Thermal analysis of elemental sulphur

255 Thermochimica Acta, 9 (1974) 255-259 .
© Elsevier Scientific Publishing Company, Amsterdam — Printed in Belgium
THERMAL ANALYSIS OF ELEMENTAL SULPHUR
B. R. CURRELL AND A. J. WILLIAMS
The Polytechnic of North London, Holloway, London, N7 8DB (England)
(Received 3 December 1973)
ABSTRACT
Differential scanning calorimetry has been applied to a study of elemental sulphur. Thermal curves have been interpreted on the basis of allotropic conversions and melting points. A method has been developed for the quantitative estimation of S_a and Sp.
INTRODUCTION
In recent years the fall in the price of elemental sulphur has made it econo­mically attractive to consider its use as a construction material. Various applications have been proposed, e.g., road striping, sulphur/aggregate concretes and foams. In many of the proposed applications elemental sulphur embrittlement due to crystalli­zation is one of the main factors inhibiting future development. Very little fundamental information is available on the rates of crystallization and allotropic conversions in elemental sulphur and also about the methods of inhibiting these processes. The objective of this work was to develop, using differential scanning calorimetry (DSC), methods for the quantitative determination of S_z and in samples containing sulphur. These methods are to be used later for the study of the rates of crystallization of sulphur materials.
The only form of sulphur thermodynamically stable at normal temperatures is the orthorhombic (SJ; other main allotropes are monoclinic (S_p) and polymeric sulphur (S_o). S_a and consist of S₈ rings, S_o of long chains up to 10⁶ atoms long. S_z refers to the sulphur melt below 159 °C (the floor temperature of S_o) which consists mainly of S₈ molecules, while S„ refers to the equilibrium mixture of S_a and free S₈ molecules which is obtained from 159 °C.
DISCUSSION AND RESULTS
The differential thermal analysis (DTA) of flowers of suiphur has been inves­tigated by Miller¹, endotherms at 111, 116, 122 and 173 °C were reported. These were assigned to melting point depression of monoclinic (111 ^CQ, fusion of orthorhombic
(Sj) (122°C) and polymerization of the monomer (173°C), while the peak at 116^CC was thought to be “a form of melting point depression of SJ’_ A sample of sulphur rapidly quenched from 200°C to liquid nitrogen temperature (— 196°C) was reported to have the following assigned endotherms: 82°C, the T_g of polymeric sulphur; 108°C, fusion of 118°C, fusion of S_s; and 173°C, polymerization. No information was given as to the composition of the flowers or quenched sulphur in terms of monomer and polymer or any indication that the transition, S₂ —»■ S_p, can be observed by DTA. The thermal analysis of sulphur has now been examined in greater detail; results and conclusions differing in important respects to those of Miller have been reached.

Fig. 1. DTA curves of; (a) single crystal of 6N sulphur (SJ; (b) pure S_#; and (c) microcrystalline S«.

A single crystal of 6N sulphur, i.e., orthorhombic S*, gives (Fig. I a) endotherms at 112^CC (melting of S₌) and 173 °C (formation of polymeric sulphur). No evidence of the enantiotropic transition, —> S_p, was observed for single crystals even when heated under isothermal conditions for I h at 100 °C. Presumably, the presence of a seed is essential for the S_x —► conversion to occur, the endotherm for it (at 100°) is seen in the curve (Fig. 1c) of microcrystalline S_a.
Monoclinic is obtained in a metastable condition by rapidly cooling a sample of 6N sulphur to ambient temperature from the melt at 120 °C. This shows (Fig. lb) endotherms at I19°C (Sp melting) and at ca. 170°C due to formation of polymeric sulphur. Note that this polymerisation endotherm is obtained whatever the allotropic pedigree of the specimen.
On storage at ambient temperature for a number of hours, Sp reverts to the thermodynamically stable form S_a. Figure 1c is the curve of microcrystalline S_a formed by allowing Sp to stand at ambient temperatures. An endotherm at 100 °C is obtained and is assigned to the phase change, S_a -» Sp; the endotherm at 119 °C being due to the fusion of Sp.
The assignment of peaks due to melting was confirmed in each case by visual observation. The assignment of the S_a -* S_fi endotherm depends on: (a) visual obser­vation that it does not involve melting, and (b) the fact that rapid cooling of a melt of the product gives metastable Sp. In addition, assignment of all peaks, both fusion and allotropic transitions, is confirmed by measurement of the relevant AH values from the peak areas; these were found to agree with those available in the literature²’³.

1. 
   fusion of S_a; AH = 66.04 J g^{_ 1};
   
2. 
   fusion of S_fi; AH = 53.42 J g"¹;
   
3. 
   enantiotropic transition, S_a —► S_A; AH = 12.54 J g~ *.
   

The curves for the flowers and CS₂ insoluble sulphur are given in Fig. 2. Three endotherms were obtained (Fig. 2a) for flowers of sulphur, attributed to:

1. 
   S_a -* Sp phase change (100°C);
   
2. 
   Sc, (polymer) melting (104°C);
   
3. 
   Sp melting (108 ^CC).
   

In all of the samples of flowers of sulphur analysed, up to 30% were found to be insoluble in carbon disulphide. This insoluble portion was shown by laser Raman spectroscopy to be polymeric sulphur. Evidence that S_o is melting (endotherm at 104 °C) and not reverting to S_a is afforded from the curves of pure polymer (Fig. 2b). This curve shows an endotherm at 115°C, visual observation of which indicated the sample was melting to give a viscous product, as compared to S_a and Sp, the fusion products of which are not viscous. This evidence does not exclude the possibility that depolymerization and melting are occurring simultaneously. However, the absence of an endotherm, on further heating to 170^cC, suggests that depolymerization to S₈ does not occur readily. Hence, contrary to Miller’s report, the presence of polymeric sulphur in flowers of sulphur and the phase change, S_a —► Sp, are observable by thermal analysis. In addition, the fusion of S_a as observed by DSC is now reported to occur at a lower temperature than the fusion of Sp. Table 1 summarizes the curve data of various allotropic forms of sulphur.
Quantitative allotropic determination of elemental sulphur
The area under the peak representing the transition, S_a —► Sp, may be used to

Temperature (°C)
Fig. 2. DSC curves of; (a) flowers of sulphur; and (b) polymeric sulphur S_e.

TABLE 1
TRANSITION TEMPERATURE OF VARIOUS ALLOTROPES OF SULPHUR, AS INDICATED BY DSC PEAK TEMPERATURES