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Cadmium Sulphide, CdS

The precipitation of yellow cadmium sulphide from solutions of cadmium salts by hydrogen sulphide was the first known fact about cadmium. CdS occurs naturally as greenockite, which is a brownish-yellow mineral crystallising in the hemimorphic group of the hexagonal system with a density of 4.9-5.0 and a hardness of 3-3.5.

Yellow amorphous cadmium sulphide has been prepared by electrolysing a solution of an alkali salt between a cathode of copper sulphide and an anode of cadmium. Its complete precipitation by hydrogen sulphide can be effected in neutral or alkaline solutions of cadmium salts, though it may be slightly soluble in ammonium sulphide when freshly precipitated. Since it is distinctly soluble in mineral acids, hydrogen sulphide will not precipitate it completely, or at all, from solutions that contain much acid, especially hydrochloric acid. According to Bruni and Padoa, hydrogen sulphide does not precipitate cadmium sulphide from acid solutions of cadmium sulphate if the surrounding space is under low pressure. Increase of pressure increases the effectiveness of precipitation. It dissolves readily in hot dilute sulphuric acid and in acidified concentrated solutions of alkaline chlorides.

The yellow variety of cadmium sulphide, especially in the nascent state, is (apparently) liable to be polymerised into a vermilion modification by dilute acids, alkalies, etc. This polymerisation frequently occurs during the precipitation of cadmium sulphide, and the red variety passes through the yellow stage when it is dissolved in acids. The yellow variety passes into the red under friction or pressure as well as under the influence of chemical reagents. The crystalline forms of the red variety differ from the hexagonal form of the yellow, and their densities are different: from 3.906 to 4.147 for the yellow, and between 4.476 and 4.513 for the red. The β- or red sulphide precipitates when relatively strongly acid solutions are precipitated by hydrogen sulphide. Precipitated cadmium sulphide is frequently a mixture of the β- and α- or yellow varieties. Prolonged heating converts the β- into the α- form, which becomes darker in colour on heating, and returns to its original yellow on cooling if it has not been oxidised.

A bulky yellow δ-sulphide is precipitated from an ammoniacal solution of cadmium hydroxide by hydrogen sulphide. The same modification is produced by fusing the a-sulphide with sulphur.

According to Egerton and Raleigh, the yellow form is obtained from cold or alkaline solutions and the red from hot or acid solutions. Crushing, they add, converts the yellow permanently into the red form, while heating only effects the change temporarily.

These varieties of cadmium sulphide are important for its use as a pigment. Both the α- and β-varieties make good covering paints. The former is slowly oxidised in light and air, but the colour is very permanent when made into paint. A brilliant yellow pigment can be prepared by electrolysing a sodium chloride solution between a platinum cathode and a cadmium anode and passing in hydrogen sulphide. Cadmium yellow is an expensive pigment that is much prized by artists. The paler shades of commerce are said not to be permanent, though modern manufacturers now claim to supply them reasonably pure and stable.

Great pressure converts a mixture of cadmium and sulphur powders into cadmium sulphide, which is less brightly yellow than the precipitated product.

Amorphous cadmium sulphide gradually becomes a crystalline mass when it is heated for a long time at 265° C. in an exhausted glass tube, and is easily converted into greenockite by heating in an electric furnace. When hydrogen sulphide acted on cadmium vapour a monoclinic variety of cadmium sulphide was produced simultaneously with the hexagonal.

The crystalline sulphide has been prepared by (a) interaction at a red heat between hydrogen sulphide and vaporised cadmium chloride, (b) fusing cadmium sulphide with potassium carbonate and sulphur, (c) subliming the amorphous sulphide in a current of hydrogen, (d) heating cadmium oxide with barium sulphate and calcium fluoride, (e) heating cadmium oxide in sulphur vapour, (f) heating cadmium sulphide with aluminium powder.

Geitner converted metallic cadmium into a mixture of the amorphous and crystalline sulphides by heating at 200° C. with a solution of sulphurous acid in a closed tube.

The artificial crystals of cadmium sulphide are usually yellow and closely resemble greenockite.

The freshly precipitated δ-sulphide goes slowly into colloidal solution when suspended in water and treated with hydrogen sulphide. Boiling the solution produces no precipitation. A similar solution is obtained by treating a solution of cadmium iodide with hydrogen sulphide. Acids and ammonium chloride immediately precipitate it. Prost obtained a colloidal solution by the former method; it is golden- yellow by transmitted light and fluorescent by reflected. When very dilute solutions of cadmium salts are treated with hydrogen sulphide, colloidal solutions of cadmium sulphide are apt to be produced. A colloidal solution has also been prepared by triturating cadmium sulphide with oil and adding light petroleum.

Boiling water has no action on cadmium sulphide, but, at a red heat, steam converts it into the oxide. When heated it is reduced to metal in hydrogen, and converted into chloride in hydrogen chloride.

It dissociates readily when heated under reduced pressure.

[Cd] + [S] (rhombic) = [CdS] (crystalline)+34.0 Cal.

Yellow cadmium pentasulphide was said to be precipitated from solutions of cadmium salts by potassium pentasulphide.

The luminescence of crystalline zinc sulphide under X-rays is increased by the presence of 1.30 per cent, of cadmium sulphide in solid solution.

Double Sulphides of Cadmium

The double sulphides, Na2S.3CdS and K2S.3CdS, have been described.

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