**3.1 Reagents**

For phase equilibrium study, reagents used should be high-purity grade otherwise the recrystallized step was needed. For the stable and metastable phase equilibria in the salt-water system (NaCl - KCl – Na2B4O7 - K2B4O7 - H2O), the chemicals used were of analytical grade, except borax which was a guaranteed reagent (GR), and were obtained from either the Tianjin Kermel Chemical Reagent Ltd. or the Shanghai Guoyao Chemical Reagent Co. Ltd: sodium chloride (NaCl, ≥0.995 in mass fraction), potassium chloride (KCl, ≥ 0.995 in mass fraction), borax (Na2B4O7·10H2O, ≥ 0.995 in mass fraction), potassium borate tetrahydrate (K2B4O7·4H2O, ≥ 0.995 in mass fraction), and were re-crystallized before use. Doubly deionized water (DDW) with conductivity less than 1.2×10-4 S·m-1 and pH 6.60 at 298.15 K was used to prepare the series of the artificial synthesized brines and chemical analysis.
