Speciation and thermodynamic properties of the cobalt(II) methanesulfonate-water-methanesulfonic acid system.

The growing interest in methanesulfonic acid (MSA) in hydrometallurgical research has motivated further study of methanesulfonate systems. Cobalt, like nickel, is an essential component of the cathode active materials used in lithium-ion batteries, yet its solution chemistry in MSA media remains insufficiently characterized. In this work, the Co(CH3SO3)2-MSA-H2O system was investigated with an emphasis on solid-liquid equilibria. The liquid phase was characterized via solubility, density, and water activity measurements over a broad range of compositions and temperatures up to 50 °C. Solid phases were investigated through thermogravimetric analysis and single-crystal X-ray diffraction. Three different hydrates were identified: Co(CH3SO3)2·6H2O, Co(CH3SO3)2·4H2O, and Co(CH3SO3)2·2H2O. The solubility was determined using two independent experimental methods, isothermal titration calorimetry (ITC) and the isothermal solubility method, with good agreement between both approaches. The solubility data reveal that cobalt(II) methanesulfonate solubility decreases with increasing MSA concentration but remains higher than that in sulfuric acid media, while temperature effects are limited within the studied range. The experimental dataset was used to develop a thermodynamic model with the Mixed-Solvent Electrolyte (OLI-MSE) framework of OLI Systems. Thermodynamic properties of the hydrates and interaction parameters for the aqueous phase were optimized to achieve an accurate calculation of the system.