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158 Ralf Steudel Sulfane monosulfonic acids H-Sx-SO3H are intermediates in the reaction at Eq. (5); see below. As indicated by Eq. (6) the elemental sulfur formed will be a mixture of homocycles Sx from S6 via S7 and the main product S8 up to traces of larger rings. To purify the sol the mixture is first separated by centrifugation and de- cantation from the mother liquor which is discarded. The solid sol is then peptized (”dissolved”) in water and precipitated again by aqueous sodium chloride followed by filtration. This procedure is repeated ten times to ob- tain a sol free of sulfate ions, SO2, H2S, and lower polythionates [22, 26]. After this procedure the average composition of the precipitated sol has been determined as Na1.64S28.6O6·6/n Sn·1.0 NaCl with the relative amounts of sulfur rings decreasing in the order S8>S7>S6>S10>S11>S12>S13>S14 [26]. The missing cations will be present as H3O+. In other words, the sol basically consists of sodium salts of higher polythionate anions which in water are ex- pected to form micelles and maybe even vesicles. Earlier analyses of freshly prepared Raffo sols showed compositions like Na2S55O6 but the analytical data for a number of different sols varied considerably and compositions with more than 100 sulfur atoms in the formula have been reported [21, 22]. The reaction mechanism for the formation of sulfur sols can be under- stood on the basis of the following sulfur-transfer reactions. While thiosul- fate ions are stable in water at 20 C, the protonated anion will be attacked by the more nucleophilic dianion resulting in a growth of the sulfur chain [27]: SSO2þHþ Ð HSSO ð7Þ 33 HSSOþSSO2 Ð HSSSOþSO2 ð8Þ 3333 HSSSOþSSO2 Ð HSSSSOþSO2 ð9Þ 3333 This process continues until long-chain sulfane monosulfonate ions HSnSO3 (n>5) have been formed from which both sulfur homocycles Sn as well as higher polythionate ions can be generated: HSn SO3 Sn þHSO3 ð10Þ 2HSnSO ! H2S þ SmO2ðm1⁄4 2n 1Þ ð11Þ 36 The reactions at Eqs. (7)–(10) are reversible: At pH values above 7 ele- mental sulfur dissolves in aqueous sulfite solutions and thiosulfate is ob- tained [28] which is produced industrially in this manner. In the concentration range of up to 15 g sulfur in 100 ml of the sol the density and the refraction index of Raffo sols increase first linearly (up to 10 g/100 ml) and at higher concentrations more or less linearly with the sul- fur content [29, 30]. The density of the dissolved particles was calculated as 1.9–2.1 g cm3 which is roughly identical to the density of liquid sulfur at 20 C. The thermal expansion coefficient, the viscosity as well as the surface tension of Raffo sols have also been studied in detail [30].PDF Image | Topics in Current Chemistry
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