混合溶剂中弱电解质解离热力学(Ⅰ)

THERMODYNAMICS OF THE DISSOCIATION OF WEAK ELECTROLYTE IN MIXED SOLVENTS (Ⅰ)

溴酚蓝(BPB)是分析化学中常用的指示剂,它有一个等吸收点的特征吸收谱图,利用这种吸收谱图可以确定溴酚蓝热力学解离常数K°.用通常的化学法和电化学法测定弱电解质的解离度,不能区分没解离的部分是分子还是离子对.用分光光度法测定解离度,可以找出这个区别,因为分子和离子有不同的吸收光谱.在溶液中离子对的形成主要靠离子间的库伦力,而光学性质通常受库伦力影响不大.因此,用具有等吸收点的吸收谱图来确定弱电解质的解离度的方法被称作真解离度测定法.在pH=2—5范围内。

The dissociation degrees of bromophenol blue at various total ionic strengths inpure water and in 15%,25 mass% ethanol-water mixed solvents were measured byU-3200 spectrophotometer,and the pH values of corresponding solution were alsomeasured by Orion EA 940 ionometer.The thermodynamic dissociation constants ofbromophenol blue in pure water and in the mixed solvents were determined by twomethods:extrapolation and polynomial approximation.The effect of medium onabsorption spectra of bromophenol blue was discussed.