Numerical evaluation of melting point and fusion heat for potassium acetate dihydrate as a latent heat storage material

To clarify the inevitability of the relationship between the melting point (T_m=291 K) and the heat of fusion (ΔH_f=135 kJ/kg) of potassium acetate dihydrate, a potential latent heat storage material, a quantitative examination of the relationship was made in terms of the structural formula. First, four pairs of salts comprising Na⁺ or K⁺ with the same given anion were investigated, for all of which the degree of hydration and the dehydration transition temperature of the hydrate was known. It was found that the dehydration temperature of the potassium salt was generally 20‒50 K lower than that of the sodium salt. It was also found that the dehydration temperature, that is T_m, was lower as the ratio of Stokes radius of the salt ion in aqueous solution to the ion radius in crystal structure decreased. Next, the entropy change ΔS_f (=ΔH_f/T_m) of the hydrate during fusion was expressed as a combination of entropy change ΔS_f,HO of water during fusion and the mass fraction Φ_w of the water of crystallization in the hydrate. With this combination, a simple relational expression between T_m and ΔH_f was derived. Application of this expression to the measured data for 18 dif-ferent hydrates, it was indicated that the numerical value of ΔH_f could be estimated with a relative standard deviation of ±0.212 when the structural formula and the T_m value of a hydrate were both known. Through this, the inevitability of the correspondence between the ΔH_f value and T_m for potassium acetate dihydrate was also confirmed.