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Low-temperature heat capacities of the solid coordination compound Zn(His)SO 4 ·H 2 O(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, T d =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 ( C p,m ) with the reduced temperatures, x ( x = f ( T )), by least square method. Enthalpies of dissolution of the ZnSO 4 ·7H 2 O(s)+His(s) (Δ d H ° m,1 ) and the Zn(His)SO 4 ·H 2 O(s) (Δ d H ° m,2 ) in 100.00 ml of 2 mol l −3 HCl(aq) at T =298.15 K were determined to be, Δ d H ° m,1 =−(13.129±0.013) kJ mol −1 and Δ d H ° 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: Δ f H ° m ( Zn ( His ) SO 4 · H 2 O , 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 [ZnSO 4 ·7H 2 O(s)+His(s)] mixture in 2 mol l −3 hydrochloric acid) and Solution A′ (from dissolution of the complex Zn(His)SO 4 ·H 2 O(s) in 2 mol l −3 hydrochloric acid). |
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