Equilibrium distribution of polysulfide ions in aqueous solutions at 25°C:: A new approach for the study of polysulfides equilibria
Equilibrium distribution of polysulfide ions in aqueous solutions at 25°C:: A new approach for the study of polysulfides equilibria
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DOI：
10.1021/es049514e
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发表时间：
2004-12-15
影响因子：
11.4
通讯作者：
Lev, O
中科院分区：
文献类型：
--
作者：
Kamyshny, A;Goifman, A;Lev, O
采用一种基于单相快速化学衍生化的新方法测定了无机多硫化物在水溶液中的歧化常数和热力学基础。这种方法解决了关于水溶液中是否存在六硫键的争议，并确定了水中存在更高的多硫链。当n=2-8时，多硫物种的Gibbs自由生成能(Gegrees(Sn)(2-))分别为77.4、71.6、67.4、66.1、67.2、70.5和73.6kJ/mol。我们的方法是基于聚硫化物与三氟甲磺酸甲酯(三氟甲酸甲酯)的单相快速甲基化，然后用高效液相色谱法测定二甲基多硫化物。为了确认观察到的二甲基多硫化物分布与水中多硫化物分布之间的定量等价性，使用了两种独立的方法：(I)对每个竞争反应步骤的动力学研究表明，衍生化反应的动力学速度快于每个竞争反应，这可能导致歧化反应，并使观察到的二甲基多硫化物的分布与水中多硫化物的分布发生偏差。(2)对两种溶液(一种含有天然同位素分布的多硫化物，另一种含有富含S-34的多硫化物)在衍生化过程中的同位素混合的测定表明，衍生化过程中多硫化物的混合量很低。根据同位素稀释试验，衍生化过程中五硫化物重新分配的系统误差为3%，根据动力学考虑，系统误差不到总零价硫的5%。
A new approach based on rapid, chemical derivatization in a single phase was used to determine the disproportionation constants and the underlying thermodynamics of inorganic polysulfides in aqueous solutions. This method resolves the dispute over the existence of hexasulficle in aqueous solutions and establishes the presence of even higher polysulfide chains in water. The Gibbs free energies of formation (Gdegrees(Sn)(2-)) for the polysulficle species are 77.4, 71.6, 67.4, 66.1, 67.2, 70.5, and 73.6 kJ/mol for n = 2-8, respectively. Our approach is based on single phase, fast methylation of polysulfides with methyl trifluoromethanesulfonate (methyl triflate) and subsequent determination of the dimethylpolysulfides by HPLC. Two independent methods were used in order to confirm quantitative equivalence between the observed distribution of dimethylpolysulfides and the polysulfide distribution in the water: (i) Kinetic studies of each competing reaction step showed that the kinetics of the derivatization are faster than each of the competing reactions that may lead to disproportionation and deviation of the observed distribution of dimethylpolysulfides from that of the aqueous polysulfides. (ii) Determination of isotope mixing during the derivatization of a mixture of two solutions, one containing polysulfide of natural isotopic distribution and the second containing S-34-rich polysulficle revealed that polysulfide mixing during derivatization is rather low. The systematic error due to redistribution of pentasulfide during derivatization is 3% based on isotope dilution tests and less than 5% of total zerovalent sulfur based on kinetic considerations.