Colloidal Sulphur

Colloidal sulphur, sometimes designated δ-sulphur, consists of sulphur in such small particles that these can remain suspended in water as a "colloidal solution." Such a suspension is readily formed by rapidly pouring an alcohol solution of sulphur into water, or similarly treating a solution of sulphur in hydrazine hydrate. Cathodic pulverisation, as applied commonly to the production of colloidal solutions of metallic substances, has also been successfully extended to sulphur.

When hydrogen sulphide is passed into a solution of sulphurous acid some of the sulphur remains in colloidal solution, the amount and degree of dispersion depending on the concentration of the sulphur dioxide, both increasing to a maximum as the latter decreases and then falling rapidly; the degree of dispersion of the sulphur thus decreases with its concentration. The formation of sulphur sol by the action of hydrochloric or sulphuric acid on aqueous solutions of sodium thiosulphate decreases with increasing concentration of the reactants and also decreases somewhat with lowering of temperature. After the formation of sulphur sol by these chemical methods, the mixture should immediately be flocculated by the addition of pure sodium chloride and the precipitate separated by centrifuging the liquid. The sulphur obtained is largely redispersible in water, forming a hydrosol, any un- dispersed portion being removable by sedimentation.

On account of the tendency of the suspended particles to coalesce, colloidal solutions of sulphur are generally short-lived. The stability of the solutions can be increased by the addition of a " protective colloid " such as albumen or gelatine. The action of hydrochloric acid on sodium thiosulphate solution yields a colloidal solution which is more stable if the reagents are used in a concentrated condition, but the life of the unstable colloidal solution obtained with dilute reagents can be extended by the addition of gelatine. Also, by preparing the sulphur in a wet way in the presence of albumen, the precipitate can subsequently be washed, dispersed in water containing a trace of alkali, and the sulphur obtained finally as a greyish-white amorphous substance by dialysis and evaporation of the colloidal solution; the product contains 95 per cent, of sulphur and on dispersion in water gives an opalescent sol.

Sulphur sols may also be prepared by intensively grinding together pure sulphur and grape sugar in an agate mortar and treating with water.

Highly dispersed sols, containing up to 0.082 per cent, of sulphur, may be obtained by passing superheated sulphur vapour, free from air, into air-free water. The sols have an acid reaction due to traces of polythionic acids and hydrogen sulphide. They are white and remain stable for several weeks.

Sulphur sols generally are opalescent, with a yellowish-white to yellow colour; when viewed by transmitted light they commonly appear bluish, but if freshly formed may exhibit successively the colours yellow, green, red, violet and blue. Such colour changes, which depend upon the degree of dispersion of the sulphur, may conveniently be shown by adding a dilute solution of phosphoric acid to N/25 sodium thiosulphate solution. The yellow or brown colour produced by sulphur in sodium-calcium silicate glass is not due to colloidal sulphur, but is probably caused by the formation of polysulphides.

Colloidal sulphur is usually hydrophilic and capable of absorbing water to a considerable degree. The absorption is favoured by the presence in solution of small amounts of acid or salts of univalent metals, but salts of bivalent metals hinder the absorption and cause precipitation. Sols prepared by von Weimarn's method, i.e. by pouring an alcohol solution of sulphur into water, are, however, completely hydrophobic. They are negatively charged and are readily coagulated by electrolytes; alkali salts have a ten to twenty times stronger coagulating action on them than on sols prepared by the interaction of hydrogen sulphide and sulphurous acid, or the decomposition of thionic acids. It is evident that the micelles in the two types of sols are not identical; the sulphur is in a highly polymerised condition and the hydrophilic colloid appears to contain pentathionic acid; this acid can be detected in the filtrate after coagulation. Moreover, hydrophilic sols are produced by reactions which yield both sulphur and pentathionic acid, as in the decomposition of sulphur monochloride by water:

5S₂Cl₂ + 6H₂O = 5S + H₂S₅O₆ + 10HCl.

A sulphur sol containing sulphuric acid and sodium sulphate, which act as stabilisers, has an electrical conductivity lower than that of a similar solution containing no colloidal sulphur; the freezing-point is also higher. If such a colloidal sulphur solution is dialysed, and then to the dialysed solution sulphuric acid and sodium sulphate are added in the amounts originally present, the electrical conductivity of the resulting solution will have the same higher value as a similar solution from which the sol has been removed. It must be assumed therefore that when the sol is first formed the sulphur reacts in some way with the electrolytes, probably causing a change in their physical nature, this resulting in a reduction in electrical conductivity and osmotic pressure. Further quantities of the electrolytes added after the formation of the colloid are not affected by the colloid.

The transformation of maleic acid to fumaric acid is accelerated by the presence of colloidal sulphur.