J O U R N A L O F THE AMERICAN CHEMICAL SOCIETY (Registered in
U. S. Patent Office) (Copyright, 1950, by the American Chemical Society)
OCTOBER 16, 1950
VOLUME 72
NUMBER 10
[CONTRIBUTION FROM THE ARGONNE NATIONAL LABORATORY]
Exchange Reactions of Uranium Ions in' Solution BY ELIZABETH RONA' Exchange reactions with heavy isotopes have received increasing attention in recent years. The experiments presented here were undertaken to obtain information on the kinetics and mechanism of the exchange of uranium in the four and six oxidation states in hydrochloric acid solution. The exchange studied is represented by U(1V)
+ U*(VI) = U*(IV) + U(V1)
where the ion with the asterisk is labelled with the artificial radioactive isotope U233. Recently Betts2 reported experiments on the exchange of U(IV) and U(V1) ions in sulfuric acid solution. King3 previously reported measurable exchange in three molar perchloric and six molar hydrochloric acid.
Experimental Materials.-Uranyl chloride solution was prepared from purified uranyl nitrate hexahydrate. A weighed amount added (in the form of uranyl nitrate hexahydrate) was converted t o uranyl chloride by repeated evaporation to dryness with hydrochloric acid. A stock solution of 0.300 molar riranyl chloride in 0.140 molar hydrochloric acid was prepared, and this solution was diluted t o the proper concentration of uranyl chloride for each experiment with 0.140 molar hydrochloric acid. Spectroscopic analysis showed that the impurities were less than 0.1%. The uranium tetrachloride was a resublimed product. Chemical analysis indicated a ratio U:C1 of 1 t o 3.94. It was dissolved in oxygen-fFee hydrochloric acid of a concentration which by trial gave the desired PH a t a time two to five min. after mixing the uranium tetrachloride solution with an equal volume of stock uranyl chloride solution. For example, 0.136 t o 0.139 molar hydrochloric acid, depending on the concentration of the U(1V) and U(V1) ions, was used t o obtain reaction mixtures of PH 0.85. The uranium tetrachloride solution was prepared immediately before the beginning of each experiment to avoid oxidation. (1) Oak Ridge Institute of Nuclear Studies, Oak Ridge, Tennessee. (2) H. H. Betts, Can. J . Research, P6, 702 (1948). (3) E. L. King, Manhattan District Declassified Document MDDC-813 (1847).
The alpha emitter Uzss was used as a tracer,' since its specific activity is high compared t o that of Uess, and a correction can be applied for the U*s8activity. The Ut88 was kindly supplied by F. T. Hagemann. Experiments.-Equal volumes of uranium tetrachloride and uranyl chloride solutions were mixed in a reaction vessel which had an inlet and outlet for nitrogen. The vessel was immersed in a water-bath, the temperature of which could be maintained within 0.05'. The exchange mixture was kept in an atmosphere of oxygen-free nitrogen, saturated with water vapor, throughout the experiments. The time of mixing was taken as the starting time of the experiment. Samples for analysis were forced by nitrogen pressure through a side arm into calibrated centrifuge cones at convenient time intervals. The reaction was stopped by chilling the sample in an ice-bath, and the sample was then analyzed. For the interpretation of the results it was im ortant to know whether U(1V) could be.oxidized t o U(V1yunder the standard conditions of the experiments. Blanks were run with uranium tetrachloride without the addition of uranyl chloride. A hydrochloric acid solution containing Uta and a few mg. of uranyl chloride was reduced t o the plus four oxidation state with a mercury cathode, in an atmosphere of oxygen-free nitrogen. This was then added t o the uranium tetrachloride solution of the required concentration. Samples were taken a t convenient time intervals, and handled in the same manner as those of the mixture. No measurable oxidation was detected in several days. Of the main factors which might influence the velocity of exchange, those of Ti(1V) and U(V1) concentrations, hydrogen ion concentration, temperature, concentration of an added inert salt, chloride ion concentration, and light were investigated. The concentrations of the U(1V) and U(V1) ions were systematically varied in the range of 0.02 to 0.15 molar. The effect of hydrogen ion concentration was studied in a PH range of 0.85 t o 1.6. The narrow range was imposed by the fact that below 0.85 the exchange is very slow, and at a pH higher than 1.6 hydrolysis becomes troublesome. The PH of the reaction mixture was measured both a t the beginning and a t the end of each experiment with a Beckman PH meter using a glass electrode. The hydrogen ion concentration became constant two or three min. after mixing the U(1V) and U(V1) ions, because the actual concentrations of the two ions remain (4) G. T. Seaborg, J. F. Gofman and R. W. Stoughton, Pkyr. Rev.. 11, 378 (1947).
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essentially constant during the experiment, and the slight hydrolysis sets in quickly after mixing. The effect of an inert salt on the exchange was determined by adding varying amounts of sodium perchlorate to the reaction mixture, and that of chloride ions by adding sodium chloride. Finally, the rate was observed under light and dark couditions. Separation of U ( N ) and U(VI) Ions.-Several methods of separation of U(1V) and U(V1) ions were tested. The method finally adopted was precipitation of U(IV) in hydrofluoric acid as the tetrafluoride. The preci itation i s instantaneous and quantitative, and all the UArI) remains in the supernatant solution. No induced exchange was observed. After separatiou, the precipitate was washed repeatedly with diiute hydrofluoric acid, and dissolved in a mixture of hydrogen peroxide and hydrochloric arid. The total activity and that of the dissolved U(IVj precipitate were determined by evaporating an aliquot of each on a platinum disc and measuring the alpha activity in an air chamber, attached to a counting circuit.5 Three aliquots were counted for each sample. All the experiments were carried out in triplicate. The limit of error for an experiment, including the analytical error, timing, pipetting and counting thr samples, ritiniated to he about *IO@;.
Results Treatment of the Results.--In treating thc results it is convenient to use the fact that the rate of appearance of the tracer in the plus four state must be first order with respect to the concentration of the tracer.6 Let a be the total concentration of U(IV), b the total concentration of U(VI), x the concentration of the tracer in the U(1V) form, and y the concentration of the tracer in the U(VI) form. The rate of the exchange reaction, R, is a function of the concentrations a and b and is a constant in a single experiment. The rate of appearance of x can be expressed h?the following differential equation
I1
A t zero time, N = 0 ; at infinite time x ;= x, and y = y m ; also, ay, = b x , and x y = x m 3. .y... Substituting these values and integrating equation (1 'i the following equation is obtained
+
t' .L- can easily be shown to he equal to the expression = I-y- ( I 1 )activity
(1
tot
