Crystalline Sulphur

The power of sulphur to crystallise in different forms was first recognised in 1823 by Mitscherlich, who described the rhombic and prismatic varieties. The classification of the chief crystalline forms was completed by Muthmann, who distinguished them by Roman numerals.

This is the form commonly occurring in nature. On account of the presence of a small but variable quantity of the μ-modification of liquid sulphur and also S_π in molten sulphur, the freezing-point of the latter is lacking in constancy and is dependent on the previous history of the liquid; crystallisation of molten sulphur generally gives rise to sulphur II. If, however, sulphur (preferably recrystallised from carbon disulphide and so containing a reduced proportion of γ- or μ-sulphur) is melted in a flask fitted with a cork carrying a zigzag glass tube, the contents of the flask may be cooled rapidly to 95° C. and then slowly to 90° C. before spontaneous crystallisation begins. By inverting the flask before solidification is complete, the octahedral crystals (sulphur I.) which form at the bottom of the flask may be examined, whilst the remainder of the sulphur is retained in the neck of the flask by the plug of solid sulphur formed in the bent outlet tube. Undercooled liquid sulphur can also be made to separate in the octahedral form by the addition of a crystal of this type, but the result is rendered more certain by working at a temperature below 95.5° C.

Crystallisation of sulphur at the ordinary temperature from solution, e.g. from carbon disulphide, yields octahedral crystals which differ from the crystals obtained by the solidification of molten sulphur in that the proportion of γ- and π-sulphur, present in solid solution, is less. Octahedral crystals of sulphur may be prepared at the ordinary temperature also by the gradual atmospheric oxidation of a solution of hydrogen sulphide in pyridine or by the much slower process of sublimation.

The foregoing crystals belong to the rhombic system, the elements a:b:c = 0.8138:l:1.9076 being subject to slight variation. This slight inconstancy is doubtless due in part to the presence of a small variable percentage of γ- and π-sulphur in the crystals, this impurity also accounting for the various figures which have been given at different times for the density, the correct value at the ordinary temperature for pure octahedral sulphur probably lying between 2.03 and 2.06.

The melting-point of octahedral sulphur is influenced in a similar manner, and indeed the effect of these almost permanent impurities in the crystalline forms of sulphur has added to the difficulties of investigation. In the following table are given the ideal temperatures for the change of state of the various forms, together with the temperatures as commonly observed with the forms as usually obtained; the ideal temperatures can be approached only with specimens prepared with extreme care.

	Ideal Temperature, ° C	Natural Temperature, ° C	Percentage of "Impurity" present in latter case.
S. I.	Freezing-point 112.8	110.2	3.4
S. II.	119.2	114.5	3.6
S. III.	106.8	103.8	3.1
S. I. ⇔ S. II.	Transformation 95.3	95.5	3.1

Octahedral sulphur is a brittle solid of hardness approximately 2.3; the colour is lemon-yellow at the ordinary temperature but darkens somewhat on warming, whilst at -50° C. it almost disappears, leaving the solid practically colourless; the refractive index for sodium light is 2.08, the mean specific heat is 0.176, and the coefficient of cubic expansion approximately 0.00022 at 20° C. The plasticity and deform- ability are not appreciably increased by heating at temperatures up to 280° C. under pressures of 1000 to 19,600 kgm. per sq. cm.; under 19,300 kgm. per sq. cm., octahedral sulphur melts at 263° C.

When a sphere cut from a crystal of rhombic sulphur is allowed to vaporise at 100° C., plane surfaces develop which correspond to the most important crystal faces.

Carbon disulphide is an excellent solvent for rhombic sulphur, the proportions soluble in 100 parts by weight of the solvent at different temperatures being given in the following table; the highest temperature is the boiling-point of the saturated solution under ordinary atmospheric pressure.

Temperature, ° C	Sulphur Dissolved (Parts by Weight).	Temperature, ° C	Sulphur Dissolved (Parts by Weight).
-11	16.5	38	94.6
0	24.0	48.5	146.2
15	37.1	55	181.3
22	46.0

Methylene iodide, aniline, and benzyl chloride, especially when warm, are good solvents for sulphur; phenol is also fairly good. Other less powerful solvents are benzene, toluene, turpentine, chloroform, ether, alcohol and acetic acid.

