Determination of dissociation constant of naproxen in different solvent mixtures

Abstract

Introduction: The dissociation constant is an important parameter in determining the behavior of compounds in various applications. Simplicity, accuracy and reproducibility of potentiometry made this method as an accepted technique for determination of dissociation constants among different methods. Aim: The aim of this project was to determine the dissociation constant of naproxen in mixtures of 1-propanol + water, 2-propanol + water and tetrahydrofuran + water in various concentrations of (10-90), (20-80), (30-70), (40-60), (50-50), (60-40), (70-30) and (80-20)% v/v at four temperatures of 298, 303, 308 and 313 K by potentiometry method using the BEST computer program. Methods: The ionic strength of solution was adjusted using potassium chloride at 0.1 mol.L-1. Naproxen titrations in different mixtures: 1-propaneol + water, 2-propaneol + water and tetrahydrofuran + water at four different temperatures of 298, 303, 308 and 313 K were done as described by Martell and Motekaitis. In order to the evaluation of the dissociation constant of naproxen, eight sample solutions in different binary solvent mixtures from 10 to 80% (v/v) were used in and the pH range of 2–12 adjusted with HCl of 0.001 mol.L-1. Results: The results showed that in different mixtures of solvents, the dissociation constant of naproxen increased with increasing the amount of organic solvent due to the dominance of the electrostatic forces in the equilibrium process and decreased by temperature increasing, because the acidity of naproxen decreases. Conclusion: The effect of temperature on the dissociation constant in three different temperatures was investigated and the related thermodynamic parameters of naproxen (ΔG°, ΔH° and ΔS°) were calculated from temperature dependence data.