Effect of freezing on the pH and composition of sodium and potassium phosphate solutions: the reciprocal system KH2PO4Na2HPO4H2O

Changes in composition caused by ice precipitation and by salt precipitation affected the pH of solutions of mono- and disodium and potassium phosphates markedly during freezing. Except for neutral and alkaline solutions with a relatively high potassium phosphate content, the pH decreased with temperature during the precipitation of ice alone. Precipitation of dibasic salts was accompanied by a decrease in pH, while precipitation of monobasic salts increased the pH. The effect of simultaneous precipitation of two salts on pH depended on the type of salts precipitating as well as the initial pH. Simultaneous precipitation of disodium and monopotassium phosphates and ice occurred over a wide range of pH and solution composition; it increased the pH of the more alkaline solutions but decreased the pH of acid solutions. Simultaneous precipitation of other two-salt combinations occurred over a rather limited range of pH and composition, and consequently the effect on pH was relatively small. The data obtained provide a relatively complete picture of the phase relations in aqueous solutions of mono- and disodium and potassium phosphates in the freezing zone. The system has eleven eutectic points at temperatures between −0.5 °C. and −17.2 °C.; the pH of the eutectic solutions ranges from 3.3 to 9.3.