Solubility isotherms determination of the H3BO3 + CsCl + H2O system at T = (273.15, 298.15, 323.15, 348.15 and 363.15) K and thermodynamic modeling

The construction of a comprehensive thermodynamic model for boron-containing salt brine is important. However, the general lack of the solubility of H 3 BO 3  + salt + H 2 O ternary systems makes it difficult to determine model parameters involving boron species. Here, we systematically determined the solubility isotherms of the H 3 BO 3  +  CsCl  + H 2 O system at T = (273.15, 298.15, 323.15, 348.15 and 363.15) K. There are three solubility branches corresponding to H 3 BO 3(cr) , CsCl (cr) and a novel adduct in the system. The determined composition of the adduct is 2H 3 BO 3 ·3CsCl. Moreover, the single crystal structure of 2H 3 BO 3 ·3CsCl is solved, suggesting that H 3 BO 3 molecules keep it native plane-triangle structure, but the O atoms coordinate to the Cs ion. We modeled these ternary solubility isotherms using the Pitzer-Simonson-Clegg (PSC) model implemented in the ISLEC program. With the help of the model, a complete polythermal phase diagram of the system was constructed for the temperature range from 250 K to 373.15 K. The determined standard thermodynamic quantities of 2H 3 BO 3 ·3CsCl (cr) from the model are Δ f G m Θ  =  − 3182.26 kJ∙mol −1 , Δ f H m Θ  =  − 3496.19 kJ∙mol −1 , S m Θ  = 485.85 J∙K −1 ∙mol −1 , C p , m Θ  = 320 J∙K −1 ∙mol −1 , respectively.