Solubility and KSP of Mg4Al2(OH)14·3H2O at the various ionic strengths

Mg4Al2(OH)14·3H2O was prepared from co-precipitation of magnesium and aluminum in mixed MgCl2–AlCl3 solutions. The solid prepared in the condition of initial CAl = 0.0625 M, Mg/Al = 2, at 323.2 K was confirmed to be Mg4Al2(OH)14·3H2O by XRD, chemical analysis, and TGA. Since it was very difficult to obtain the supernate through filtration in order to determine the solubility of Mg4Al2(OH)14·3H2O in pure water, the solubility of Mg4Al2(OH)14·3H2O in 0.1–2.0 M NaCl solutions was measured in the temperature range of 298.2–333.2 K. The log KSP for the reaction Mg4Al2(OH)14 ⋅ 3H2O ⇔ 4Mg2 + + 2Al(OH)4− + 6OH− + 3H2O at 298.2 K was determined to be − 49.09. The activity coefficients of ions and activity of water in the Mg4Al2(OH)14·3H2O-saturated NaCl solutions were calculated by the Bromley-Zemaitis and Meissner equations with the help of the OLI software. The solubility product increases linearly with the inverse of temperature suggesting a constant enthalpy of reaction at 298.2–333.2 K. On the basis of solubility product regression, the free energy of formation, ΔGf, 298.15K0(Mg4Al2(OH)14 ⋅ 3H2O), was obtained with a value of − 6365.50 kJ·mol− 1. The enthalpy and entropy of reaction, ΔHr0 and ΔSr0, were also calculated from linear regression to be − 145.57 kJ·mol− 1 and − 1429 J·mol− 1, respectively. Using values for log KSP, ΔHr0, ΔSr0 and partial molal quantities for the constituent ions, the enthalpy of formation ΔHf, 298.15K0 and the entropy of formation ΔSf, 298.15K0 were obtained and were − 7145.05 kJ·mol− 1 and 1241 J·mol− 1·K− 1, respectively. The heat capacity of Mg4Al2(OH)14·3H2O was determined to be − 485 J·mol− 1·K− 1 assuming ΔCp, r0 = 0. In addition, the precipitation diagrams of Al(OH)3, Mg4Al2(OH)14·3H2O, and Mg(OH)2 were successfully constructed with the aid of the newly obtained KSP of Mg4Al2(OH)14·3H2O. These show that Mg4Al2(OH)14·3H2O can only be obtained from mixed MgCl2–AlCl3 solutions (Mg/Al ratio = 2) with magnesium concentration < 1 M and pH above 6.