In an aqueous mixture to be processed by Evatherm Ltd., three salts were present: sodium chloride (NaCl, A), sodium sulphate (Na2SO4, B) and sodium tetraborate (Na2B4O7, C). The customer was primarily interested in the composition of the liquid phase in the invariant point at different temperatures. The invariant point is where the solution is saturated for all the three salts. We have collected the physicochemical data of the components and the experimental solubility data published for the system NaCl, Na2SO4, Na2B4O7 and water (D), including the location of the invariant point.

We measured the compositions of the quaternary invariant point of the components for the aqueous system of NaCl-Na2SO4-Na2B4O7 by the isothermal solution method at three different temperatures. Solutions with an excessive quantity of all the three salts were shaken for several days in a constant temperature water bath. Three parallel measurements were performed at every temperature. The approach of the equilibrium was tracked by measuring the conductivity of liquid samples. At the end of the measurement, the composition of the liquid phase was determined with chemical analysis by the experts of the Department of Inorganic and Analytical Chemistry. Our results showed that the concentration of NaCl slightly decreases, that of Na2B4O7 considerably increases, while that of Na2SO4 has a maximum with temperature.

More information: László Hégely PhD

Equilibrium measurement in a shaking water bath