The stability of the magnesium carbonate ion pair from 10 degrees to 90 degrees C

Abstract

The dissociation constants of the aqueous MgCO/sub 3//sup 0/ ion pair were determined experimentally from 10 to 90/sup 0/C at 15/sup 0/ intervals. Absolute pH measurements in MgCl/sub 2/--KHCO/sub 3/ solutions being titrated with K/sub 2/CO/sub 3/ were used to calculate the results. The experimental pK/sub MgCO/sub 3//sup 0// (-log K/sub dissociation/) at 25/sup 0/C is 2.98 +- 0.03, and the pK values rise smoothly, as a function of temperature, to a value of 3.41 +- 0.067 at 90/sup 0/C. The temperature variation is in excellent agreement with that predicted by electrostatic ion pair theory. The derived values of ..delta..G/sub R//sup 0/, ..delta..H/sub R//sup 0/, and ..delta..S/sub R//sup 0/ of dissociation at 25/sup 0/C are 17050 +- 125 J mol/sup -1/ (4075 +- 30 cal mol/sup -1/, -8439 J mol/sup -1/ (-2017 cal mol/sup -1/), and -85.48 J deg/sup -1/ mol/sup -1/ (-20.43 cal deg/sup -1/ mol/sup -1/), respectively.