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Liquid Sulfur 97 Table 2 Molecular composition of liquid sulfur determined by vibrational spectroscopy of quenched melts (T: equilibration temperature; concentrations in mass %; Sx are soluble rings larger than S8; the homocycles S6, S7, and Sx are the constituents of Sp) [88] T (C) 115 130 145 159 250 350 S8 (%) Sm (%) 95 0.05 94 0.2 93.4 0.4 90.4 2.4 43.8 51.3 42.8 52.0 Sp (%) 4.9 5.9 6.2 7.2 4.9 5.2 S6 (%) 0.6 0.9 0.9 1.4 0.9 1.0 S7 (%) 2.8 3.6 4.0 4.7 3.3 3.5 Sx (%) 1.5 1.4 1.3 1.1 0.7 0.7 The concentration of Sx was obtained as the difference between the total sul- fur concentration and the sum of the concentrations of S6, S7, and S8. To double-check the results the molecular mass of the S8+Sp solution was deter- mined osmometrically and the result compared with the molecular mass cal- culated from the analytical composition data assuming an average value of x=25; excellent agreement was obtained [88]. The final results were then used to calculate the composition of the melt with the assumption that the quenching procedure does not change the composition. These analytical re- sults are given in Table 2. The data in Table 2 demonstrate that the concentration ratio c(Si)/c(S8) in- creases with increasing temperature for rings with i<8 but decreases for rings with i>8 as is to be expected from the impact of the entropy. The data obtained for the Sp content are in agreement with earlier values derived from the depression of the melting temperature of liquid sulfur (see above). The maximum amounts of pure sulfur allotropes [71] isolated from 400 g liquid sulfur quenched from 160 C, extracted at 20 C by CS2 followed by fractional crystallization are as given below [88]: – S7:0.8g S12:0.25g S18:80mg S20:40mg The maximum amount of Sx isolated from 400 g melt was 1.2 g. By chromatography on silica gel 60 at a column temperature of 40 C the Sx mixture has been separated into six fractions; the mean molecular mass of the sulfur rings dissolved in these fractions ranged from 1100 (first frac- tion) to 733 (last fraction). In other words, Sx must contain rings with up to 35 atoms at least [88]. A more sophisticated chromatographic separation will be described in the following section. 3.2 Analysis of Quenched Sulfur Melts by HPLC High-performance liquid chromatography (HPLC) using the reversed phase technique is used to separate the most complex mixtures of organic and in- organic molecules with base-line separation quality. In the case of sulfur ho-PDF Image | Topics in Current Chemistry
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