Low‐temperature Heat Capacity and Standard Molar Enthalpy of Formation of Potassium L‐Threonate Hydrate K(C4H7O5)·H2O

Abstract

AbstractThe solid potassium L‐threonate hydrate, K(C4H7O5)·H2O, was synthesized by the reaction of L‐threonic acid with aqueous potassium hydrogen carbonate and characterized by means of chemical and elemental analyses, IR and TG‐DTG. Low‐temperature heat capacity of K(C4H7O5)·H2O has been precisely measured with a small sample precise automated adiabatic calorimeter over the temperature range from 78 to 395 K. An obvious process of the dehydration occurred in the temperature region of 364–382 K. The peak temperature of the dehydration of the compound has been observed to be (380.524±0.093) K by means of the heat capacity measurements. The molar enthalpy, ΔdHm, and molar entropy, ΔdSm, of the dehydration of K(C4H7O5)·H2O were calculated to be (19.655±0.012) kJ/mol and (51.618±0.051) J/(K·mol) by the analysis of the heat‐capacity curve. The experimental molar heat capacities of the solid from 78 to 362 K and from 382 to 395 K have been respectively fitted to two polynomial equations of heat capacities against the reduced temperatures by least square method. The constant‐volume energy of combustion of the compound, ΔcUm, has been determined to be (−1749.71±0.91) kJ·mol−1 by an RBC‐II precision rotary‐bomb combustion calorimeter at 298.15 K. The standard molar enthalpy of formation of the compound, ΔfH⊖m, has been calculated to be (−1292.56±1.06) kJ·mol−1 from the combination of the standard molar enthalpy of combustion of the compound with other auxiliary thermodynamic quantities.