Phase Equilibria for the Aqueous Reciprocal Quaternary System K+, Mg2+//Cl-, Borate-H2O at 298 K

摘要

The phase equilibria of the quaternary system K+, Mg2+//Cl-, borate-H2O were studied at T = 298 K using an isothermal dissolution method. On the basis of the experimental data, the stable equilibrium phase diagram and the diagrams of the density versus composition and refractive indices versus composition of this quaternary system at 298 K were constructed. This phase diagram consists of three invariant points, seven univariant curves, and five crystallization fields corresponding to the single salt MgB4O7 center dot 9H(2)O, KCl, MgCl2 center dot 6H(2)O, K2B4O7 center dot 4H(2)O, and the double salt KCl center dot MgCl2 center dot 6H(2)O. The largest crystallization field is MgB4O7 center dot 9H(2)O; the smallest crystallization field is MgCl2 center dot 6H(2)O. By comparison with the stable phase diagram of system K+, Mg2+//Cl-, borate-H2O at 288, 298, and 323 K, the crystalline forms of salts are not changed, and the crystalline form of borate are B4O5(OH)(4)(2-). The crystallization field of MgB4O7 center dot 9H(2)O decrease obviously at 323 K. The crystallization region of KCl and K2B4O7 center dot 4H(2)O increases evidently, and the crystallization region of MgCl2 center dot 6H(2)O and KCl center dot MgCl2 center dot 6H(2)O changes slightly, meaning temperature has little impact on the solubility of KCl-MgCl2 center dot 6H(2)O and MgCl2 center dot 6H(2)O.