ELECTRICAL CONDUCTANCES OF AQUEOUS SODIUM IODIDE AND COMPARATIVE THERMODYNAMIC BEHAVIOR OF AQUEOUS SODIUM HALIDE SOLUTIONS TO 800 DEGREES AND 4000 BARS

From electrical conductance measurements on dilute (0.001-0.10 m) aqueous solutions, the ionization behavior of NaI was studied in the temperature range 0-800° and at pressures to 4000 bars. Both the conventional (K) and complete (K0) ionization constants were calculated for comparison with published values for NaCl and NaBr. As expected, NaI ionized to a greater extent than either NaBr or NaCl, the order being directly proportional to the anion size. The net change (k) in waters of solvation on ionization decreased from 10.2 for NaCl to 9.7 for NaI. For the temperature range 400-800°, the van't Hoff isochore yielded standard thermodynamic functions for the complete equilibrium, NaX(solvated) + kH2O ⇌ Na+(solvated) + X-(solvated). The ΔH° obtained, approximately constant with temperature, was essentially the same as found for NaBr and NaCl. With the calculated values of ΔG°and an average value for ΔH° of -7.0 kcal mol-1, standard entropy changes of -88.3, -86.2, and -82.9 cal mol-1 deg-1 for the complete reactions were obtained for NaCl, NaBr, and NaI, respectively. The negative values of ΔS° show that order is increased by additional solvation on ionization of the electrolyte. Thus, at high temperatures, inclusion of the solvent as a reactant provides a simple description whereby each equilibrium can be described by only three values (k, ΔH°, and ΔS°) which are essentially independent both of temperature and pressure. The closeness in values of k and ΔS° for the three halides suggests that averaged values from these salts may be used to estimate the equilibrium properties of most 1-1 salts between 400 and 800°.