Solid-liquid equilibria of Mg(OH)2(cr) and Mg2(OH)3Cl•4H2O(cr) in the system Mg-Na-H-OH-O-Cl-H2O at 25°C

The solubility of crystalline Mg(OH)(2)(cr) was determined by measuring the equilibrium H+ concentration in water, 0.01-2.7 M MgCl2, 0.1-5.6 m NaCl, and in mixtures of 0.5 and 5.0 m NaCl containing 0.01-0.05 m MgCl2. In MgCl2 solutions above 2 molal, magnesium hydroxide converted into hydrated magnesium oxychloride. The solid-liquid equilibrium of Mg-2(OH)(3)Cl.4H(2)O(cr) was studied in 2.1-5.2 m MgCl2. Using known ion interaction Pitzer coefficients for the system Mg-Na-H-OH-Cl-H2O (25degreesC), the following equilibrium constants at I = 0 are calculated: Mg(OH)(2)(cr) + 2 H+ double left right arrow Mg2+ + 2 H2O log Kdegrees(s) = 17.1 +/- 0.2 Mg-2(OH)(3)Cl.4H(2)O(cr) + 3 H+ double left right arrow 2 Mg2+ + Cl- + 7 H2O log Kdegrees(s) = 26.0 +/- 0.2 The experimental results are discussed with regard to discrepancies in frequently used databases and computer codes for geochemical modeling, such as EQ3/6, Geochemist's Workbench and CHESS. Copyright (C) 2003 Elsevier Ltd.