Cadmic Hydroxide, Cd(OH)₂

Cadmium oxide, amorphous or crystalline, is converted into white cadmium hydroxide, Cd(OH)₂, by prolonged treatment with water or solutions of either alkalies or salts. This amorphous hydroxide is often prepared by precipitating solutions of cadmium nitrate with a carbonate-free alkali hydroxide and washing the precipitate free from alkali. If the cadmium nitrate solution is concentrated a basic salt may contaminate the precipitated hydroxide. A pure product is obtained by electrolysing a solution of an alkali salt (such as sodium nitrate) between a platinum cathode and a cadmium anode. Cadmium is dissolved by the anions, and precipitation finally occurs, more readily if the solution is agitated, through the formation of hydroxyl ions at the cathode.

According to Buchner, a form of cadmium hydroxide that gives yellow cadmium sulphide with sodium sulphide is precipitated by adding sodium hydroxide to solutions of cadmium salts, and a form giving red cadmium sulphide with sodium sulphide by either performing this precipitation in the heat or adding the cadmium salt solution to the sodium hydroxide.

A solution of ammonia precipitates cadmium hydroxide from solutions of cadmium salts, but the precipitate is soluble in excess of the reagent. This solution probably contains the complex group Cd(NH₄)₄, which is less stable than the corresponding group containing zinc. Cadmium hydroxide can be precipitated in the amorphous form from its solution in ammonia by potassium hydroxide.

Flattened hexagonal crystals of cadmium hydroxide, of density 4.79 at 15° C., have been obtained by heating a solution of cadmium iodide with excess of potassium hydroxide till the precipitated hydroxide dissolved, and cooling. It has also been obtained crystalline by leaving cadmium and iron in contact under a solution of ammonia.

Cadmium hydroxide is very slightly soluble in water but more soluble in solutions of some salts - including ammonium chloride. It is less basic than zinc hydroxide.

[Cd(OH)₂]+2HCl.Aq. = CdCl₂.Aq.+20.29 Cal.
[Cd(OH)₂]+2HBr.Aq. = CdBr₂. Aq. +21.56 Cal.
[Cd(OH)₂]+2HI.Aq. = CdI₂. Aq. +24.21 Cal.

The corresponding figure for HF is 25.6 Cal.

An unstable potassium cadmate may be formed when cadmium oxide is dissolved in molten caustic potash and the melt treated with aqueous potash.

An amorphous basic chloride, Cd(OH)Cl, has been obtained by adding dilute ammonia to a boiling concentrated solution of cadmium chloride.