双波长分光光度法测定有机弱酸弱碱的解离常数

本文提出了一种测定有机一元弱酸弱碱解离常数的新方法———双波长分光光度法。该方法是在选定的一对工作波长下测定相同浓度不同pH值的有机一元弱酸弱碱水溶液的吸光度,利用所推导的公式作图求得pKa(或pKb)。利用本方法测定了甲基橙、溴甲酚绿、苯酚和苯胺等多种常见的有机一元弱酸弱碱的解离常数,结果十分满意。与传统的单波长分光光度法相比,双波长法测定结果的精密度和准确度更高。

Determination of Dissociation Constant of Organic Weak Acids or Bases by the Dual-wavelength Spectrophotometry

A dual-wavelength spectrophotometric method was developed for the determination of dissociation constant of organic weak acids or bases.In this new method,a pair of working wavelengths was selected,and the absorbance of the organic weak acid or base solutions with the same concentration but different pH was measured at the selected working wavelength pair.The measured data were used to plot against solution pH based on the mathematic equation,leading to a linear line.The intercept of the line at the horizontal axis was just the pKa value of the organic acid or the conjugated acid of the tested base.This new method was used to determine pKa values of several organic acids or bases,including methyl orange,bromocresol green,phenol,and aniline,etc.,achieving satisfactory results.Compared with the traditional single-wavelength spectrophotometry,the dual-wavelength spectrophotometry allows easy selection of the working wavelengths,and produces better precision and accuracy.