Solubility and thermodynamic properties of sodium theophylline salt in aqueous sodium hydroxide solution at different temperatures

Abstract

The solubility data of sodium theophylline salt in the water and aqueous sodium hydroxide systems with molarities of 0.00, 0.01, 0.10, 0.20, 0.49, and 0.96 mol kg−1 at temperatures ranging from 283.15 to 368.15 K are determined via the equilibrium method. The experimental results show that the solubility of sodium theophylline increases on increasing the temperature at the same concentration of NaOH solution and decreases with an increase in the NaOH concentration at the same temperature. These results correlate well with the polynomial empirical equation, the modified Apelblat equation, and the λh equation. At the same time, it is found that the modified Apelblat equation gives better correlation results than the polynomial empirical and the λh equations. The results show that the three models correlate well with the solubility data of sodium theophylline. The modified Van’t Hoff equation is used to estimate the thermodynamic properties including the dissolution enthalpy, the dissolution entropy, and the Gibbs free energy change of sodium theophylline when dissolved in aqueous NaOH solution at different concentrations. The solubility data and correlation results obtained from the experiments can provide basic data for the cooling crystallization, separation, and purification of sodium theophylline and are expected to be of great significance for the research and development of sodium theophylline and for caffeine production.