Metastable phase equilibria of the quaternary system Na+, K+//Cl-, B4O72--H2O at 273 K

The metastable phase equilibria of the quaternary system Na+, K+//Cl-, B4O72--H2O were studied at 273 K by isothermal evaporation method. Solubilities and densities of the equilibrium solution of this system were determined. According to the experimental data, the metastable equilibrium phase diagram was plotted. The phase diagram of the system consists of five univariant curves, four crystallization fields and two invariant points. No double salt or solid solution was formed. The four crystallization fields were corresponding to single salt NaCl, KCl, K2B4O7 center dot 4H(2)O and Na2B4O7 center dot 10H(2)O, respectively. Invariant point E-1 was saturated with salt KCl, NaCl and Na2B4O7 center dot 10H(2)O, the mass fraction composition of the equilibrium solution corresponding to E-1 is w(Cl-)=29.15%, w(B4O72-)=0.64%, w(K+)= 5.97%, w(Na+)=15.55%, while that of point E-2 was w(Cl-)=22.84%, w(B4O72-)=10.98%, w(K+)=28.01%, w(Na+)=1.53%. Point E-2 is saturated with salt KCl, Na2B4O7 center dot 10H(2)O and K2B4O center dot 4H(2)O. Compared with the stable phase diagram at 298 K, the crystallization field of Na2B4O7 center dot 10H(2)O became large at 273 K in this system, whereas the crystallization fields of K2B4O7 center dot 4H(2)O and NaCl became small.