EQUILIBRIUM PARTIAL PRESSURES OF STRONG ACIDS OVER CONCENTRATED-SOLUTIONS .3. THE TEMPERATURE-VARIATION OF HNO3 SOLUBILITY

The mole fraction Henry's Law constant (KHx/atm-1) of nitric acid is described by the equation KHx = xH+ · xNO3 - · f± */pHNO3, where f± * is the mean rational activity coefficient, prefix 'x' indicates mole fraction (calculated on the basis of complete dissociation in solution), and 'p' denotes partial pressure. From 215<T<400 K, ln(KHx) is given by: 385.972199-3020.3522/T - 71.001998 ln T + 0.131442311 T - 0.420928363 × 10-4 T2. The mean activity coefficient of pure aqueous HNO3 is described from 0 to 100% HNO3 by an empirical extension to the Pitzer and Simonson thermodynamic model, allowing activities of both HNO3 and H2O to be easily calculated. This approach, in addition to its utility at normal and high temperatures, offers a considerable improvement over the methods previously used for estimating the partial pressure of HNO3 over the low temperature, highly concentrated aerosols found in the stratosphere. © 1990.