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Speciation and Thermodynamics of Sulfur Vapor 121 Table 2 Concentrations (c) of the various sulfur species (mmol l1) in the saturated vapor at 500 C [23], calculated from the thermodynamic data of Rau et al. [24]. Total pressure: 0.2 MPa. The molar ratio refers to c(S8)=100 Species Concentration S2 1.58 S3 0.41 S4 0.34 S5 0.91 S6 8.13 S7 9.58 S8 10.5 2.3 Pressure and Density Measurements Molar ratio 15 4 3 9 77 91 100 The most elaborate analysis of the composition of sulfur vapor was pub- lished by Rau et al. in 1973. These authors first measured the pressure of sat- urated sulfur vapor up to 1273 K and estimated the critical temperature Tc as 1313 K (pc=182 bar) [27]. The mean number of atoms per molecule at the critical point was calculated as 2.78. In a second publication [24] the densi- ties of saturated and unsaturated sulfur vapor in the temperature range of 823–1273 K were reported. Using literature data for the entropy of S8 a set of equations was derived which allow the partial pressures of the molecules S2 to S8 to be calculated as a function of total pressure and temperature (real gas corrections included). The obtained reaction enthalpies for the equilib- ria (Eq. 1) are listed in Table 1 (calculated from the data in Table 2 of [24]). Although one has to take into account that these data apply to 298 K, the agreement with the results of Berkowitz et al. and Detry et al. is rather poor except for the formation of the larger molecules S6 and S7 from S8. On the other hand, the ratio of the partial pressures calculated from Rau et al.s enthalpies for the saturated vapor at 300 C (S8:S7:S6:S5:S4:S3:S2= 1:0.36:0.43:0.024:0.001:0.001:0.005) is in excellent agreement with the corre- sponding ratios calculated from the data of Detry et al. as given above. The absolute concentrations of the various sulfur species in the saturated vapor at 500 C (0.2 MPa) calculated from the data of Rau et al. [24] are given in Table 2. It is not easy to locate the origin of the deviations between the various sets of thermodynamic data listed in Table 1. Rau et al. mentioned that the enthalpies of all reactions except for the formation of S2 could be varied slightly without much effect on the total sulfur pressure which, on the other hand, was taken as the final proof of the consistency of the data set. In addi- tion, the entropy of S8 used by these authors (430 J mol1 K1 at 298 K) does not agree well with the experimental value of 423 J mol1 K1 (298 K) derived from the geometrical data and the vibrational spectra [34].PDF Image | Topics in Current Chemistry
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