Determination of structural, thermodynamic and phase properties in the Na2S–H2O system for application in a chemical heat pump

Structural, thermodynamic and phase properties in the Na2S–H2O system for application in a chemical heat pump have been investigated using XRD, TG/DTA and melting point and vapour pressure determinations. Apart from the known crystalline phases Na2S·9H2O, Na2S·5H2O and Na2S a new phase Na2S·2H2O has been proven to exist. Na2S·12H2O is not a phase but a 3:1 mixture of Na2S and Na2S·2H2O, presumably stabilised by very slow dehydration kinetics. The vapour pressure–temperature equilibria of the sodium sulphide hydrates have been determined and a consistent set of thermodynamic functions for these compounds has been derived. XRD measurements indicate the topotactic character of the transitions between the hydration states.