The
solubility relations and water activities in NaNO3, NaNO2, and NaOH solutions are used in many applications,
including the management of alkaline high-level nuclear waste. Only
limited water activity data is available at temperatures above ambient
for NaNO3 and NaNO2 aqueous solutions, so water
activity was measured by isopiestic methods at 50 and 100 °C
in this study. The results are consistent with the limited experimental
data previously available. These data are used to parameterize the
Pitzer electrolyte solution model for these electrolytes and NaOH
in water between 0 and 100 °C. Neutral ion pairs between Na+ and NO3
– as well as Na+ and NO2
– were required in order to
fit the data adequately. Ternary Pitzer ion interaction parameters
for the interactions between the electrolytes were also required and
developed to model solubility in mixtures of the electrolytes. These
results provide a strong foundation for modeling the effect of NaNO2, NaNO3, and NaOH on the solubility of other key
electrolytes of interest to the management of high-level nuclear waste.