Sulphur

Physical Properties.

Commercial sulphur melts at 113° C if quickly heated and boils at C under atmospheric pres sure. The orange-yellow vapour darkens with increase of tempera ture, being deep red at 50o°, but above that it becomes lighter again and is straw-yellow at 65o°. At 200° (under reduced pres sure) the vapour density corresponds to 58 molecules; at 500° to at about I,000° to S2 and at about 2,000° the molecules are completely dissociated to atoms. In carbon disulphide solution sulphur exists as 58, as shown by boiling-point and freezing-point determinations. Sulphur is a bad conductor of electricity and be comes negatively electrified by friction. It is insoluble in water, it ignites in air at 363° and in oxygen at 280°, burning with a characteristic blue flame and forming sulphur dioxide, a gas with a choking smell, and traces of the trioxide. Sulphur also combines directly with most elements to form sulphides.

Allotropic Modifications.

Sulphur can assume a number of forms differing in external appearance, e.g., crystal structure, and in physical properties, e.g., density. Of these the two most im portant are rhombic (a, Sc, or ) and monoclinic ((, Ss, or since they provided the first instances of what E. Mitscher lich (1822) termed "polymorphism," but is now called "allotropy" (q.v.). Rhombic sulphur is readily obtained by allowing a so lution of sulphur in carbon disulphide to crystallise. The mono clinic variety is formed as fine needles on the inside of the crust on molten sulphur which has been allowed to cool slightly. Both these varieties are soluble in carbon disulphide and sulphur chlo ride; above 96° S« is more or less rapidly converted into Ss, whereas below 96° S$ changes gradually to Sa . If S« is heated quickly it melts at 114° before it has had time to change into Ss, but if it is heated slowly the change is completed and the Sp formed then melts at 119°. The density of S« is 2.07 and that of Sft 1.96. Under higher pressures the two forms can only co-exist at a definite temperature for any particular pressure (i.e., the transition point is raised), and if the pressure is raised while the temperature is kept constant the rhombic form is pro duced, since it is the denser. This balanced state of affairs con tinues up to 151° and 1,288 atmospheres, above which the mono clinic form cannot exist at all.

The fact that, by rapid heating, the rhombic variety persists up to (under ordinary pressure) without changing into the monoclinic is due to this change being slow and not instantaneous.

Although the foregoing account covers the more obvious phe nomena, the relationships are, in fact, extremely complicated, and at least three varieties of sulphur (called Si,, S.-, and SA) are involved. The first, Sx, is almost certainly and constitutes the greater part of both the rhombic and monoclinic varieties, but the other two (possibly S4, or form solid solutions with Sx, and all three are involved in very complex equilibria in both the liquid and the solids. Owing to the slowness with which

these equilibria are established, the physical conditions may be somewhat indefinite; thus pure Sx is calculated to have an "ideal" freezing point of 119°, but the presence of S, and depresses this to a varying extent (but never below according to the rate of cooling, i.e., according to whether the time has sufficed for them to attain their equilibrium values.

Other forms differing in external appearance are the nacreous (or pearly) variety (S111), formed when solutions of sodium thiosulphate and potassium bisulphate interact, or when hydrogen disulphide is shaken with alcohol or ether, and a tabular variety (S iv), formed in the aerial oxidation of alcoholic solutions of ammonium polysulphides. Both these are monosymmetric but have different angles from ; they change rapidly to at the ordinary temperature, or to above 96°, but S111 may be melted at 107° if rapidly heated.

Engel's variety ) is obtained by mixing sodium thiosulphate solution with hydrochloric acid, filtering, extracting the filtrate with chloroform, and evaporating the extract. It appears to be but its connection with the other forms has not been estab lished. It is intermediate in colour between Sx and S, (i.e., and and slowly changes into a mixture of these.

Apart from varieties possessing obvious external differences, more subtle differences have been detected. When sulphur melts the liquid is pale yellow, but on further heating it darkens, and at 160° C it rapidly becomes more viscous. At 180° the liquid is dark red and the viscosity attains a maximum. This behaviour has been traced to the existence of three different types of mole cules in the liquid—Si, Sr and ; if the molten liquid is poured into water, one obtains clear and elastic strings of plastic sulphur. From 5 to 20% of this (depending upon the temperature and time of heating of the liquid) is found to be insoluble in carbon disulphide and is S, and the rest is a mixture of Sx and ST. The material becomes opaque and brittle after a few days owing to the conversion of most of the S, and some of the S. into crystalline S x or rhombic sulphur; the proportion of S, and Sx can be ascertained by taking advantage of the fact that at —8o° C the latter becomes insoluble in carbon disulphide whereas the former is still soluble. The proportion of S, in sulphur increases to about 6% at 2oo° from about 3.5% at and it has been shown to be tetratomic, i.e., S4. Owing to its insolubility, the molecular weight of has not been determined ; it is the chief constituent of the so-called amorphous sulphur (Sy) and of milk of sulphur, and because of its increasing production at higher temperatures it occurs in greater proportion in sulphur which has been rapidly condensed, as in flowers of sulphur.