Amorphous Sulphur

"Insoluble Amorphous Sulphur" is produced, usually together with some sulphur soluble in carbon disulphide, by many reactions in which sulphur is set free. The slow decomposition of sulphur chloride or bromide by water, which may require several days for completion, gives a very stable form of insoluble sulphur. The action of mineral acids on solutions of thiosulphates and polythionates, the oxidation of hydrogen sulphide by sulphurous acid or concentrated sulphuric acid, the oxidation of hydrogen sulphide or metallic sulphides by nitric acid or halogens, and the incomplete combustion of sulphur compounds such as hydrogen sulphide or carbon disulphide, all yield some of this sparingly soluble sulphur. It is interesting to note that all these reactions occur in the presence of free acid which, as has been already mentioned, is favourable to the existence of μ-sulphur.

In all probability the primary product when sulphur is precipitated by chemical action consists entirely of the insoluble γ-sulphur, but, as soon as formed, this begins to change rapidly into the more stable soluble crystalline form in a fine state of division. The presence of halogens retards this change, so that a concentrated solution of bromine or iodine, acting on a solution of a sulphide or polysulphide, will give a product containing a higher proportion of insoluble sulphur than will a more dilute solution. Hydrogen ions also exert a retarding effect, so that whereas the interaction of acetic acid or dilute hydrochloric acid with a solution of a thiosulphate or polysulphide yields a product entirely soluble in carbon disulphide, the precipitate obtained with concentrated hydrochloric acid contains a considerable proportion of insoluble y-sulphur.

"Flowers of sulphur", when freshly prepared, commonly contains about 30 per cent, of insoluble sulphur, but this percentage may vary considerably. However, if sulphur vapour is condensed on the surface of cold water or especially on the surface of a cold aqueous solution of a mineral acid such as sulphuric acid, the proportion of the insoluble amorphous modification in the deposit may be so high as to render it the main constituent.

The insoluble sulphur obtained by these various methods is somewhat inconstant in physical properties, but all the varieties are included under the designation γ-sulphur. They are generally loose amorphous powders of pale yellow colour, the density ranging from 1.85 to 2.05. They all have a tendency to change spontaneously into soluble crystalline sulphur, the change at 110° C. being accompanied by a slight evolution of heat. Contact with a cold aqueous solution of hydrogen sulphide or of an alkali sulphide has a marked accelerating effect on the change, as also has compression, or heating, either alone or with alcohol or an animal or vegetable oil; if the y-sulphur is heated until it becomes fluid, the phenomena observed will be those already described for liquid sulphur. The products from " plastic sulphur " and from sodium thiosulphate are amongst the least stable forms of γ-sulphur whilst those from the sulphur halides are relatively very stable. By treatment with an aqueous solution of a mineral acid such as sulphuric or sulphurous acid, the stability of the products may be increased.

That under suitable conditions γ-sulphur can retain its individuality for a considerable period is clearly demonstrated by the fact that "insoluble sulphur" has been found in a specimen of "flowers of sulphur" prepared more than fifty years previously. Indeed, the presence of "insoluble sulphur" serves as a trustworthy characteristic of genuine "flowers of sulphur" by which the fraudulent substitution of powdered sulphur may easily be detected.

When prepared by chemical methods, γ-sulphur is frequently accompanied by an apparently amorphous powder which is readily soluble in carbon disulphide. This has been regarded, by some investigators, as a definite form of sulphur and given the name "soluble amorphous sulphur"; in reality, however, it consists of minute spheroidal crystals of rhombic sulphur possibly together with nacreous; sulphur.3 Another so-called modification of amorphous sulphur, described as soluble in carbon disulphide but becoming insoluble on evaporation of the solvent, is probably no distinct form, but only a mixture of γ-sulphur with finely divided crystalline sulphur.

It is probable that the inconstancy of the results obtained by various investigators for the specific gravity and other characteristics of γ-sulphur is due to contamination of the substance examined with other forms of sulphur or with foreign substances. Such a probability is increased by the description of a hydrated form of amorphous sulphur the composition of which points to its being a definite hydrate, S₈.H₂O; this compound is stated to be obtainable by the interaction of hydrogen sulphide and sulphurous acid in aqueous solution.

"Milk of Sulphur" or "Precipitated Sulphur" of commerce, which is prepared by precipitating with acid a solution of calcium polysulphide (obtained by heating an aqueous emulsion of slaked lime with finely divided sulphur) or of "liver of sulphur" (obtained by fusing together potassium carbonate and sulphur), is also a mixture of finely divided rhombic sulphur with the insoluble amorphous y-sulphur. It owes the paleness of its colour and its especial suitability for internal medicinal application to its very fine state of division. γ-Sulphur in reactivity is very little different from ordinary crystalline sulphur.

π-Sulphur, a deep yellow amorphous form of sulphur soluble in (carbon disulphide, has already been described.

Other forms of amorphous sulphur, which have been described at various times in chemical literature, are the so-called "blue sulphur" and "black sulphur"; our present state of knowledge of these is far from satisfactory, and their existence as definite modifications of pure sulphur is questionable, especially in the case of the latter variety, the colour of which appears to be due to small quantities of carbon or of metallic sulphides.

When a concentrated solution of ferric chloride is rapidly mixed with fifty to one hundred times its volume of aqueous hydrogen sulphide solution the liquid assumes a transient blue colour, sulphur subsequently precipitating in the ordinary yellowish-white form. Sulphur with a blue coloration is also obtained in the interaction of carbon disulphide and sulphur chloride, for the preparation of carbon tetrachloride, under the catalytic influence of ferric chloride,

CS₂ + 2S₂Cl₂ = CCl₄ + 6S.

By submitting various metallic sulphides, e.g. those of bismuth, silver, cadmium or zinc, to the action of a solution of sulphur chloride in benzene or toluene, a greenish-blue precipitate of sulphur is obtainable, but the product invariably contains several units per cent, of mineral impurity. The suggestion that Ultramarine owes its colour to the presence of a blue variety of sulphur appears to have little probability, especially in view of the stability of this substance towards heat, and indeed the true nature of the blue- or green-coloured precipitates of sulphur, obtained by any of the afore-mentioned methods, requires much more experimental investigation before the existence of a blue or green modification of sulphur can be accepted.

Sulphur, however, dissolves in certain organic liquids, for example, hot glycerol or ethylene glycol, and in pyrosulphuric acid, yielding clear blue solutions. Cryoscopic measurements with solutions in the latter solvent indicate that the sulphur molecules are diatomic, and it is to be assumed that this is also the case with the blue solutions in organic solvents. Thus, in such solutions the sulphur is more highly dispersed than in most organic solvents, in which the molecules are octatomic.

The existence of a black form of sulphur was first suggested by Mitseherlich in 1856, who observed that in the presence of very slight traces of organic impurities, plastic sulphur could be obtained with a very deep or even black colour; the necessity for the presence of foreign matter, however, greatly discredits any claim of this variety to be a distinct allotropic form.

Sulphur thus presents a most remarkable example of polymorphism. Indeed, on account of the complexity of its behaviour in this direction, much further investigation will be required before any attempt at a full and general interpretation of the phenomena will be possible.