Solutions of cadmium sulphate are readily obtained by dissolving metallic cadmium, cadmium sulphide, or cadmium oxide in sulphuric acid. Salts of cadmium with volatile acids can also be heated with sulphuric acid. The hydrate, CdSO₄.8/3H₂O, or 3CdSO₄.8H₂O, crystallises from such solutions under ordinary conditions: this is the cadmium sulphate of commerce, the most common and important salt of the metal. This hydrate is also precipitated when alcohol is added to a solution of cadmium sulphate.

Cadmium can be obtained in a pure form by crystallisation as this salt. The crystals are monoclinic and are stable in ordinary dry air, though they effloresce rapidly if they are powdered.

A very pure sample has been prepared by converting purified cadmium bromide into the nitrate by nitric acid, the nitrate into the sulphate by sulphuric acid, and recrystallising.

The density of the crystals is about 3.0, and their specific heat between 17° C. and 21° C. 0.200.

They have been said to lose 4 molecules of water at 150° C., but they require heating at 700° C. in air containing sulphur trioxide to make them anhydrous. The deliquescent compounds 3CdSO₄.8HCl, 3CdSO₄.4H₂O,4HCl, and, ultimately, cadmium chloride, have been obtained by the action of hydrogen chloride on 3CdSO₄.8H₂O at different temperatures.

A saturated solution of 3CdSO₄.8H₂O contains 43.35 parts of CdSO₄ in every 100 parts of solution at -18° C., 44.99 parts at 60° C., 46.6 parts at 73.5° C., 37.8 parts at 100° C., and 2.3 parts at 200° C. The diminution in solubility above 74° C. corresponds to the formation of the monohydrate, CdSO₄.H₂O, which separates when solutions of the ordinary hydrate are evaporated at temperatures above 80° C. The monohydrate has been obtained by evaporating a solution of cadmium sulphate containing an excess of sulphuric acid. The crystals are monoclinic, and their density at 15° C. is 3.786. They retain their water at 100° C., and slowly combine with water at ordinary temperatures to form the ordinary hydrate.

The Weston standard cell consists of the system mercury - mercurous sulphate (made into a paste with some cadmium sulphate) - cadmium sulphate crystals - cadmium sulphate solution - cadmium sulphate crystals - cadmium amalgam. It has been suggested that certain irregularities noted in the behaviour of some Weston cells are due to a change undergone by 3CdSO₄.8H₂O at 15° C., two different modifications of the salt being possible below that temperature. Careful determinations of the solubility between 13.7° C. and 25° C. did not indicate a transition-point at 15° C.

The heptahydrate, CdSO₄.7H₂O, is left as a granular powder when the cryohydrate of cadmium sulphate is melted. Above 4° C. it is slowly converted into the ordinary hydrate. Its solubility slowly rises from 44.45 per cent. CdSO₄ at -17° C. to 48.7 at -4.5° C.

Various other hydrates that have been reported do not seem to exist.

[CdSO₄]+400Aq. = CdSO₄.Aq. + 10.74 Cal.
[CdSO₄.H₂O]+400Aq. = CdSO₄.Aq.+6.05 Cal.
[3CdSO₄.8H₂O] +400Aq. = CdSO₄.Aq. +2.66 Cal.

Cadmium sulphate is moderately ionised in aqueous solution. Ionisation has been said to proceed according to the equation

3CdSO₄ ⇔ [Cd₂SO₄]^••+[Cd(SO₄)₂]''.

The basic sulphates, 4CdO.SO₃ and 2CdO.SO₃.H₂O, have been reported.

Cadmium sulphate triammoniate, CdSO₄.3NH₄, is formed when cadmium sulphate absorbs ammonia gas. It decomposes into CdSO₄.NH₄ at 100° C.

The hydrated compounds 2CdSO₄.4NH₄.5H₂O and CdSO₄.4NH₄.2H₂O have been prepared.

Cadmium sulphate, similarly to zinc sulphate, forms a series of double salts corresponding to the general formula M₂SO₄.RSO₄.6H₂O (M = NH₄, Th, or any alkali metal except Li or Na, and R = a divalent metal).

The following double sulphates have also been described: Na₂SO₄.CdSO₄, Na₂SO₄.3CdSO₄, 3Na₂SO₄.CdSO₄; Na₂SO₄.CdSO₄.2H₂O; KSO₄.2CdSO₄, KSO₄.3CdSO₄; KSO₄.CdSO₄.1½H₂O, KSO₄.CdSO₄.2H₂O; (NH₄)₂SO₄.2CdSO₄; 3(NH₄)₂SO₄.CdSO₄.10H₂O; MgSO₄.CdSO₄.14H₂O; 3CdSO₄.Ce₂(SO₄)₃.6H₂O; (Cd,Fe)SO₄.7H₂O, (Cd,Fe)SO₄.2½H₂O.