The apparent molar heat capacity of aqueous hydrochloric acid from 10 to 140°C

The apparent molar heat capacity of aqueous HCl, C_p,, has been measured at temperatures of 25, 76, 103, 125 and 140°C and molalities from 0.1 to 1.02 mol-kg^–1 using a Picker flow microcalorimeter. The results were analyzed using the Pitzer and the Helgeson-Kirkham-Flowers models to derive standard state heat capacities. The fitted parameters were also used to extrapolate the standard EMF of the silver-silver chloride reference electrode at steam saturation from 0 to 200°C and the mean ionic activity coefficient, _± (HCl,aq) to 225°C, with an accuracy at the highest temperature of 2 mV and 4%, respectively. The results confirm that experimental values of C_p, to just over 100°C can be used to extrapolate standard state and excess Gibbs energies above 250°C, when the corresponding enthalpies at 25°C are accurately known.