Eutectic composition of a salt hydrate mixture of sodium carbonate and disodium hydrogen phosphate

Based on the idea that eutectic formation is a useful technology for controlling the melting points of latent heat storage materials over a wide temperature range, experimental studies were carried out on the pseudobinary mixtures of sodium carbonate decahydrate and disodium hydrogen phosphate dodecahydrate, both of which are also heat storage materials. From the distribution of melting points of the mixtures of varying composition determined from the heating curve, the phase diagram was developed as a single eutectic type, and the melting point of the eutectic was found to be 298 K. For specifying the eutectic composition, two methods were examined: one by using the empirical formula of the melting point depression, and the other by utilizing the change in the latent heat of fusion. The eutectic composition as a mole fraction of disodium hydrogen phosphate dodecahydrate was estimated to be 0.40 mol/mol by the former method and 0.37 to 0.38 mol/mol by the latter. These values closely match the 0.4 mol/mol derived from the hypothesis that the eutectic would be formed by equivalent exchange of dissociated ions in the melt of salt hydrate mixture, proposed by Watanabe and Hirasawa (2018).