Activity-product constants of brucite from 10° to 90°C.

The activity-product constant of brucite, K//B equals left bracket Mg**2** plus right bracket left bracket OH** minus right bracket **2 (where the brackets denote activities), was determined experimentally at 10 degree , 25 degree , 40 degree , 55 degree , 70 degree , and 90 degree C. The values obtained for minus log K//B are 10. 89 plus or minus 0 . 16 (10 degree ), 10. 88 plus or minus 0. 10 (25 degree ), 10. 90 plus or minus 0. 10 (40 degree ), 10. 90 plus or minus 0. 10 (55 degree ), 10. 99 plus or minus 0. 14 (70 degree ), and 11. 10 plus or minus 0. 14 (90 degree ). Using National Bureau of Standards Technical Note 270 data for the ions, the free energy of formation ( DELTA G degree //f) at 25 degree for brucite is minus 831. 486 joules per mole ( minus 198,730 cal mol** minus **1). Using data compiled by R. A. Robie and D. R. Waldbaum in 1968 for the ions, this DELTA G degree //f is minus 823,323 J mol** minus **1 ( minus 198,930 cal mol** minus **1). Values of minus log K//B are also calculated for the temperature range 0 degree to 200 degree 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.