DSC STUDY OF MELTING AND SOLIDIFICATION OF SALT HYDRATES

Most salt hydrates, especially those proposed for thermal-energy-storage applications, melt incongruently. In static systems, this property often leads to differences between the enthalpy of fusion and enthalpy of solidification. By means of differential scanning calorimetry (DSC), these differences have been determined for several salt hydrates. For Na2SO4 · 10 H2O, the enthalpy of solidification at or near the peritectic temperature is never more than 60% of the enthalpy of fusion; further cooling leads to a second phase transition at a temperature corresponding to eutectic melting of mixtures of ice and this hydrate. This asymmetrical melting and freezing behavior of Na2SO4 · 10 H2O decreases its potential as an energy-storing medium and also limits its usefulness for temperature calibration of DSC instruments. Sodium pyrophosphate decahydrate, Na4P2O7 · 10 H2O, although in some ways a higher temperature analog of Na2SO4 · 10 H2O, exhibited a smaller discrepancy between the enthalpies of fusion and of solidification; its relatively high transition temperature permits a more rapid solidification reaction than is the case for Na2SO4 · 10 H2O. For Mg(NO3)2 · 6 H2O, a congruently melting compound, the magnitude of ΔH of crystallization equalled ΔH of fusion, even when supercooling occurred; a solid-state transition at 73°C, with ΔH = 2.9 cal g-1, was detected for this hydrate. MgCl2 · 6 H2O, which melts almost congruently, exhibited no disparity between ΔH of crystallization and ΔH of fusion. CuSO4 · 5 H2O and Na2B4O7 · 10 H2O exhibited marked disparities. Na2B4O7 · 10 H2O formed metastable Na2B4O7sd 5 H2O at the phase transition; this was derived from the transition temperature and verified by relating the observed ΔH of transition to heats of hydration. Peritectic solidification of hydrates can be viewed as a dual process: crystallization from the liquid solution and reaction of the lower hydrate (or anhydrate) with the solution; where ΔH of solidification appears to be less in magnitude than the ΔH of fusion, the difference can be attributed to slower reaction rate between solution and the lower hydrate. New or previously unreported values for ΔH of fusion obtained in this study were, in cal g-1: Mg(NO3)2 · 6 H2O, 36; Na4P2O7 · 10 H2O, 59; CuSO4 · 5 H2O, 32; Na2B4O7 · 10 H2O, 33. © 1979.