Solid-liquid equilibria of Mg(OH)2(cr) and Mg2(OH)3Cl·4H2O(cr) in the system Mg-Na-H-OH-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 Mg2(OH)3Cl·4H2O(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 (25°C), the following equilibrium constants at I = 0 are calculated: Mg(OH)2(cr) + 2 H+ ⇔ Mg2+ + 2 H2O log K°s = 17.1 ± 0.2 Mg2(OH)3Cl·4H2O(cr) + 3 H+ ⇔ 2 Mg2+ + Cl− + 7 H2O log K°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.