POTENTIOMETRIC MEASUREMENTS OF THE FIRST HYDROLYSIS QUOTIENT OF MAGNESIUM(II) TO 250-DEGREES-C AND 5 MOLAL IONIC-STRENGTH (NACL)

The first molal hydrolysis quotient, Q(1,1), of Mg2+ was measured pote ntiometrically from 1 to 250 degrees C at ionic strengths of 0.11, 0.3 1, 1.01, and 5.0 mol-kg(-1) in an aqueous NaCl medium using a hydrogen -electrode, concentration cell. Only hydrolysis of the first four perc ent of the magnesium in solution could be followed before precipitatio n of brucite, Mg(OH)(2)(cr), occurred. The log Q(1,1) values were fitt ed as a function of temperature and ionic strength using four adjustab le parameters. The resulting constants are compared with the limited e xisting low temperature data. At infinite dilution and 25 degrees C th e following quantities are reported: log K-1,K-1 = -11.68+/-0.05, Delt a H degrees = 70.1+/-1.2 kJ-mol(-1), Delta H degrees = 11+/-4 J-K-1-mo l(-1), and Delta C-p(o) = 0 J-K-1-mol(-1). At each ionic strength, inc luding the values extrapolated to infinite dilution, the heat capacity change for the hydrolysis reaction was zero, i.e., log Q(1,1) was fou nd to be a linear function of the reciprocal temperature in Kelvin, at least over the measured range of 1-250 degrees C. The hydrolysis cons tants at infinite dilution were modeled to 550 degrees C and two kbar pressure with a function incorporating solvent density using published results obtained at these extreme conditions.