Low-temperature heat capacity and standard molar enthalpy of formation of potassium L-threonate hydrate K(C4H7O5)•H2O

The solid potassium L-threonate hydrate, K(C4H7O5)(H2O)-H-., 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)-H-. 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, Delta H-d(m), and molar entropy, Delta S-d(m), of the dehydration of K(C4H7O5)(H2O)-H-. 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, Delta U-c(m), 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, Delta H-f(m)theta 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.