Activity behavior of the NdNO3--HNO3--H2O--HDEHP--AMSCO System

The thermodynamic behavior of the two-phase NdNO3--HNO3--H2O--HDEHP--AMSCO system has been studied to provide experimental data for use with a model to predict the distribution coefficients of neodymium. The nitrate ion activities in the aqueous phase were measured with a nitrate ion selective electrode taking into account the presence of high hydrogen concentration by calibration of the nitrate electrode with nitric acid. The nitrate ion concentration in the organic phase due to the extraction of neodymium complexes by HDEHP was determined by back extraction of the organic phase with 3 M sulfuric acid, diluted and analyzed with a nitrate ion electrode calibrated for different nitrate-sulfate concentration. The amount of the nitrate species extracted into the organic phase increases with increase in the initial neodymium nitrate concentration. The neodymium ion concentration in the organic phase was measured by back extraction with 6 M nitric acid, removal of the neodymium by adsorption with the cation exchange resin, DOWEX 50 x 8 and titration of the neodymium free aqueous phase with NaOH using phenolphtalein as the indicator. The hydrogen ion activity in the equilibrated aqueous phase was determined by direct pH measurements since the polyvalent neodymium ions were found to have very negligible effect on the pH electrode. Vapor pressure measurements to estimate the activity coefficients of the HDEHP dimer resulted in non-reproducible values because of the nonuniform outgassing of the most highly volatile portion of the HDEHP diluent, AMSCO. The activity coefficients of the HDEHP dimer were estimated from the data of Baes who made isopiestic comparisons with triphenylmethane in n-octane as the reference solution assuming that the activity coefficient of triphenylmethane is unity