Abstract A method is proposed to calculate the dissociation constant of the indicator without measuring the concentration of the indicator and just by measuring the absorbance of the indicator. The dissociation equilibrium constant of bromothymol blue indicator is determined to be pKa=7.24 by adjusting the pH value of the solution to make the indicator present different colors. The results are consistent with those of literature value pKa=7.30. Through this experiment, students can not only master the chemical dissociation equilibrium of indicators, but also better understand and grasp the working principle and operation method of ultraviolet-visible spectrophotometer. By organically combining the principles of chemical analysis with instrumental analysis, a simple and efficient method is successfully developed to determine the dissociation constant of indicator by spectrophotometry experiment. This method not only improves the students’ practical ability, but also cultivates their innovative ability and enhances their learning interest.
XIONG Yan, LI Xiang, LI Jun, CHEN Jie, FANG Shen-Wen, DUAN Ming, LUO Xin. Improvement and Practice of Experimental Teaching for Spectrophotometric Determination of Indicator Dissociation Constant[J]. Chinese Journal of Chemical Education, 2020, 41(6): 31-35.