10.4314/bcse.v31i1.11

Analytical measurement of materials requires exact knowledge of their acid dissociation constant (p K 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 p K 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 (p K 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 p K a values of xylenol orange. The results show that the calculated p K a values have a comparable agreement with the experimentally determined p K 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