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162 Ralf Steudel have previously been acidified with sulfuric acid [42]. In the first case, H2S is slowly bubbled into a concentrated cold solution of SO2 in water until no SO2 can be detected any longer in the headspace [4, 43]. The reactions taking place are not completely understood [44] but polythionates and elemental sulfur are formed in a series of redox reactions, e.g.: H2S þ 3SO2 ! H2S4O6 ð17Þ 2H2S þ SO2 ! 3=8 S8þ2H2O ð18Þ Other lower oxoacids of sulfur including sulfane monosulfonic acids must be intermediates which then undergo reactions according to Eqs. (7)–(12). The freshly prepared sol is purified by precipitation with saturated aqueous NaCl, isolated by centrifugation, peptized in water and precipitated again. This procedure is repeated several times until no sulfate ions can be detected in the aqueous phase any longer. Selmi sols are yellow and clear at low concentrations but turbid at higher concentrations. They have properties very similar to those of Raffo sols and sometimes these two types of hydrophilic sulfur sols are summarized as OdØn sols [4]. Sols prepared by hydrolysis of S2Cl2 also resemble Raffo sols since the hydrolysis in cold water produces H2S, SO2, HCl and polythionic acids in the first place [4]. The particles of Selmi sols prepared by the method of Janek [42] have been investigated by infrared and Raman spectroscopy as well as by ion-pair chromatography. They consist of polythionates SmO62 with m=4–16 as well as of sulfur homocycles Sn (n=6–10), but depending on the method of prepa- ration and purification a certain amount of salts (Na2SO4, NaCl) will be pres- ent in addition. The sulfur atoms in the zero oxidation state are distributed between elemental sulfur and polythionates in an approximate ratio of 2:1 [45]. Sols obtained from H2S and aqueous SO2 and purified by repeated precip- itation with saturated aqueous NaCl are polydispersed and metastable at 20 C for a long time but are decomposed by NaOH, NH3, or H2S. They con- sist of negatively charged particles of ca. 0.2 mm diameter which can be pre- cipitated by cations. Most effective in this respect are the chlorides of Ce3+, Al3+, Sm3+, and Th4+ followed by Ba2+, Sr2+, and Ca2+ [4, 31]. If H3O+, Li+, or Na+ ions are present higher concentrations of the precipitating agents are necessary. For this antagonistic effects of these and other cations, see [46] and also the analogous behavior of Raffo sols above. OdØn has shown that Selmi sols can be separated into fractions of differ- ing particle size by fractional precipitation with increasing concentrations of NaCl. The larger the particles the less NaCl is needed for precipitation [38]. Selmi sols are metastable for many weeks [4], but on aging the sol slowly decomposes to elemental sulfur (which precipitates as S8 crystals) and lower polythionates (which remain dissolved in water) in a similar fashion as Raffo sols do, but it should be pointed out that the relative content of elemental sulfur in Selmi sols is higher from the beginning compared to Raffo sols [4].PDF Image | Topics in Current Chemistry
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