Phase Equilibria and Thermodynamic Properties of Phases in the Manganese Methanesulfonate–Methanesulfonic Acid–Water System: Experiment and Calculation

The section of the phase diagram of the Mn(CH₃SO₃)₂–CH₃SO₃H–H₂O system at 298.15 K was obtained by the isothermal solubility method. It was shown that the dihydrate Mn(CH₃SO₃)₂⋅2H₂O is in equilibrium with solutions containing 0–58 wt % acid. The bulk properties of solutions were obtained for a number of compositions at 298.15 K for this ternary system and in the temperature range 288.15–323.15 K for its subsystem Mn(CH₃SO₃)₂–H₂O. The bulk properties are described using the Laliberté semi-empirical approach. Based on the results of the saturated vapor pressure measurement by the static method, the water activity was calculated in the temperature range of 288.15–323.15 K. The possibility of estimating the solubility and water activity using only the binary parameters of the Pitzer–Simonson–Clegg model for the liquid phase and the known solubility constant at 298.15 K was tested for the Mn(CH₃SO₃)₂–CH₃SO₃H–H₂O system. It is shown that using the binary parameters alone is not enough for adequate prediction of solubility; at least one parameter of the ternary interaction must be taken into account.