Octahedral sulphur will form mixed crystals with selenium containing up to 35 per cent, of the latter, although no corresponding crystalline form of pure selenium has been isolated.

This is a variety of monosymmetric crystalline sulphur obtained ordinarily when sulphur crystallises at a temperature near the melting-point from the molten condition. The usual procedure in order to obtain distinct crystals is to allow molten sulphur to cool undisturbed until approximately one-half has crystallised, and then, after breaking the crust, to pour away the remaining liquid. Crystallisation by the rapid cooling of hot solutions (see the following) sometimes also yields this modification, although more frequently the variety obtained is S. III.

The crystals consist of long slender needles belonging to the mono- clinic system, the parameters being a:b:c = 0.99575:1:0.99983; β = 84.23°. When prepared from molten sulphur this variety frequently possesses a pale amber colour, which is probably due in part to π-sulphur present as impurity, since crystals obtained from solution are of a much paler colour. The density, 1.957 at 25° C., is lower than that of the octahedral form, whilst the specific heat is higher. As is the case for sulphur I., the presence of variable proportions of π- and γ-sulphur in the crystals affects the constancy of the melting-point, so that whereas as usually prepared the melting-point is 114.5° C., the melting-point of pure monoclinic sulphur should be 119.2° C..

At the ordinary temperature the clear transparent crystals become opaque and friable, the change occupying from several hours to several days, and the alteration being due to a transformation into sulphur I.; if the needle crystals are not crushed they will retain their external form, although they are built up of minute octahedral crystals of the denser modification. Conversely, if octahedral sulphur crystals are maintained at a temperature within a few degrees of the melting-point, they gradually undergo conversion into sulphur II.

The transition point, at which ordinary sulphur I. and sulphur II. are in equilibrium, is at 95.5° C., although if no trace of π- or γ-sulphur were present the temperature would be slightly lower. The afore-mentioned changes can occur a few degrees above or below the stated temperature and are considerably facilitated by contact with a little of the stable form; below 95.5° C. sulphur I. is the stable modification and sulphur II. is unstable, whilst above 95.5° C. and up to the point of fusion, the position is reversed. It is possible for each of the forms to exist for a considerable period within its unstable range of temperature, and it is obvious that but for this fact it would be impossible to determine the melting-point of octahedral sulphur. Specimens of sulphur II. may be kept unchanged for years between a microscopic slide and a cover glass. The actual velocity of the transformation is dependent on several factors, such as the temperature, the previous history of the sulphur, or in other words its content of γ- and π-sulphur, subjection to mechanical stress, and contact with the stable form or with a solvent; actinic light is said also to have an acceleratory influence on the transformation.

Alteration of pressure affects the transformation in a more marked manner by actually influencing the transition temperature itself. Increase in pressure causes a rise in the transition temperature to an extent of approximately 0.05° C. per atmosphere. However, increase in pressure also raises the melting-point of each form, and the relation between the variations of the three temperatures with pressure is such that under 1288 atmospheres the transition temperature and the melting-points of the two forms coincide at 151° C.

Relationship between Sulphur I; and Sulphur II;

These relationships are represented in an exaggerated manner in fig. The lines AO, BC and OD represent the vapour pressures of sulphur I., sulphur II. and molten sulphur, respectively; OE and CF show the effect of pressure change on the melting-points of the two crystalline forms, whilst the effect of pressure on the transition temperature is indicated by BF.

Determination of the molecular weight by cryoscopic and ebullioscopic measurements in various solvents, including carbon disulphide, various hydrocarbons, and metallic chlorides such as antimonic or stannic chloride, indicates the value corresponding to S₈ both for octahedral and prismatic sulphur. This is not surprising, because in the molten condition these two modifications of sulphur are identical, and their difference in crystalline form and other physical properties may be attributed to variation in the manner in which the octa-atomic molecules are grouped together in the crystal.

As is to be expected with modifications showing the mutual relationship described, the less stable form is more soluble in ordinary solvents; above 95.5° C., therefore, the octahedral form is the more soluble, whilst below this temperafure prismatic sulphur possesses the greater solubility. At the ordinary temperature the solubility ratio for most solvents; is approximately 1.3:1.

