Low-temperature heat capacities and standard molar enthalpy of formation of the coordination compound Zn(His)SO4·H2O(s) (His=l-α-histidine)

Low-temperature heat capacities of the solid coordination compound Zn(His)SO4·H2O(s) were precisely measured by a precision automated adiabatic calorimeter over the temperature range between T=78 and 373 K. The initial dehydration temperature of the coordination compound was determined to be, Td=324.20 K, by analysis of the heat-capacity curve. The experimental values of molar heat capacities were fitted to a polynomial equation of heat capacities (Cp,m) with the reduced temperatures, x (x=f(T)), by least square method. Enthalpies of dissolution of the ZnSO4·7H2O(s)+His(s) (ΔdH°m,1) and the Zn(His)SO4·H2O(s) (ΔdH°m,2) in 100.00 ml of 2 mol l−3 HCl(aq) at T=298.15 K were determined to be, ΔdH°m,1=−(13.129±0.013) kJ mol−1 and ΔdH°m,2=−(11.271±0.012) kJ mol−1, by means of a homemade isoperibol solution-reaction calorimeter. The standard molar enthalpy of formation of the compound was determined as: ΔfH°m (Zn(His)SO4·H2O,s, 298.15 K)=−(1827.61±2.31) kJ mol−1 from the enthalpies of dissolution and other auxiliary thermodynamic data through a Hess thermochemical cycle by using the homemade isoperibol calorimeter. Furthermore, the reliability of the designed Hess thermochemical cycle was verified by comparing between UV/Vis spectra and the refractive indexes of Solution A (from dissolution of the [ZnSO4·7H2O(s)+His(s)] mixture in 2 mol l−3 hydrochloric acid) and Solution A′ (from dissolution of the complex Zn(His)SO4·H2O(s) in 2 mol l−3 hydrochloric acid).