Low temperature heat capacities and standard molar enthalpy of formation of the coordination compound Zn(His)(NO3)(2)center dot 1/2H(2)O(s) (His = L-alpha- 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 373K. 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,C-m) with the reduced temperatures, x (x = f(T)), by least square method. Enthalpies of dissolution of the ZnSO4 . 7H(2)O(s) + His(s) (Delta(d)Hm,1degrees) and the Zn(His)SO4 . H2O(s) (Delta(d)H(m,2)degrees) in 100.00 ml of 2 mol l(-3) HCl(aq) at T = 298.15 K were determined to be, Delta(d)H(m,1)degrees = -(13.129 +/- 0.013) kJ mol(-1) and Delta(d)H(m,2)degrees = -(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: Delta(f)H(m)degrees (Zn(His)SO4 . H2O, s, 298.15 K) = - (1827.61 +/- 12.3 1) 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 . 7H(2)O(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). (C) 2003 Published by Elsevier B.V.