The solid -solid state coordination reaction enthalpy of CaO with ascothic acid has been determined by solution calorimetry in an isoperibel reaction calorimeter at 298.2K. The calorimietric solvent is 2mol/L HCL A thermoohemical cycle has been designed,and the standard Enthalpy of formation for Ca (C6H7O6)2·4H2O (s)has been calculated. The recommended values of standard enthalpy of formation ΔfHm^Θ [ Ca( C6H...