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Aqueous Sulfur Sols 161 erties and composition of the resulting “acidoid” sols. Pauli et al. [40] per- formed electric conductivity measurements, titrations with NaOH, Ba(OH)2, BaCl2, AgNO3, and HgNO3, reactions with I2 and NaOH as well as heating, freezing and coagulation of the sol with salts. All reported observations can be explained by the model described above, i.e., with particles consisting mainly of long-chain polythionic acids (as oxonium salts) with a certain amount of sulfur homocycles, but the original authors came to a totally dif- ferent conclusions and proposed a model in which elemental sulfur is the main component and adsorbed thiosulfuric acid as well as traces of poly- thionic acids were assumed to make these particles hydrophilic. The acidoid sol may be titrated with NaOH or Ba(OH)2 like a normal mineral acid, the endpoint can been observed conductometrically. Addition of BaCl2 sets the hydrogen ions free (by ion exchange) which then can be titrated acidimetri- cally with NaOH: SnðSO3HÞ2þBa2þ !SnðSO3Þ2Baþ2Hþ ð13Þ The reaction with AgNO3 (or HgNO3) is slow and proceeds according to Eq. (14): SnðSO3HÞ2þ2AgNO3þ2H2O ! Ag2S þ 2HNO3þ2H2SO4 þ ðn 1Þ=8 S8 ð14Þ The freshly prepared sol did not react with iodine which indicates that no -SH groups and therefore no sulfane monosulfonic acids HSnSO3H are pres- ent. However, the following reaction takes place slowly: SnðSO3HÞ2þI2þ2H2O ! 2H2SO4þ2HIþn=8 S8 ð15Þ In the reactions at Eqs. (14) and (15) the SSO3 bonds are hydrolytically cleaved because these are the longest and obviously weakest SS bonds in polythionate anions [41]. Sodium hydroxide destroys polythionic acids in a similar reaction: Sn ðSO3 HÞ2 þ5NaOH ! 2Na2 SO4 þðn 1Þ=8 S8 þ3H2 O þ NaHS ð16Þ Pauli et al. studied all of these reactions quantitatively but interpreted the results erroneously in terms of the presence of H2S2O3 [40]. They also ob- served that the conductivity of the acidoid sol increased with aging which can be understood in terms of the reaction at Eq. (6) in which lower poly- thionate anions are formed which are soluble in water in contrast to the very long-chain anions in the sol particles. Coagulation by electrolytes was most efficient with AgNO3 and La(NO3)3 followed by BaCl2. 3.3 Selmi Sols Selmi sols are obtained either by reaction of gaseous H2S with aqueous SO2 [4] or by reaction of aqueous Na2S with aqueous Na2SO3, if both solutionsPDF Image | Topics in Current Chemistry
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