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Aqueous Sulfur Sols 157 ranging from 1 (trithionate) via 2 (tetrathionate) to probably more than 50 (polythionates) [18]. On the other hand, the free polythionic acids are unstable and subject to slow disproportionation on attempts to prepare them from their salts by ion exchange, for instance. Therefore, the structures of these species have not been determined experimentally but the lower members as well as their monoanions have been studied by ab initio MO calculations (for n=2–4) [19]. The structures of the corresponding dianions are known from the X-ray structural analyses of a number of salts [20]. Long-chain polythionate anions can be dispersed in water to form emul- sions called hydrophilic sulfur sols [21]. Depending on how these anions are synthesized from small precursors two types of sols are known. Raffo sols result from the acid decomposition of sodium thiosulfate by concentrated sulfuric acid, while Selmi sols are obtained by comproportionation of sulfide and sulfite ions in acidic solution or by the reaction of H2S with an excess of SO2 in water, respectively. Bassett and Durrant [22] have been the first to recognize “OdØns colloidal sulfur” (hydrophilic sulfur sol) as the “soluble sodium salt of a sulfur-polythionate complex” although these authors be- lieved that only one polythionate, namely hexathionate, was present which they thought to be “the highest polythionate capable of existence” at that time. More recent analyses using modern chromatographic techniques have shown that the sulfur is present as a number of different homocycles and a mixture of many different long-chain polythionate ions (or their sodium salts). The latter can be separated by ion-pair chromatography [23], while the sulfur rings may be analyzed and determined using reversed-phase HPLC [24, 25]. UV absorbance detectors are employed in both cases since all compounds containing bonds between two-valent sulfur atoms show a strong absorption near 230 nm. Linear calibration functions have been ob- tained for the concentration dependence of the UV absorption of polythion- ates and of sulfur homocycles allowing quantitative analyses. 3.2 Raffo Sols So-called Raffo or LaMer sulfur sols are used most frequently. These can be prepared by dropwise addition of aqueous sodium thiosulfate to concentrat- ed sulfuric acid followed by cooling and precipitation of the hydrophilic sol by a saturated solution of sodium chloride. The sol originates from the spontaneous decomposition of the primary product thiosulfuric acid (H2S2O3) which disproportionates in a series of complex redox reactions producing elemental sulfur, hydrogen sulfide, sulfur dioxide and polythionic acids: Na2S2O3þH2SO4 ! H2S2O3þNa2SO4 ð4Þ xH2S2O3 ! yH2OþzSO2þH2SmO6 ð5Þ H2SmO6 ! H2SmnO6þSnðn> 5Þ ð6ÞPDF Image | Topics in Current Chemistry
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