Activity-product constants of brucite from 10 degrees to 90 degrees C

The activity-product constant of brucite, K_B= [Mg²⁺][OH^-]² (where the brackets denote activities), was determined experimentally at 10°, 25°, 40°, 55°, 70°, and 90°C. The values obtained for -log K_B are 10.89±0.16 (10°), 10.88±0.10 (25°), 10.90±0.10 (40°), 10.90±0.10 (55°), 10.99±0.14 (70°), and 11.10±0.14 (90°). Using National Bureau of Standards Technical Note 270 data for the ions, the free energy of formation (ΔG°_t) at 25° for brucite is 831.486 joules per mole ( 198,730 cal mol^-1). Using data compiled by R. A. Robie and D. R. Waldbaum in 1968 for the ions, this ΔG°_t is -823,323 J mol^-1 (-198,930 cal mol^-1).

Values of log KB are also calculated for the temperature range 0° to 200°C, by using thermochemical data listed by V. B. Parker, D. D. Wagman, and W. H. Evans in 1971 and E. G. King, M. J. Ferrante, and L. B. Pankratz in 1975 and the partial molal heat capacities estimated by C. M. Criss and J. W. Cobble in 1965. The experimental values are similar to the calculated values, especially at lower temperatures.