Characteristics of melting point depression in a melt system of disodium hydrogen phosphate dodecahydrate

Technology employing melting point depression can be useful for extending the applicable temperature range of a latent heat storage material. In this study, the melting points T _m and latent heats of fusion ΔH _f were measured in systems of disodium hydrogen phosphate dodecahydrate (with measured values of T _m,0 =308.5 K and ΔH _f,0 =227 kJ/kg), which is especially utilized as a medical heat-retention material, with five additives that are readily soluble in the dodecahydrate melt: tripotassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, ammonium dihydrogen phosphate, and sodium dihydrogen phosphate dihydrate. By using the simplified empirical formula proposed previously expressing the relation between additive concentration and the degree of melting point depression (Watanabe and Hirasawa, 2017) or the degree of latent heat decrease (Watanabe, 2017), the abilities of each additive to depress melting point and decrease latent heat were numerically expressed. These values differed from one additive to another and appeared to be specific to each. However, the numerical relation expressed in the equation below was found to hold almost invariably regardless of the additive(forumala Presented).