Heat Capacities of Aqueous Ions

1 Heat Capacities of Aqueous Ions The difference between the specific heat of a dilute solution of an electrolyte and that of water can be obtained by flow microcalorimetry that requires also knowledge of the corresponding densities. Extrapolation to infinite dilution of these differences yields the standard partial molar (constant pressure) heat capacity of the electrolyte, Cp. Alternatively, the heat of solution of an electrolyte in water to form a dilute solution can be measured calori-metrically at several temperatures and exhapolation of the temperature coefficients of these heats of solution to infinite dilution yields the same quantity, but somewhat less accurately. A recent review of the experimental methods by Hakin and Bhuiyan [63] may be consulted for details. Determinations of CZ are accurate to 1 to 

The uncertainty in the absolute values of C j is large (see above, 14J K mol ). but the uncertainties for the comparison of ions of the same sign of charge is much lower, being near those for the experimental C, 1-3 J K mol , for most ions except for the lanthanides, for which it is 20-30J K mol , showing disagreement between the values in Refs. 21 and 64 for La and Gd (the only ones in the latter publication). 

The partial molar heat capacities of electrolyte solutions at appreciable concentrations are obtained from electrolyte-specific parameters reported by Criss and Millero [74, 75], The semiempirical Helgeson-Kirkham-Flowers [76] expression may be used for similar calculations according to Schock and Helgeson [65], particularly useful for elevated temperatures. 

---