This paper presents a new set of Pitzer ion interaction model parameters for the binary NaOH-H 2 O system for concentrations up to over 30 molÁkg -1 and temperatures ranging from 273.15 to 523.15 K. Assuming that the electrolyte is only partially dissociated, the model requires the adjustment of (i) the three classical binary interaction parameters b (0) , b (1) and C / , (ii) the equilibrium constant of formation of the aqueous complex NaOH 0 (aq), and iii) one binary (k NaOH/NaOH ) and one ternary (f NaOH=Na þ =OH À ) interaction parameter. This approach, which provides much better results than the approach of treating NaOH as a fully dissociated electrolyte, was chosen to extend the description of the system to high temperatures and high concentrations. The temperature functions of the solubility products of anhydrous NaOH(cr) and five hydrated salts, NaOHÁnH 2 O(cr) (where n = 1, 2, 3.11, 3.5, 4a), were determined. In order to evaluate the quality of the new set of Electronic supplementary material The online version of this article (parameters, several tests were run on various properties using various literature data. These include the boiling point elevation in the NaOH-H 2 O system and the phase diagrams of the two ternary systems NaOH-NaCl-H 2 O and NaOH-LiOH-H 2 O. Interaction parameters for the two related binary systems NaCl-H 2 O and LiOH-H 2 O were taken from previous studies. To ensure consistency, four new mixing parameters were revised (f Na þ =Cl À =NaOH and W Cl À =OH À =Na þ for the ternary system NaOH-NaCl-H 2 O and k Li þ =NaOH and W OH À =Na þ =Li þ for the ternary system NaOH-LiOH-H 2 O). Consistent with Pitzer's equations, our new set of parameters can be used to satisfactorily describe the quaternary Na ? -Li ? -Cl --OH --H 2 O system to very high concentrations and temperatures.
