Dissolution enthalpies and the thermodynamic properties of sodium metaborates

Abstract

The thermodynamic properties including enthalpy, entropy and Gibbs free energy are important parameters, which can provide a theoretical basis for utilizing salt lake resources. In addition, the dissolution equilibrium constant (K), activity coefficient (γ) and osmotic coefficient (Φ) of inorganic salt solution, especially for sodium borate, are critical and fundamental, but scarce owing to the difficulty to measure them. These data are not only important theoretical basis in analytical and physical chemistry, but also reliable information in determining chemical production processes. In this paper, the dissolution enthalpies (∆Hsθ) of sodium metaborate tetrahydrate (NaBO2·4H2O) and sodium metaborate dehydrate (NaBO2·2H2O) were determined using a microcalorimeter TAM IV. Based on our experimental data and the Pitzer electrolyte model, values of dissolution equilibrium constant (K, 8.7986 mol2·kg−2 for NaBO2·4H2O, 9.9560 mol2·kg−2 for NaBO2·2H2O), activity coefficient (γ, 0.2932), osmotic coefficient (Φ, 0.7999) of NaBO2 were measured with precision. According to the designed thermochemical cycle and the basic formula of thermodynamics, the standard enthalpy of formation (∆Hfθ), the standard entropy of formation (∆Sfθ), the standard Gibbs free energy (∆Gfθ), the standard enthalpy of dissolution (∆Hsθ), the standard entropy of dissolution (∆Ssθ), and the dissolution Gibbs free energy (∆Gsθ) of NaBO2·4H2O and NaBO2·2H2O at 298.15 K, which are not reported in the literature, were obtained with exactitude. These results indicate that the dissolution of both hydrates are all endothermic, spontaneous and entropy driven processes.