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Aqueous Sulfur Sols 155 takes about 1 week to be completed and can be accelerated by centrifugation [4, 10]. To obtain a monodispersed sol it is necessary to add the water to the or- ganic sulfur solution and not the other way round. The observed particle size, derived from higher order Tyndall spectra, ranges from 0.20 to 0.45 mm if the initial sulfur concentration in ethanol is between 0.25 and 2.9 mg l1 [8]. The pH value of a freshly prepared Weimarn sol, obtained from ethanolic solutions of recrystallized S8 and deionized and doubly distilled water, was 5.4 and this value remained practically constant when the sol was stored un- der nitrogen. However, in the presence of air the pH slowly decreased to 5.2 after 7 days and to 4.3 after 17 days. The most likely explanation is the fol- lowing disproportionation reaction: 3=8S8þ2H2O Ð 2H2SþSO2 ð1Þ Since both H2S and SO2 are weak acids in water the drop in pH can be understood by the following reactions: H2OþH2S Ð H3Oþ þHS ð2Þ 2H2OþSO2 Ð H3OþþHSO3 ð3Þ The driving force of the reaction at Eq. (1) comes from the thermody- namic instability of elemental sulfur in water. The Pourbaix diagram of the S8/H2O system at 20 C shows elemental sulfur to exist as a separate phase only at very low pH values and redox potentials in the range 0.1–0.3 V [11]. Electrophoretic measurements have demonstrated that the particles of Weimarn sols are negatively charged. This charge originates from the ad- sorption of anions. Therefore, the charge can be increased by adding OH ions to the aqueous phase [4]. The anions HS and HSO3 resulting from the reactions at Eqs. (2) and (3) will also be partly adsorbed by the liquid S8 par- ticles and in this way contribute to their negative charge. As a consequence of the negative charge of the sol particles they can be precipitated by cations the efficiency of which increases in the order H3O+, K+, Na+, Li+PDF Image | Topics in Current Chemistry
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