MECHANISM OF CATHODIC AND ANODIC POLARIZATION OF THE URANYL ION IN SULFURIC ACID MEDIA AT DIFFERENT CURRENT DENSITIES

E. K. Spirin

(Presented by the scientific-methodological seminar of the Chemical-Technological Faculty of TPI)

Cathodic waves of uranium were obtained by cathodic polarization of the system electrode—investigated solution \((4.0 \cdot 10^{-3}\ \text{g-ion}\cdot\text{l}^{-1}\) uranyl sulfate against a background of \(0.1 \div 2.5N\ \mathrm{H_2SO_4})\). This relatively high concentration of uranyl ion was chosen in order to obtain a large current, considerably exceeding the intrinsic current of the background.

At low current densities against a background of \(0.2N\) sulfuric acid, three waves are formed with peak potentials of \(-0.3\ \mathrm{V}\), \(-0.9\ \mathrm{V}\), and \(-1.04\ \mathrm{V}\), the second and third waves partially overlapping one another (Fig. 1). The investigations of Sinyakova and Gokhshtein [1, 2] established that the following electrode reactions correspond to these waves:

\[ \begin{aligned} 1)\quad &\mathrm{UO_2^{2+}} + e \rightarrow \mathrm{UO_2^{+}},\\ 2)\quad &\mathrm{UO_2^{+}} + e \rightarrow \mathrm{UO^{2+}},\\ 3)\quad &\mathrm{UO^{2+}} + e \rightarrow \mathrm{UO^{+}}. \end{aligned} \tag{1} \]

Fig. 1.

As the concentration of the background increases, the height of the second cathodic wave gradually decreases, while that of the first increases; however, the sum of the currents of these waves remains constant up to a background concentration of \(1.0N\) (Table 1). The increase in the height of the first cathodic wave becomes understandable if the following is taken into account.

The pentavalent uranium formed as a result of the first electrochemical process disproportionates according to the following scheme:

\[ 2\mathrm{U}^{+5} \rightarrow \mathrm{U}^{+6} + \mathrm{U}^{+4}. \tag{2} \]

The uranium (6+) obtained as a result of reaction (2) is again subjected to electroreduction according to scheme 1.1, as a result of which the height of the wave corresponding to this stage increases. The wave corresponding to process 1.2 disappears almost completely when the concentration of the background exceeds \(1.0N\). This fact is explained by the circumstance that, at a background concentration of \(0.5N\) and higher, the rate of reaction 2 so exceeds the rate of reaction 1.2 that the main mass of tetravalent uranium ions is formed electrochemically. The magnitude of the third cathodic peak with change in the concentration of the background practically

remains constant (Table 1). In fact, the third cathodic peak corresponds to the transition of tetravalent uranium to trivalent uranium, and it is immaterial by what route—electrochemical or chemical—the initial tetravalent form was produced; only its total amount in the solution is important.

Anodic polarization of the electrode—test solution system immediately after preliminary accumulation, at any concentration of the sulfuric-acid background, makes it possible to obtain only two anodic peaks (Fig. 2) with potentials of \(-1.0\) and \(-0.2\) V, corresponding to the stages:

Table 1

Effect of background concentration on the height of the cathodic uranium peaks

1) \(U^{3+} \to U^{4+}\),

2) \(U^{4+} \to U^{6+}\). \(\tag{3}\)

Fig. 2.

The absence of the stage \(U^{4+} \to U^{5+}\) can also be explained by the occurrence of process 2: pentavalent uranium disproportionates before the electrooxidation \(U^{4+} \to U^{5+}\) takes place in amounts sufficient for its registration on the polarogram.

Let us now consider the formation of cathodic and anodic waves under those conditions when uranium reduction during preliminary electrolysis occurs under the action of a high current density (more than \(4\ \mathrm{A/cm^2}\)).

