Thermodynamics of (NH4)2SO4(aq) and Solubilities of (NH4)2MII(SO4)2·6H2O (M = Mg, Fe, Zn, Cu) Tutton Salts with Implications to Thermochemical Storage

Abstract Available thermodynamic data for (NH 4 ) 2 SO 4 (aq) have been critically evaluated and used to parameterize an extended ion interaction (Pitzer) model. The model satisfactorily represents the available activity and thermal data from below the eutectic temperature to 110 °C. At near ambient temperatures (5–40 °C) the model accurately predicts the water activities in supersaturated solutions to about 30 mol·kg –1 . The model has been used to determine the solubility products of (NH 4 ) 2 SO 4 (s) from the eutectic temperature to 110 °C. Solubilities in the ternary Tutton salt forming systems (NH 4 ) 2 SO 4 –MgSO 4 –H 2 O, (NH 4 ) 2 SO 4 –FeSO 4 –H 2 O, (NH 4 ) 2 SO 4 –ZnSO 4 –H 2 O, and (NH 4 ) 2 SO 4 –CuSO 4 –H 2 O have been used to determine the ternary interaction parameters in the Pitzer model and to calculate the solubilities and deliquescence humidities of the Tutton salts (NH 4 ) 2 M II SO 4 ·6H 2 O (with M = Mg, Fe, Zn, Cu). Using additional literature data of decomposition vapor pressures, the phase diagrams of the Tutton salts have been established and the suitability of the salts as thermochemical heat storage materials has been critically evaluated.