Activity Coefficients of Nitrate Uranyl and Nitric Acid in Mixed Solutions

Abstract

Aqueous solutions are frequently used in chemical technology. So numerous reference books contain basic data on major properties of binary electrolyte solutions. Usually they include densities, molar or molal concentrations, activity coefficients, water activities and sometimes osmotic coefficients. Unfortunately, most solutions are mixed ones. Therefore, it is necessary to use data of binary solutions in order to calculate thermodynamic properties of mixed solutions. For example, the aqueous phase in extraction reprocessing of nuclear reactor fuel can be considered as mixed solution of nitric acid and uranyl nitrate in first approximation. Thus, in order to calculate equilibrium during extraction of uranium with TBP it is necessary to create calculations algorithm of activity coefficients of uranyl nitrate and nitric acid with different ratios of their concentrations. Usually the integration of Gibbs-Duhem equation with some conditions is used. The first ones to offer this approach were Mc Kay and Perring in 1953. Practical implementation shows that it is recommended to integrate the equation under isopiestic conditions, meaning under constant osmotic coefficients. Zdanoskiy’s rule states, that the sum of ratios of molal concentrations in mixed solution to molal concentrations in binary solutions is equal to 1. Mikulin’s equations for the systems that abide by Zdanovskiy’s rule allow calculations of molal coefficients of activity for such systems. Molal activity coefficients of nitrate uranyl (0.1-2.0 mole/kg) and nitric acid (0-5.0 mole/kg) in mixed solutions have been calculated using Zdanovskiy’s rule and Mikulin·s equations. Analytical equations were found in order to calculate these values under different concentrations. Literature data of activity coefficients for binary solutions can be combined with calculation results from equations.