Barium Sulphate, BaSO₄

Barium Sulphate, BaSO₄, occurs naturally as the mineral barytes or heavy spar. It was the first known of the compounds of barium- It crystallises in the rhombic system and is usually white and opaque, but it sometimes forms transparent crystals which are colourless, or have brownish, greenish, or bluish shades. The finest specimens come from the Auvergne, Hungary, Bohemia, and Cumberland. A monoclinic form of the naturally occurring compound found in Quebec has been described. The mineral barytocelestine is an isomorphous mixture of barium and strontium sulphates. The water of a chalk well in Harrow has been found to contain barium sulphate.

Barium sulphate may be prepared by the precipitation of a soluble barium salt with a soluble sulphate or sulphuric acid. There is no action if pure sulphuric acid be placed in contact with anhydrous barium oxide, a little water being necessary. Spring obtained it by compressing a mixture of sodium sulphate and barium carbonate under 6000 atmospheres.

The precipitated sulphate is amorphous, or composed of crystals of microscopic dimensions. Larger crystals may, however, be obtained by very slow precipitation in very dilute solutions; by heating the precipitate in a sealed tube with hydrochloric acid or sodium bicarbonate at 250° C.; by fusion of potassium sulphate with barium chloride, of barium fluoride with calcium sulphate in presence of potassium chloride and sodium chloride, or of barium sulphate with barium chloride at 1150 ° C., or with sodium sulphate at 1130° C., which is simply crystallisation from a solvent; and, finally, by evaporation of a sulphuric acid solution of barium sulphate at, or a little below, the boiling-point. Manross found the density of the crystallised product to be 4.179. According to von Weimarn, a substance usually precipitated in the crystalline state may be obtained in the colloidal form under conditions which diminish the solubility and thus increase the number of centres of precipitation. Barium sulphate may, therefore, be prepared in a colloidal form by precipitation from solutions containing methyl or ethyl alcohol, glycerol, amyl alcohol, or an alkaline solution of casein. In the latter case crystallisation is probably prevented by the coating of the particles of barium sulphate with casein. Colloidal barium sulphate is also obtained by precipitating a very concentrated solution of barium acetate with aluminium sulphate, or with magnesium or sodium sulphate.

The melting-point of barium sulphate is 1580° C., at which temperature partial decomposition also takes place. The specific heat increases linearly from 0.1137 at 150° C. to 0.1486 at 1050° C.

There is apparently a transition point from β-BaSO₄ to α-BaSO₄ on heating above 1149° C. Strontium and barium sulphates are completely miscible in all proportions in both α- and β-forms; barium and calcium sulphates are isodimorphous, but the miscibility is at most 5 gram-molecules per cent, of calcium sulphate in barium sulphate.

Barium sulphate alone does not fluoresce under the influence of the cathode rays, but will do so if a little bismuth be present.

The effect of reducing agents on barium sulphate has already been described. By fusion with alkali carbonates, or boiling with solutions of the latter, it is transformed into the carbonate. This reaction was Guldberg and Waage's classic example in their deduction of the law of mass action. When heated with ammonium chloride, barium chloride is formed.

The solubility of barium sulphate in water is exceedingly small. From electrical conductivity determinations Kohlrausch found for precipitated barium sulphate 0.0197 milli-equivalents, or 2.30 mgm. per litre at 18° C., and 0-0226 milli-equivalents for the natural barytes. The higher value for the latter may be due to impurities in the mineral. Melcher obtained the following values at different temperatures: -

Temperature, °C.	18	25	50	100
Milli-equivalents per litre	0.0190	0.0212	0.0288	0.0334

By calculation from the change of solubility with temperature the heat of solution is -6.0 Cal. Thomsen, from precipitation data, deduced the value -5.580 Cal.

Higher values for the solubility have been obtained owing to super- saturation. According to Hulett, the solubility depends on the size of the solid particles in equilibrium with the solution. By grinding so finely that the diameter of a particle is less than 0.001 mm., the solubility may reach 4.15 mgm., or 0.0355 milli-equivalents per litre at 25° C., or with barytes 6.18 mgm.

The solubility is reduced by salts with a common ion, including sulphuric acid if dilute. Other salts and acids increase it.

The solubility is increased in 40 per cent, hydrobromic and hydriodic acids, and in metaphosphoric acid solutions. Barium sulphate is very soluble in concentrated sulphuric acid, and also to a considerable extent in hydrogen peroxide. The latter fact has a bearing on the preparation of hydrogen peroxide.

Owing to its insolubility barium sulphate is used in quantitative analysis for the estimation of barium or sulphate, and the conditions necessary for satisfactory precipitation have been studied by numerous investigators. The chief sources of error are naturally the increased solubility of the precipitate due to the presence of other substances in solution, and the adsorption of foreign substances by the precipitated sulphate.

Barium sulphate has the power of precipitating colloidal solutions, for example colloidal gold or arsenious sulphide. The action is mechanical.

Barium sulphate alone, on account of its slight solubility, does not go through the same setting and hardening process as calcium sulphate, but hardening may be effected by mixing finely divided barytes with a 10 per cent, solution of aluminium, iron, or magnesium chloride, which acts by increasing the solubility.

According to Tingle there are three hydrates of barium sulphate: 10BaSO₄.H₂O, separating from a hot neutral solution; 10BaSO₄.2H₂O, from a hot acid solution; and 10BaSO₄.4H₂O, from a cold acid solution.

Barium sulphate is used in the production of permanent white and lithopone, and also for glazing and weighting paper.

Several double salts are known, for example BaSO₄.SbSO₄.6H₂O, 2BaSO₄.3TiSO₄, and BaSO₄.Sn(SO₄)₂.3H₂O. The freezing-point curve of mixtures of barium and sodium sulphates apparently indicates the existence of the compound BaSO₄.6Na₂SO₄, but it has not been isolated, and, from the form of the curve, it is very easily dissociated.