Sulfur is a very important element in today’s world. Its most important use is in the manufacture of sulfuric acid (H2SO4). Sulfur exists in two allotropic forms. The two forms of sulfur are known as α-form (Orthorhombic crystal structure) and β-form (Monoclinic crystal structure). Both allotropes are yellow, with the α-form a brighter yellow and the β-form a paler, whitish-yellow.
In the present study, the thermal analysis technique has been used to investigate the phase transition behaviors of elemental sulfur. This technique provides qualitative and quantitative information about physical and chemical changes that involve endothermic or exothermic processes or changes in heat capacity using minimal amounts of sample.
Fig.2 Shows the DTA thermogram of the elemental sulfur at the heating rate of 40°C in presence of an inert gas such as Argon.
The peak at 113oC corresponds to the solid-phase change from the rhombic to the monoclinic form. The peak at 128oC corresponds to the melting point of the element. Liquid sulfur is known to exist in at least three forms, and the peak at 197oC apparently involves these transitions, whereas the peak at 440oC corresponds to the boiling point of sulfur.
Fig.3 shows the TGA thermogram of sulfur with different heating rate as shown in the graph. As the heating rate is increased, the onset of decomposition is moved to higher temperatures. The figure shows that the vaporization temperature increases gradually from 320 to 440oC as the heating rate increases from 5oC/min to 40oC/min.
Fig.4 shows the DSC thermograms of elemental sulphur on different heating rates which resolute the individual phase transformation temperatures upon a lower heating rate. The temperatures corresponding to peaks in the DSC curves increase by raising the heating rate which further indicates that the transformations are temperature dependent.
On the other hand, when sulfur finally volatilizes from liquid state to vapor state, there occurs a rapid mass loss which is reflected from the derivative wt. curve with temperature in Fig.5.
The activation energy for the above processes has been estimated by the Kissinger relation as follows:
Where E is the activation energy for the transformation, β is the heating rate, Tm is the peak temperature and R is the gas constant. Activation energies may be estimated from the plot of versus . Fig.6 presents the Kissinger plots for the first three phase transformations. The activation energies derived from this plot are 219.3, 298.8 and 160.2 kJ/mole, respectively.
Fig.7 shows the Kissinger plot for the liquid to vapor phase transformation. The activation energy derived from the plot is 65 kJ/mole which is close to that reported in the literature.
Apart from the above analysis, TGA study is found to be an important tool for determining purity of elemental sulphur. Fig.8 presents the TGA curves for different sulfur samples for which the purity is presented within the graph. It is determined from the weight loss that occurred at high temperature under the controlled atmosphere.