The Solubilities and Physicochemical Properties of the Aqueous Quaternary System Li+, K+, Rb+//Borate–H2O at 348 K

The solubility values and physicochemical properties (pH values, refractive indices, and densities) of electrolyte mixtures in quaternary system containing lithium, potassium, rubidium, and borate were investigated at 348 K by isothermal dissolution method. The space phase diagram, the planar projection diagram, the water content diagram, and the diagrams of physicochemical properties depending on the composition were obtained using the measured data. The phase diagram of this quaternary system contains one invariant point, three univariant curves, and three single salts corresponding to lithium tetraborate trihydrate (Li2B4O7·3H2O), potassium tetraborate tetrahydrate (K2B4O5(OH)4·2H2O), and rubidium pentaborate tetrahydrate (RbB5O6(OH)4·2H2O). This quaternary system is of a simple cosaturation type, no double salt or solid solution formed. The scope of areas of crystallization of salts is such that RbB5O6(OH)4·2H2O > Li2B4O7·3H2O > K2B4O5(OH)4·2H2O, which demonstrates the salt RbB5O6(OH)4·2H2O can be more easily separated from the solution than the other coexisting salts at 348 K. The physicochemical properties of the solutions at equilibrium on the univariant curve CE increase obviously with increasing Jänecke index, J(K2B4O7); whereas on the univariant curve AE or BE, the physicochemical properties change slightly along with the changes of J(K2B4O7).