Determination of the pKa of the acid-base indicator Bromothymol Blue

Abstract

The dissociation constants of weak acids (or their pKa values) are physical-chemical parameters widely used in various fields of research and of interest in various branches of science. They indicate the extent of ionization of acid groups in molecules at different pH values and play a particularly fundamental role in studies of analytical procedures. They also find application in many chemical and biochemical processes and can be considered a key factor for understanding (and quantifying) chemical phenomena associated with biological activity. Several instrumental methods have already been used to determine pKa values of weak acids, but UV-Visible spectrophotometry, associated or not with chemometric strategies, is by far the simplest, most direct technique and the one that involves the lowest costs. The spectrophotometric determination of the pKa of an acid-base indicator is a widely used experiment in teaching laboratories because only easily prepared solutions, general-purpose glassware, and basic laboratory equipment (such as balances, pH meters and spectrophotometers) are employed. This article details a simple procedure, often used for the spectrophotometric determination of the pKa of acid-base indicators in teaching laboratory, in which the indicator used was bromothymol blue and the experimental data were obtained by undergraduate students during a laboratory class, under author’s the supervision. The results agree with those found in the literature.