In the polarogram in \(2N\) sulfuric acid, in this case only one cathodic tooth is observed (\(\varphi_p = -1.5\) V), and, as shown in (3), 6 electrons participate in the electrode process:

\[ U^{6+} \to U_{\mathrm{met}}. \tag{4} \]

With a decrease in the background concentration, the height of the tooth decreases. At \(0.7N\) the tooth disappears completely. The disappearance of the tooth is explained by the occurrence of the hydrolysis reaction of metallic uranium (4), as a result of which its trivalent forms are formed, losing the capacity for further reduction (1).

With an increase in the background concentration above \(2.2N\), the depth of the tooth also decreases, but more gradually, which is apparently associated with an increase in the viscosity of the solution.

Anodic dissolution of the uranium amalgam formed as a result of preliminary electrolysis gives two anodic teeth on the polarogram, with peak potentials of \(-1.42\) V and \(-0.2\) V, corresponding to the stages

1) \(U_{\mathrm{met};am} \to U^{4+}\),

2) \(U^{4+} \to U^{6+}\). \(\tag{5}\)

(The fact that the processes proceed precisely in this manner was shown by us earlier [3]).

The depth of the anodic notch corresponding to stage 5,1 remains practically constant in the supporting-electrolyte concentration interval \(1.8—2.2N\), and decreases above and below these limits.

To establish the reversibility of the oxidation and reduction processes of uranium, its cathodic and anodic peaks were compared (Table 2). The degree of reversibility of the cathodic process can theoretically be characterized by the formula

\[ \varphi_{\mathrm{p}/2}^{\mathrm{k}}-\varphi_{\mathrm{p}}^{\mathrm{k}}=2.2\,\frac{RT}{nF}. \tag{A} \]

Approximately, for a reversible anodic notch an analogous relation may be taken as valid:

\[ \varphi_{\mathrm{p}}^{\mathrm{a}}-\varphi_{\mathrm{p}/2}^{\mathrm{a}}=2.2\,\frac{RT}{nF}. \tag{B} \]

Table 2

Reversibility of cathodic and anodic processes for uranium

(However, the question of the possibility of applying equations (A) and (B) to determine the reversibility of stepwise electrode processes is currently debatable).

The reversibility of the stage \(\mathrm{U}^{6+}\rightarrow\mathrm{U}^{5+}\) and the irreversibility of the reverse process may possibly be explained by the fact that these processes proceed differently, for example, through different complexes.

Recently, a number of investigators [5, 6] have shown that the presence of double notches on the polarogram can be explained not only by the stepwise character of the electrode processes taking place, but also by the formation, during preliminary electrolysis, of solid films on the cathode, which, upon their subsequent electro-dissolution, give additional notches on the polarogram along with the notches corresponding to the transition of the amalgam metal into one of the oxidized states. This possibility was taken into account in the present work, and measures were adopted to create the necessary conditions excluding it (selection of a supporting electrolyte with whose anion uranium does not form an insoluble compound, thorough purification of reagents from impurities capable of giving solid films with the supporting-electrolyte anion, and so forth).

As a result of the measures taken, it may be assumed that the formation on the cathode of solid films in quantities sufficient for recording the processes of their electro-dissolution on the polarogram did not occur.

Conclusions

1. The mechanism of the cathodic and anodic polarization of uranium in sulfuric acid media has been examined at different current densities over a wide range of concentrations of the indifferent electrolyte.

2. The reversibility of the cathodic and anodic electrode processes for uranium on a stationary mercury drop electrode has been established.

References

1. S. I. Sinyakova. Abstract of a doctoral dissertation. GEOKhI. Moscow, 1962.
2. Ya. P. Gokhshtein, Gao Tsei-shen. ZhNKh, 6, 1821, 1961.
3. E. K. Spirin, A. G. Stromberg. Collection of Works by Postgraduate Students of TPI, 1965, in press.
4. A. P. Vinogradov. Physicochemical Methods for Control of Uranium Production, Moscow, 1955.
5. Kh. Z. Breinina, T. A. Rygailo, V. V. Belyavskaya. Zav. lab., 29, 393, 1963.
6. Kh. Z. Breinina, E. M. Roizenblatt, V. V. Belyavskaya. Zav. lab., 28, 1047, 1962.