共聚焦拉曼光谱研究Mg^2+和SO4^2-缔合平衡作用

Studies on association equilibrium of Mg and SO by confocal Raman spectroscopy

硫酸镁溶液是一种应用很广的电解质溶液,Mg^2+和SO4^2-缔合平衡的特征与规律一直是人们关注的热点,具有重要的现实意义。用拉曼光谱法结合高斯-洛伦兹去卷积分峰拟合程序考察了不同浓度MgSO4溶液在不同温度下Mg^2+和SO4^2-缔合结构特征,计算了其缔合平衡常数KR,对标准状态下Mg^2+与SO4^2-离子缔合反应及其生成的总接触式离子对CIP’的表观热力学参数进行了推测计算。结果表明,MgSO4溶液中SO4^2-浓度与ν1-SO4^2-特征峰的峰面积和峰强度之间均存在较好的线性相关关系。自由态SO4^2-是ν1-SO4^2-特征峰的主要结构。随溶液温度升高,1.5和2.0 mol/L MgSO4溶液ν1-SO4^2-特征峰的峰面积和峰强度有减小的趋势,峰位发生红移,峰的对称性发生变化,其中自由态SO4^2-结构比例呈下降趋势,而单齿接触式离子对结构比例相应增加,双齿接触式离子对和SO4^2-离子团簇结构比例变化规律不明显。Mg^2+和SO4^2-的缔合平衡随温度升高向右移动,KR逐渐增大。在较稀MgSO4溶液中,几乎没有双齿接触式离子对和SO4^2-离子团簇结构,ν1-SO4^2-特征峰缔合结构及KR随温度变化无明显规律。相同温度条件下,KR随MgSO4溶液浓度升高呈现指数降低的趋势。

Magnesium sulfate solution is an important electrolyte solution which is widely used in many fields.The characteristics and the association equilibrium of Mg^2+ and SO4^2- have been the focus of attention,which has important application values.In this work,the characteristic of ion association structure of Mg^2+ and SO4^2- in different concentration MgSO4 solutions at different temperature were studied by Raman spectroscopy combined with Gauss-L orentz peak fitting program.Furthermore,the ion association equilibrium constant KR of MgSO4 solutions were calculated.Furthermore,the apparent thermodynamic parameters of the associated reaction of Mg^2+ and SO4^2- and the product of the total contact ion pair CIP’under standard state were inferred.The spectra experiments results showed that the peak area and peak intensity of theν1-SO4^2-characteristic peaks in MgSO4 solution were linearly associated with the concentration of SO4^2-.The free SO4^2- ion was the main structure ofν1-SO4^2-characteristic peaks.During the process of the temperature increasing for high concentration of 1.5 mol/L and 2.0 mol/L MgSO4 solution,the peak area and peak intensity of theν1-SO4^2-characteristic peaks decreased,red shift phenomenon occurred in the peak position,and the symmetry of the peak changed.Accordingly,the content of free SO4^2-ions decreased,the chance of monodentate contact ion pairs(CIP)increased,but the content of bidentate CIP and SO4^2- groups structure changed irregularly.Likewise,the ion association equilibrium constant KR of MgSO4 solution increased which showed that the ion association equilibrium moved to the right with the temperature increasing for 1.5 mol/L and 2.0 mol/L MgSO4 solution,thus more CIP generated.In dilute MgSO4 solutions,there were almost no bidentate CIP and SO4^2-groups structure detected,the ion association structure ofν1-SO4^2-characteristic peaks and KR value changed irregularly.At the same temperature,the KR value decreased exponentially with the increasing of the concentration of MgSO4 solution.