DETERMINATION OF ACIDITY CONSTANTS OF ACID-BASE INDICATORS BY MOLECULAR ABSORPTION SPECTROPHOTOMETRY

Abstract

The determination of acidity constants (pKa) of acid-base indicators is essential for understanding their physicochemical properties and their applications in analytical chemistry. In this study, the pKa values of four commonly used indicators, including Methyl orange (MO), Bromophenol blue (BPB), Bromocresol green (BCG), and Bromothymol blue (BTB), were determined in aqueous solution using UV-Vis molecular absorption spectrophotometry. Two graphical methods were applied, including the intersection method based on absorbance-pH plots and the logarithmic ratio method derived from the Beer-Lambert and Henderson-Hasselbalch equations. The obtained pKa values were 3.45 and 3.39 for MO, 4.12 and 4.01 for BPB, 4.75 and 4.86 for BCG, and 7.29 and 7.36 for BTB using the first and second methods, respectively. The results obtained from both methods show close agreement with those previously reported in the literature. This study demonstrates that UV-Vis molecular absorption spectrophotometry provides a simple, rapid, reliable, and cost-effective approach to accurately determining the acid constant of acid-base indicators.