The Isotopic Exchange Reaction between U(IV) and U(VI) Ions in a Mixed System of Acidic Solution and Cation Exchange Resin, (II):Mechanism of Exchange Reaction in Homogeneous Phase

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The mechanism of the exchange of uranium isotopes between U(IV) and U(VI) ions in resinous phase was investigated, and analysis of the results also led to a clearer understanding of the nature of isotopic exchange in solution systems containing no resin. From the consideration that the rate of isotopic exchange in the homogeneous phase is dependent on the degree of "hydrolysis of tetravalent uranium : U+4+n•H2O ?? U(OH)n+(4-n)+n•H+" the following approximate equation was derived for the apparent exchange rate constant: k=Kn, pk4/[H+]v[1+ ?? Kn, n/[H+]n]The experimental and analytical results are as follows : (1) At moderate acidities, the apparent exchange rate is inversely proportional to the third powerof hydrogen ion concentration in both the resinous phase and the solution system. Isotopic exchange occurs most readily between U(VI) and the hydrolyzed complex U(OH)3+ of U(IV). (2) The apparent rate constant for the isotopic exchange in the resinous phase is larger than that in the solution system by 3 to about 4 orders of magnitude. The cation exchange resin used seems to have a catalytic property that accelerates the isotopic exchange.
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