At least two other varieties of monoclinic sulphur are known to exist; the better known was discovered by D. Gernez in 1884, who, from the pearly lustre of the crystals, gave it the name "nacreous sulphur." This monosymmetric form of sulphur can be obtained by many different methods. Sulphur which has been heated in a glass tube to 150° C. and subsequently cooled slowly in a water-bath to 98° C. without crystallisation, can, by applying local cooling or by rubbing the interior of the tube with a glass rod already in the liquid, be made to give crystals of this modification. Hot concentrated solutions of sulphur in carbon disulphide, benzene, toluene, turpentine and other solvents, prepared by heating sulphur with the solvent, preferably in a sealed tube, until no solid remains undissolved, generally deposit nacreous crystals on being rapidly cooled. Similar crystals can also be obtained by the precipitation of sulphur from carbon disulphide solution by the addition of ether, and also by subliming sulphur, e.g. from one watch glass into another acting as cover. Pure gas-free sulphur prepared by distillation first in a stream of carbon dioxide and then in high vacuum yields only nacreous sulphur on solidifying.

Under suitable conditions, certain chemical reactions will give rise to nacreous sulphur; the most satisfactory result is obtained by allowing slow inter-diffusion of solutions of sodium thiosulphate and potassium hydrogen sulphate to occur. Another method involves the gradual decomposition of sulphur chloride or bromide by the vapour of water or methyl alcohol at the ordinary temperature. The decomposition of calcium polysulphides by hydrochloric acid, and of hydrogen per-sulphide by the addition of alcohol, ether, ethyl acetate or other organic solvents, also yields sulphur of the desired modification.

Nacreous sulphur forms clear, pale yellow (almost colourless), doubly refractive leaflets of the monoclinic class, a:b:c = 1.06094:1:0.70944; β = 88.2°. The degree of pearly lustre is largely dependent on the method of preparation. This form of sulphur is unstable and tends to change more or less rapidly, without alteration in external form, into sulphur I., i.e. octahedral sulphur. The natural melting-point is 103.8° to 103.9° C., but, on account of its tendency at this temperature to undergo conversion into sulphur II. (ordinary monoclinic sulphur), nacreous sulphur, when heated slowly, may give a melting-point approaching that of sulphur II. In any case the crystals contain small percentages of γ- and π-sulphur; the ideal melting-point for pure nacreous sulphur would approximate to 106.8° C.

Although no crystalline variety of selenium is known corresponding to nacreous sulphur, this modification of sulphur can form mixed crystals containing between 35 and 66 per cent, of selenium.

This is another variety of monoclinic or monosymmetric sulphur which is exceedingly unstable and has, in consequence, been little investigated. When the clear liquid obtained by saturating an alcohol solution of ammonium or sodium sulphide with sulphur is diluted with four times its bulk of alcohol and then allowed to stand at the ordinary temperature, nacreous sulphur (sulphur III.) separates. At temperatures below 14° C., however, e.g. at 5° C., the deposit consists of a mixture of nacreous sulphur with the fourth crystalline form, or may even consist entirely of the latter.

In addition to the foregoing crystalline modifications of sulphur, at least two others have been observed. When using boiling sulphur as the heating agent in the outer jacket of a Victor Meyer vapour density apparatus, Friedei, in 1879, noticed the formation of very unstable triclinic crystals inside the upper part of the jacket; these crystals rapidly underwent change into ordinary octahedral sulphur. Various crystalline stuctures may be observed when sulphur condenses in droplets on a glass plate and solidifies. Such crystals are also unstable.

By extracting with chloroform the aqueous solution obtained by mixing concentrated solutions of sodium thiosulphate and hydrochloric acid at 10° C., a solution is obtained, which, after evaporating off the solvent chloroform, leaves rhombohedral orange-yellow crystals of sulphur. This form of sulphur is commonly known as Engel's rhombohedral sulphur, or S_φ. The crystals are usually prisms of specific gravity 2.435. Although it corresponds with the rhombohedral forms of selenium and tellurium, rhombohedral sulphur is very unstable and, in the course of a few hours, changes into a mixture of octahedral and insoluble sulphur. By crystallising sulphur from chloroform solution containing rubber as a thickening agent and a few drops of benzonitrile, two other forms, also claimed to be distinct allotropes, have been obtained, designated respectively ζ- and η-sulphur. Both forms are practically colourless and the crystals are doubly refracting; the former crystallises in rhombic plates and the latter in hexagonal plates.