Calcium Monosulphide, CaS

Calcium sulphide has been found in the natural state in a meteorite fallen in India. It may be prepared by any of the following methods: -

1. The action of sulphuretted hydrogen on calcium carbonate or sulphate at red heat.
2. The action of sulphuretted hydrogen on calcium hydroxide. The reaction begins at 60° C. Anhydrous calcium oxide does not absorb the dry gas at low temperatures, but at red heat gives the sulphide.
3. The reduction of calcium sulphate by carbon, moist hydrogen, water gas, carbon monoxide, or sulphur. This method is the most convenient technically, especially the reduction by carbon.
4. The action of sulphur on calcium carbonate, and on anhydrous calcium oxide at red heat in a current of hydrogen.
5. The action of sodium sulphide on calcium carbonate at red heat.
6. The action of carbon bisulphide on quicklime at high temperatures. The last method produces the purest sulphide.

Pure calcium sulphide is white, but the commercial product is generally coloured yellow, or yellowish red, by impurities in the original materials. It is usually obtained as an amorphous powder of density 2.25, but by fusion in the electric furnace, or by preparation through reduction of the sulphate by carbon in the electric furnace, brilliant cubical crystals of density 2.8 at 15° C. are produced.

The molecular heat of formation from the metal and solid sulphur is 94.3 Cal., from gaseous sulphur 114.82 Cal.

Calcium sulphide is stable in air, and more readily fusible at high temperatures than the corresponding strontium and barium compounds.

It is only slightly soluble in water, 0.212 grm. dissolving in one litre at 20° C., but it is readily hydrolysed with the formation of the more soluble products calcium hydroxide and hydrosulphide. The crystalline sulphide is more readily attacked than the amorphous compound. The molecular heat of solution is 6.3 Cal. The solubility is greatly increased by the presence of sulphuretted hydrogen through the formation of hydrosulphide. At 20° C., and under a pressure of gas of 760 mm., the solubility calculated as sulphide is 206.5 grm. per litre. This affords a means of purifying calcium sulphide by extracting the crude material with sulphuretted hydrogen solution under pressure in absence of air, and then precipitating pure calcium sulphide by removing the gas from the solution at low pressures.

Calcium sulphide is a by-product of the Le Blanc process, and is treated for the recovery of sulphur.

Calcium, sulphide as usually prepared - that is, containing a certain amount of impurity - phosphoresces after exposure to a bright light or some other exciting agent, such as cathode rays. It was used in very early times in Bacchanalian rites, and later became known as Canton's phosphorus. The pure compound does not possess this property, which is apparently associated with the presence of minute quantities of certain heavy metals, notably bismuth, copper, manganese, nickel, vanadium, tungsten, molybdenum, and the rare earths. The nature of the impurity affects the colour of the phosphorescence, and the quantity influences the intensity, an optimum value being found. The presence of a little alkali sulphate or carbonate is apparently also necessary.

According to Waentig, phosphorescence is conditioned by the presence of the heavy metal, or phosphorogen, in the form of a solid solution, the intensity increasing so long as the solution is homogeneous. This would explain the existence of a maximum value with increasing quantity of the heavy metal. The solubility is very small, but increases with temperature, so that the phosphorescent sulphides are supersaturated solutions - temperature, duration of heating, and rate of cooling being important factors in their preparation. The presence of a fusible alkali salt is favourable, because it aids the solution of the phosphorogen and hinders its separation during cooling.

Vanino and Zumbusch made a careful study of the factors influencing the phosphorescing power of calcium sulphide or Bolognian stones. They drew the following conclusions: The most favourable quantity of bismuth for example, as phosphorogen, is of the order of 0.000135 grm. per grm. of sulphide. An alkali salt of low melting- point is better than one of high, and it is possible to use too large a quantity, 2 per cent, of lithium carbonate, for instance, is better than 12 per cent. The physical structure of the lime used for preparing the sulphide (by heating with sulphur) influences the final product. For example, the oxide from the hydroxide or carbonate is better than that from the nitrate - it probably influences the amount of polysulphide. A mixture of the alkaline earths gives a more intense phosphorescence than any one alone. The proportion of sulphur may vary between 12 and 33 per cent., but with more the luminosity is considerably diminished. The texture of the sulphide is important, if hard and stony it is non-luminous. There is no connection between colour photo-sensitiveness and phosphorescing power. Finally, the presence of a reducing agent, for example 4 per cent, starch, is advantageous, although larger quantities may completely destroy the phosphorescence.

Mourelo observed that some specimens of calcium sulphide change colour under the influence of light and that, although the phosphorogen is also the phototrope, this phototropic property is apparently independent of the phosphorescing power of the compound. The change in colour is confined to the surface exposed to light, and no regularity can be observed between the colour assumed and the composition. The intensity of phototropy increases as the percentage of phosphorogen diminishes from 0.1 to 0.0001, but beyond this it decreases and soon disappears.

Lenard and Saeland regard photo-electric action as intimately connected with the phosphorescence of sulphides and as localised in certain centres which are also centres of light emission. Excitation by light or cathode rays consists in the loss of an electron from an atom of the foreign metal, and the resulting phosphorescence is due to the recombination of the metal with electrons. In this connection Perrin's view is interesting. He considers that chemical reaction takes place under the influence of the exciting rays, and that this is reversed in the dark with the emission of energy as phosphorescent radiation.

The electrical conductivity of calcium sulphide is affected by exposure to light, and there appears to be a connection between conductivity and phosphorescence.

Observations on phosphorescence spectra at temperatures down to -259° C. show that the bands become sharper and narrower as the temperature falls, and that different bands belong to different temperature ranges.