Theoretical study (ab initio and DFT methods) on acidic dissociation constant of xylenol orange in aqueous solution

Abstract

Analytical measurement of materials requires exact knowledge of their acid dissociation constant (pK_a) values. In recent years, quantum mechanical calculations have been extensively used to study of acidities in the aqueous solutions and the results were compared with the experimental values. In this study, a theoretical study was carried out on xylenol orange (in water solution) by ab initio method. We calculated the pK_a values of xylenol orange in water, using high-level ab initio (PM3), DFT (HF, B3LYP/6-31+G(d)) and SCRF methods. The experimental determination of these values (pK_a,s) is a challenge because xylenol orange has a low solubility in water. We considered several ionization reactions and equilibriums in water that constitute the indispensable theoretical basis to calculate the pK_a values of xylenol orange. The results show that the calculated pK_a values have a comparable agreement with the experimentally determined pK_a values. Therefore, this method can be used to predict such properties for indicators, drugs and other important molecules.

KEY WORDS: Ab initio, DFT method, Ionization constant, Xylenol orange, Atomic charge

Bull. Chem. Soc. Ethiop. 2017, 31(1), 127-136.

DOI: http://dx.doi.org/10.4314/bcse.v31i1.11