Cadmium Carbonate, CdCO₃

Solutions of cadmium salts are less liable than those of zinc to give basic carbonates when precipitated with alkali carbonate solutions.

The normal anhydrous carbonate, CdCO₃, mixed with some of the basic sulphate, 2CdO.SO₃, is precipitated by adding cadmium sulphate solution to dissolved sodium carbonate, and it has been prepared by adding a large excess of concentrated ammonium carbonate solution to dissolved cadmium chloride. A pure product was secured by dissolving the precipitated carbonate in hydrochloric acid and re-precipitating - repeating three times.

Rhombohedra of CdCO₃, similar to those of calcite, were obtained by heating the white amorphous carbonate with water and ammonium chloride in a tube at 150°-180° C. Similar crystals, of density 4.960, were formed by adding excess of ammonium carbonate to a solution of cadmium chloride, dissolving the precipitate in just enough ammonia solution and heating on the water-bath.

Cadmium carbonate is only slightly soluble in water and is not very soluble in carbonated water, even when the carbon dioxide is forced in under pressure. It is more or less soluble in solutions of ammonium salts, and slightly in dissolved alkaline carbonates. The precipitation of cadmium carbonate and its ignition to oxide provides a method for estimating cadmium.

The mineral otavite, crystallised in minute curved octahedra, is a basic cadmium carbonate containing 61.5 per cent, of cadmium.