Abstract
An extended UNIQUAC-Debye-Hückel model is used to predict vapour-liquid equilibria for nitric acid-water-nitrate salt systems over the whole concentration range. Both phase equilibria and chemical equilibria (acid-base dissociation and acid self-dissociation) are taken into account. It is found that VLE-data for the binary HNO 3-H 2O system are represented well by this scheme using a simple activity coefficient model. Nitrate salts added to the HNO 3-H 2O system are assumed to be completely dissociated, and solvation of ions is not taken into account by chemical theory. In order to represent the VLE data for solutions with nitrate salts at high concentrations, it is necessary to introduce concentration-dependent ion-solvent interaction parameters and to use artificially large volume and surface area parameters for the cations. As a result, it is possible to model the strong “salting out” effect of Mg(NO 3) 2 and Ca(NO 3) 2 on nitric acid.
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