Methodology for experimental determination of equilibrium water vapor pressure in magnesium chloride hydrates

Thermochemical energy storage (TCES) based on the reversible reaction of salt hydrates is a promising heat storage technology due to its high energy density and negligible long-term heat losses. A key parameter in TCES is the equilibrium water vapor pressure, which determines the direction of hydration and dehydration reactions under given conditions. Although the Clausius-Clapeyron relation provides a theoretical description of equilibrium conditions, experimental validation is required. This work describes an experimental method for determining the equilibrium water vapor pressure of salt hydrates using absolute pressure measurements. The method was applied to investigate the reversible reaction MgCl₂·6H₂O ↔ MgCl₂·4H₂O + 2H₂O. Equilibrium pressure versus temperature curves were obtained over the temperature range of 35-70°C. Measurements were performed on four independent samples to verify repeatability. The results show high consistency and good agreement with the literature data.