Studies On The Metastable Phase Equilibrium In The Quinary System K⁺, Na⁺//CO₃^2-, SO₄^2-, B₄O₇^2--H₂O At 273K

In this paper, phase equilibria of quinary reciprocal system K⁺, Na⁺//CO_<sup>2-, SO₄^2-, B₄O₇^2--H₂O and its subsystems (two ternary systems and three quaternary systems) at 273 K were studied by isothermal evaporation method. Solubilities and densities in the solution of these systems were determined. According to the experimental data, equilibria phase diagrams and density-composition diagrams of the ternary systems were plotted; equilibrium phase diagrams, corresponding water content diagrams and density-composition diagrams of the quaternary systems were finished; metastable phase diagrams (saturated with Na₂B₄O₇), density-composition diagrams and corresponding water and sodium content diagrams of the quinary systems were also plotted.Potassium and Sodium Carbonate double salt K₂CO₃·Na₂CO₃·12H₂O is formed in the ternary system K⁺, Na⁺//CO₃^2- -H₂O. Potassium carbonate (K₂CO₃) has strong salting-out effect on sodium carbonate (Na₂CO₃). Due to the higher concentration, salt potassium carbonate (K₂CO₃) has strong effect on the physico-chemical properties in the solution of these equilibrium systems, which presents alkalescence. The curves in the density-composition diagrams ascend with the increase of the concentration of K₂CO₃. In the ternary system Na⁺//CO₃^2-, SO₄^2--H₂O, double salt 2Na₂SO₄·Na₂CO₃ is formed, and sodium sulfate (Na₂SO₄) has salting-out effect to sodium carbonate (Na₂CO₃).Double salt K₂CO₃·Na₂CO₃·12H₂O was found in the quaternary system K⁺, Na⁺//CO₃^2-, B₄O₇^2- -H₂O and 2Na₂SO₄ 2222 Na₂CO₃ was found in the quaternary system Na^//CO₃^2-, SO₄^2-, B₄O₇^2--H₂O. In the two quaternary systems, sodium borate (Na₂B₄O₇) has salting-out effect to sodium carbonate (Na₂CO₃). The two double-salts are also formed in the quaternary system K⁺, Na⁺//CO₃^2-, SO₄^2--H₂O. Sodium sulfate (Na₂SO₄) has salting-out effect to sodium carbonate (Na₂CO₃) in this system.The phase diagram of the quinary system K⁺, Na⁺//CO₃^2-, SO₄^2-, B₄O₇^2--H₂O at 273K (saturated with Na₂B₄O₇) consists of seven crystallization fields, which correspond to the salt Na₂SO₄·10H₂O,K₂SO₄,K₂B₄O₇·4H₂O,2Na₂SO₄·Na₂CO₃,Na₂CO₃·10H₂O,K₂CO₃·3/2H₂O and K₂CO₃·Na₂CO₃·12H₂O, twelve univariant curves and six invariant points. The constitutions of sodium and potassium borate are Na₂B₄O₅(OH)4·8H₂O and K₂B₄O₅(OH)₄·2H₂O, respectively. In contrast to the ternary and quaternary systems, the crystallization area of K₂CO₃·3/2H₂O increases clearly in the quinary system.