Physical Properties of Sulphur

Physical Properties of Sulphur in solid state does not conduct electricity appreciably and so forms a convenient insulating material for some purposes, although it is more frequently used only as an ingredient of insulating compounds or mixtures. When rubbed, sulphur becomes negatively charged, although on exposure to radium radiation it becomes charged positively. Molten sulphur conducts electricity appreciably, although impurities may contribute in part to this characteristic behaviour.5 The conductivity of the liquid increases to a maximum at about 160° C., falls to a minimum at about 185° C., and thereafter increases. The specific resistance at 163° C. is approximately 7.5×10¹⁰ ohms. The dielectric constant of solid sulphur is approximately 2.7, although variation occurs according to the axis along which the measurement is made. For the liquid, supercooled between 150° and 95° C., the Clausius-Mossotti law is followed. Sulphur is diamagnetic.

On account of its low heat conductivity solid sulphur breaks readily when warmed, the uneven expansion caused by the warmth of the hand being sufficient to cause an audible sound of cracking. The following values for the thermal conductivity of sulphur over the temperature range 20° to 210° C. have been obtained:

Form.	Temperature, ° C	Thermal Conductivity.
Rhombic	20	0.00065
Rhombic	40	0.00061
Rhombic	60	0.00058
Rhombic	80	0.00055
Rhombic	95 transition point	0.00054
Monoclinic	100	0.00037 to 0.00040
Plastic	20	0.0002
Liquid	115 (m.pt.)	0.00031
Liquid	120	0.00031
Liquid	140	0.00032
Liquid	160 - Transition point region	0.00033
Liquid	165 - Transition point region	0.00033
Liquid	170 - Transition point region	0.00034
Liquid	190	0.00036
Liquid	210	0.00037

The latent heat of fusion of solid sulphur varies with the different allotropic forms and with the temperature; it is lowest for octahedral sulphur; it is also influenced by the proportion of insoluble sulphur (γ- or μ-) in the original solid and in the liquid formed. By electrical heating at the melting-point, the latent heat of fusion of monoclinic sulphur has been found to be 8.85 gm.-cals. per gm.

The latent heat of evaporation of sulphur (within an estimated accuracy of 2 per cent.) is 79.

Physical Properties of Sulphur in molten condition related with that on crystallisation is of a complex nature and dependent largely on the previous temperatures, the rate of cooling and the temperature at which crystallisation begins. Occasionally crystallisation occurs in a rhythmic manner, giving rise to an annular appearance in the structure of the solid mass. The crystallisation of undercooled molten sulphur is stated to be accelerated by the action of radium radiation, probably by the β-rays.

In the molten condition the density of sulphur is dependent only on the temperature, provided that the equilibrium condition between λ-sulphur and μ-sulphur has been attained; at 113° C. the density is 1.811, the value then rising somewhat up to about 160° C., then steadily falling to 1.480 at 446° C. The coefficient of expansion has been measured by several investigators, but on account of the complications introduced by the gradual readjustment of the equilibrium at new temperatures, the results are not altogether concordant, although they supply confirmatory evidence of the existence of the equilibrium. The mean specific heat of mobile sulphur is 0.220 and of viscous sulphur 0.290.

The remarkable variations in the viscosity of molten sulphur have already been mentioned. Definite measurements have been made at a series of temperatures by the method of rotating cylinders, and it has been found that exposure of the liquid to air, especially below 160° C., has a marked effect on the viscosity from 160° C. onwards. The viscosity of purified (twice distilled but not gas-free) sulphur has a value at 123° C. of 0.1094 C.G.S. units; this falls to a minimum of 0.0709 at 150° C., rises gradually up to about 159° C., then rapidly increases above this temperature; an exact transition point is not observed, however. The maximum for purified unexposed (gas-free) sulphur occurs at about 200° C. and has a value of 215 C.G.S. units. For purified (not gas-free) sulphur after prolonged exposure to the air, the maximum occurs at about 190° C. and may have a value as high as 800 C.G.S. units. Such high viscosity appears to be due to impurities, the chief being sulphuric acid, resulting from exposure to the air; sulphur dioxide and ammonia in solution also have an appreciable effect.

The surface tension of liquid sulphur has been determined for different temperatures by several workers, whose results, however, are not in harmony. Kellas, in disagreement with the data of earlier workers, maintains that the surface tension of sulphur falls continuously from the melting-point to the boiling-point, and gives the following values:

119.4° C	60.46 dynes per cm.
156 ° C	56.38 dynes per cm.
280 °C	48.2 dynes per cm.
445 ° C	39.4 dynes per cm.

Although sulphur exerts an appreciable vapour pressure at the ordinary and slightly elevated temperatures, the boiling-point is not reached until 444.60° C. (at 760 mm.). This temperature is a definite constant and enables sulphur to be used as a solvent for the ebullioscopic determination of molecular weights, the following "molecular formulae" having been determined experimentally for the respective elements dissolved in sulphur: Se_2.4, Te_1.3, As_1.0, and Sb_1.2. The boiling-point (£) at various pressures may be obtained from the equation:

t = 444.60 + 0.0910(p-760) – 0.000049(p-760).

The refractive index of liquid sulphur has been investigated and is found to decrease up to 160° C. and then increase above that temperature. With regard to the spectrum of sulphur, the most persistent lines in the emission spectra are of the following wave-lengths (Å): 1807.4, 1820.5, 1826.4, 4694.2, 4695.5, 4696.3, 9212.8, 9228.2, 9237.7.

On vaporising sulphur in a cooled hydrogen flame, or on passing feeble electric sparks through the vapour contained in a Geissler tube, a band spectrum is obtained consisting of a series of bands sharp on the violet side but shading off towards the red and extending right across the visible region.

The changes in the absorption spectrum of sulphur vapour over the range 400° to 1200° C. have already been mentioned. In the ultra-violet region the spectrum has been investigated over the range 100° to 1000° C. under low pressures (0.5 to 53 mm.) using a continuous spark under water as a background. Below 250° C. continuous absorption occurs between 2700 and 2300 Å, but at higher temperatures, as S₂ molecules appear, and under 0.5 mm. pressure, a band spectrum between 2927 and 2713 Å appears, which, with rise in temperature, continues to extend until it covers the region 3700 to 2475 Å. It is then composed of three distinct sets of bands, viz.:

1. 3700 to 2794 Å, bands of fine structure;
2. 2794 to 2592 Å, narrow bands without fine structure;
3. 2592 to 2475 Å, broad continuous bands.

There is a band of maximum absorption at 2750 Å.

The fluorescence spectrum of sulphur vapour under excitation by light from a mercury arc gives a series of bands extending over the visible region.

The absorption of light by thin layers of sulphur (0.3 mm.) at 0° C. is continuous from the ultra-violet to 4080 A, and extends a further 20 A towards the red for every 10° rise in temperature up to 300° C., no discontinuity, such as might be expected, being observed in the neighbourhood of 160° C.

The absorption of ultra-violet light by colloidal solutions of sulphur has been examined, and is found to vary with the size of the colloid particle, approaching a limit corresponding with the amount absorbed by a molecular solution of sulphur in alcohol.