The oxidation-reduction couples U(IV)-U(VI) and Fe(II)-Fe(III) in phosphoric acid solutions

The potentials of the U(IV)-U(VI) and the Fe(II)-Fe(III) couples in 1.2 to 7.5 M phosphoric acid were measured at 25° and formal potentials E′ were calculated. Plots of E′_U and E′_Fe vs. log αH₃PO₄, the phosphoric acid activity, were linear, with slopes of (+3.2 ± 0.2) × 0.60/2 mv. and (-1.3 ± 0.1) × 60 mv., respectively. For 1 M phosphoric acid the values are E′_U = -475 ± 10 mv. and E′_Fe = -555 ± 10 mv. vs. the normal hydrogen electrode. Solutions of U(IV), in sharp contrast to Fe(II), in phosphoric acid are very stable to air oxidation. The behavior of iron and uranium ions, singly or mixed, in phosphoric acid solutions was found to be as might be predicted from the single formal potentials. At phosphoric acid concentrations above 2.4 M Fe(II) reduces U(VI), and at concentrations below 1.2 M Fe(III) oxidizes U(IV).