On the Hydrolysis of the Tetravalent Uranium Ion U⁴⁺

Abstract

The formation of the hydrolytic complexes of U(IV) has been investigated at 25.00°C by measuring the Emf of a cell containing inert metal sponge and glass electrodes, in 3 mol·dm⁻³ aqueous NaClO₄ solutions containing both U(VI) and U(IV) at acidities ranging between 10^−0.5 and 10^−1.8 mol·dm⁻³. The lower limit of acidity is imposed by the precipitation of the sparingly soluble oxides of U(IV) whereas the upper limit has been chosen in order to avoid large corrections for the liquid junction potential. The experimental data have been treated using classical graphical methods (Rossotti, F.J.C., Rossotti, H.: The Determination of Stability Constants. McGraw Hill Book Company, Inc. New York (1961)) and by the computerized least-squares program LETAGROP-ETITR (Sillen, L.G., Warnqvist, B.: Arkiv för Kemi. 31, 315–39 (1969)). The results can be explained by assuming the formation of two mononuclear complexes, U(OH)³⁺ and $\mathop {{\rm U(OH)}}\nolimits_4^0$. The obtained formation constants in the 3 mol·dm⁻³ NaClO₄ ionic medium are: ${\rm log}_{10}^\ast\beta_{1}=-1.62\pm 0.02$ and ${\rm log}_{10}^\ast\beta_{4} = -7.70 \pm 0.04$. The formal standard potential ^* E _6,4 for the couple UO₂ ^{2 +}/U^{4 +} at the same conditions is 335.7± 0.5 mV.