ISOPIESTIC MOLALITIES FOR AQUEOUS-SOLUTIONS OF THE ALKALI-METAL HYDROXIDES AT ELEVATED-TEMPERATURES

Isopiestic molalities of aqueous solutions of the alkali metal hydroxi des have been measured at T = (383.25, 413.12, and 443409) K. The isop iestic standard for calculation of osmotic coefficients was NaCl(aq). The ion-interaction model gave an excellent fit with the experimental osmotic coefficients and provided parameters for the calculation of ac tivity coefficients. Osmotic and activity coefficients both decrease w ith increasing temperature in this temperature range. As at T = 298.15 K, the order of the results is the reverse of that for the alkali met al chlorides. The model of Simonson et al. (J. Chem. Thermodynamics 19 89, 21, 561-584) is an excellent predictor of the experimental results for NaOH(aq). A simple model is presented from which activity and osm otic coefficients (at p(sat)) can be calculated as a function of molal ity for T = 273.15 K to T = 523.15 K.