PRODUCTION AND PROPERTIES OF ANODIC URANIUM PEAKS IN THE METHOD OF AMALGAM STRIPPING POLAROGRAPHY

E. K. Spirin, A. G. Stromberg

(Submitted by the scientific-methodological seminar of the Faculty of Chemical Technology)

On the basis of a number of studies [1], we undertook an attempt to obtain anodic uranium peaks in the ASP method by carrying out preliminary electrolysis at high current density and studying the influence of various factors on the anodic uranium peak.

To isolate uranium into an amalgam, it is necessary to have a sufficiently negative potential. But this potential lies in the region of hydrogen evolution from 2 n H₂SO₄, which we used as the supporting background. Therefore, in order to attain it, it is necessary to reach a certain current density of hydrogen evolution. For technical reasons, in the region of hydrogen evolution it is not possible to obtain a stable and reliable value of the electrolysis potential. In this connection, in practice it is more convenient in this case to characterize the electrolysis conditions by the magnitude of the hydrogen-evolution current density (we mean the average current density over the entire electrode surface).

As the cathode, an extruded mercury drop [2] and a mercury film electrode [3] were used; their surfaces were identical and amounted to 0.01 cm². The reference electrode (anode) was an external two-normal sulfate electrode, the potential of which was measured precisely relative to the saturated calomel electrode. The working volume of the solution was 10 ml; preliminary accumulation was carried out for (10–300) minutes with continuous stirring of the solution by a propeller stirrer.

As a result of the subsequent anodic dissolution of the amalgam formed, anodic uranium peaks were obtained (peak potential approximately −1.42 V relative to the saturated calomel electrode).

The factors affecting the height of the anodic peak are considered below.

Electrolysis current density. A change in the current density at the cathode is achieved by changing the voltage of the current source at constant cell resistance. The anodic uranium peak reaches its maximum value at a current density at the cathode of (5.8–6.3) A/cm². In this interval, the peak height is practically independent of current density. At a current density below 4.7 A/cm², the uranium peak disappears. Increasing the current density above 6.3 A/cm² entails a decrease in peak height (Fig. 1), which is caused by an increase in the intensity of hydrogen evolution, the bubbles of which screen the cathode.

Electrolysis time. The dependence of the height of the anodic uranium peak on time has the form of the curve shown in Fig. 2. The cause of the deviation from a straight line is depletion of the solution [4].

The condition for depletion is fulfillment of the inequality \(\sigma \geqslant 0.1\), where

\[ b=\frac{K_1}{zF}\,S\frac{\tau}{v}, \tag{1} \]

\[ K_1=\frac{q}{SC\tau} \tag{2} \]

(\(K_1\) is the electrolysis constant, A·cm/g-ion; \(z\) is the number of electrons participating in the anodic reaction; \(F\) is the Faraday number, coulomb; \(S\) is the electrode surface, cm\(^2\); \(\tau\) is the electrolysis time, sec; \(v\) is the volume of the solution being analyzed, cm\(^3\); \(q\) is the area under the anodic peak, coulomb).

Fig. 1. Dependence of the depth of the anodic uranium peak on the electrolysis current density (background—1.8 N H\(_2\)SO\(_4\); \(\tau_\mathrm{e}=10\) min; \(t=20^\circ\)C; \(C=0.5\cdot10^{-6}\) g-ion/l; film electrode).

Fig. 2. Dependence of the depth of the anodic uranium peak on the electrolysis time (\(C=1\cdot10^{-8}\) g-ion/l; 2N nitric acid; \(t=19^\circ\)C; film electrode).

The value of \(q\), determined by the weighing method, proved to be equal to \(1.2\cdot10^{-6}\) coulomb.

Substituting the values of the remaining quantities entering into equation (2), we obtain

\[ K_1=2\cdot10^4\ \text{A·cm/g-ion}. \]

For this value of the electrolysis constant and a preliminary accumulation time of \(6\cdot10^3\) sec, we have

\[ b=0.1, \]

i.e., depletion occurs in the solution being analyzed.

Under conditions of depletion of the solution, an increase in the electrolysis time will often not produce any appreciable effect in increasing the depth of the anodic peak.

Indeed, the limiting depth of the anodic peak (\(J_{\mathrm{lim}}\)) is determined not by the accumulation time, but by the amount of microimpurity (\(cv\)) contained in the volume \(v\) of the solution being analyzed.

Then

\[ q_{\text{lim.}} = zFcv \tag{3} \]

and

\[ I_{\text{lim.}} = q_{\text{lim.}}\frac{l}{q}, \tag{4} \]

where \(l\) is the depth of the peak, and \(q\) is the area under it at any value of \(\tau\), since we are interested not in the absolute values of \(l\) and \(q\), but in their ratio.

For the case considered above, we have

\[ I_{\text{lim.}} = 4.5\ \mu\text{A}. \]

Uranium concentration and electrode shape. The values of the minimum uranium concentrations amenable to determination on a mercury-film electrode and on an extruded mercury drop were preliminarily determined for a specified sensitivity of the current-recording instrument \((4 \cdot 10^{-8}\ \text{A/mm})\) and a relative error of determination not exceeding 30%. They proved to be, respectively, \(1.1 \cdot 10^{-8}\) and \(7 \cdot 10^{-7}\) g-ion/l.

In the concentration range studied, the increase in the depth of the anodic uranium peak proved to be directly proportional to the change in uranium concentration in the solution (Fig. 3).

Electrolysis temperature. To obtain reproducible results, it is necessary to maintain the electrolysis temperature strictly, since the diffusion coefficients of metals in solution and the kinematic viscosity depend on temperature.

In the range \(10\text{–}90^\circ\text{C}\), over the concentration interval studied, the temperature gradient of the anodic uranium peak is 1.8% per \(1^\circ\text{C}\). As follows from Fig. 4, increasing the temperature from 10 to \(90^\circ\text{C}\)

![Figure 3 and Figure 4]

Fig. 3. Dependence of the depth of the anodic uranium peak on the uranium concentration in solution (2 N sulfuric acid, \(\tau_{\text{e}} = 10\) min; \(t = 23^\circ\text{C}\)): a) film electrode — ○, b) mercury drop — ◐.

Fig. 4. Effect of temperature on the anodic uranium peak \((C = 2.5 \cdot 10^{-8}\ \text{g-ion/l};\ \tau_{\text{e}} = 10\ \text{min};\ 26\text{ N sulfuric acid};\ \text{film electrode})\).

entails a threefold increase in the depth of the peak. Accordingly, the sensitivity of the determination increases.

Conclusions

1. It has been established that uranium gives anodic peaks in the ASP method when preliminary electrolysis is carried out at a high current density at the cathode (about 6 A/cm²).

2. The influence of current density, electrolysis time, uranium concentration, and electrode shape on the height of the anodic uranium peak has been studied.

3. It has been established that, under the conditions studied, the minimum determinable concentration of uranium in solution is about 0.002 μg/ml.

Literature

1. R. Groves, A. S. Russel. J. Chem. Soc., 2805, 1931.

2. A. G. Stromberg, E. A. Zakharova. Zav. lab., 30, No. 4, 1964.

3. V. A. Itolinsky, A. G. Stromberg. In: Methods for the Analysis of Chemical Reagents and Preparations, IREA Publishing House, issue 5–6, 29, 1963.

4. A. G. Stromberg. Izv. SO AN SSSR, 5, 76, 1962.