Liquid-liquid equilibrium analysis in perspective. part 1. slope analysis of the extraction of uranyl nitrate from nitric acid by di-2-ethylhexylsulfoxide

Slope analysis is critically examined as a stand-alone method for analyzing the representative extraction system uranyl nitrate, nitric acid, sodium nitrate, water, di-2-ethylhexylsulfoxide (DEHSO), n-dodecane at 25 °C. The use of detailed computer calculations of free extractant concentrations and activity coefficients has permitted asscssment of commonly employed assumptions of slope analysis in the data treatment. On the basis of slope analysis alone, the extraction of nitric acid in the absence of U(VI) can be partly described by the formation of the 1:1 adduct (DEHSO) (HNO₃), but complications due to nonideality effects and the formation of at least two other species hinder further reliable analysis. Under the limiting conditions of low DEHSO molarity, low aqueous nitric acid molarity, and low U(VI) loading, a slope-analysis treatment of U(V1) extraction data shows unambiguously that the predominant extracted species of U(VI) is UO₂ (NO₃)2(DEHSO)₂(H2O)_w (w not determined). However, it is difficult to treat graphically the complicated effects arising from multiple equilibria and changing activity coefficients that occur under any but the limiting conditions, and typical simplifying assumptions applied without appropriate corrections produce misleading results. In fact, a cancelling of effects obscures the true power dependence of the U(V1) distribution coefficient D_U on DEHSO concentration at DEHSO concentrations greater than ca. 0.03 M. Thus, the advantage of simplicity in slope analysis is balanced by its limited applicability and reliability. However, with due care, slope methods of equilibrium analysis may furnish a valid, though limited, description of an extraction system, aiding further study by other techniques such as computer modeling.