PHYSICAL CHEMISTRY

D. V. Kokoulina, P. I. Dolin, and Academician A. N. Frumkin

THE ACTION OF RADIATION ON THE POTENTIAL OF A PLATINUM ELECTRODE IN A SULFURIC ACID SOLUTION

The appearance of products of water radiolysis under the action of ionizing radiation on a solution should lead to a change in the oxidation–reduction properties of the medium and affect the electrochemical and corrosion behavior of metals present in the irradiated solution. In those cases where the solution contains no substances entering into oxidative or reductive reactions with the products of water radiolysis, the radiolysis products may interact directly with the electrode, causing the establishment of a definite potential. V. I. Veselovskii and Ts. I. Zalkind first found that, upon irradiation of nitrogen-saturated solutions of H₂SO₄, a potential close to the reversible hydrogen potential is established on Pt, while on Au a potential of ~0.95 V is established. Thus, in the system Pt/solution of H₂SO₄ saturated with nitrogen/Au, a potential difference of ~0.9 V was obtained. These effects were explained by the authors as due to the selective interaction of Pt with the reducing components of water radiolysis (mainly H atoms), and of the gold electrode with the oxidizing components (mainly OH radicals) of water radiolysis (¹). According to Clark’s data (²), a smooth Pt electrode is much more sensitive to the action of radiation than a platinized one, although in principle their behavior is similar.

Developing Allen’s idea concerning the onset of an equilibrium ratio of the concentrations of the oxidized and reduced forms of a substance during prolonged irradiation of a solution, Daynton and Collinson suggested that the state of equality of the rates of oxidation and reduction is characterized by an equivalent oxidation–reduction potential of irradiated water (³). Henderson et al. (⁴) found that, in the presence of an oxidation–reduction indicator (mainly KJ), the steady potential of a Pt electrode in an irradiated solution assumes a value of 0.85 V relative to the hydrogen electrode in the same solution. This value does not depend on the nature of the oxidation–reduction indicator, the dose rate, or the pH, and is taken by the authors to be equal to the oxidation–reduction potential of irradiated water.

In our work the aim was to determine the conditions under which the hydrogen potential and a potential close to 0.85 V are realized on Pt, and to evaluate the role of radical and molecular products in establishing the potential under irradiation. The behavior of a smooth Pt electrode in an H₂SO₄ solution was studied over a wide range of radiation doses and dose rates. Irradiation was carried out with X-rays (voltage 80 kV, maximum current 200 mA) in glass cells of two types (Fig. 1, I and II), in which maximum dose rates of respectively ~3·10¹⁷ and ~7·10¹⁶ eV/cm³·sec could be attained. In cell I, owing to the large meniscus of the liquid and the small thickness of the solution layer (2–3 mm), the hydrogen formed during radiolysis could be removed from the solution during the experiment; in cell II the meniscus of the liquid is very small, and therefore the escape of hydrogen into the gas phase is hindered. In both cells it was possible to replace the solution during the experiment by forcing it from the reservoir vessel B, where it had previously been saturated with the corresponding gas. In some

In the experiments, the cell was a vertical thin-walled tube 7 mm in diameter, through which the solution flowed; a part of the cell 3 mm high, where the electrode was located, was exposed to a horizontal beam. Solutions of 0.8 N H₂SO₄ were prepared from twice-distilled H₂SO₄. Hydrogen and nitrogen for saturating the solutions were purified of traces of oxygen and impurities. Measurements of the potential were made with the aid of a cathode voltmeter; the reference electrode was either a hydrogen electrode or a mercury-sulfate electrode. The ignited Pt electrode was etched in hot aqua regia and washed in hot twice-distilled water; before the experiment the electrode was subjected to alternating cathodic and anodic polarization in the same or in a separate apparatus.

Fig. 1. Schematic representation of the cell for measurements. A — measuring vessel with the solution under investigation; B — auxiliary vessel for preliminary saturation of the solution with the required gas. The diagram does not show the inlet into the measuring cell for entry and exit of gas and the inlet for draining the solution from the cell, which is a thin tube reaching to the bottom, with a stopcock.

Figure 2 shows the dependence of the potential of the Pt electrode on the time of action of the radiation at different dose rates. As is seen from the figure, the potential of Pt in a solution saturated with nitrogen, under the action of radiation, first shifts in the negative direction (section a) and reaches values close to the reversible hydrogen potential (section b). However, the hydrogen potential on Pt is not stable, since with continued irradiation the potential again shifts in the positive direction (section b) to values of ~0.85 V.

Fig. 2. Dependence of the potential of a platinum electrode on the time of action of radiation at different dose rates (in eV/cm³·sec). 1 — 2·10¹⁷, 2 — 5·10¹⁶, 3 — 2·10¹⁶. Solution 1 saturated with nitrogen. ↓ — irradiation switched on, ↑ — irradiation switched off. Measurements in cell I.

During irradiation the potential reaches hydrogen values at an absorbed dose of 3—5·10¹⁸ eV/cm³, independently of the dose rate*. If the yield of molecular hydrogen in 0.8 N H₂SO₄ is taken to be 1.0 molecule/100 eV [5], the concentration of molecular hydrogen in the solution at a dose of 4·10¹⁸ eV/cm³ will be

\[ 1.0 \cdot 4 \cdot 10^{18} \cdot 10^3 / 100 \cdot 6 \cdot 10^{23} = 6.6 \cdot 10^{-5}\ \text{mol/l}. \]

The concentration of hydrogen in H₂SO₄ solution when the solution is saturated with hydrogen at atmospheric pressure and room temperature is ~6·10⁻⁴ mol/l. Since in our experiments the Pt potential during irradiation did not reach the equilibrium hydrogen potential by 10—40 mV, and the temperature of the solution during irradiation rose to 40°, we believe that the observed value of the potential corresponds to the concentration of molecular hydrogen formed during radiolysis. This conclusion is confirmed by the following observations.

1. If irradiation is interrupted when the most negative value of the potential has not yet been reached, the potential continues to shift in the neg—

* Approximately the same dose is required according to the data of Ts. I. Zalkind and V. I. Veselovskii with γ-radiation from Co⁶⁰, in whose experiments the dose rate was 2·10¹⁵ eV/cm³·sec.

in the negative direction; analogous phenomena had previously been observed in our laboratory in nitrate solutions (6). In cell I the potential then shifts to 0.85 V, whereas in cell II the potential remains near the hydrogen value for a long time. This indicates that the potential is established in accordance with the concentration of the molecular hydrogen being formed, and that the stationary state at the electrode is not established instantaneously.

Fig. 3. Dependence of the Pt potential on time during irradiation of a flowing solution (flow rate 0.2 cm³/sec, linear velocity 0.42 cm/sec). The solution is saturated with nitrogen. At point A the flow of the solution was stopped; at point B it was switched on again

Fig. 4. Dependence of the potential of a Pt electrode in 0.8 N H₂SO₄ solution on the concentration of H₂O₂: 1—a nitrogen-saturated H₂O₂ solution is added to a hydrogen-saturated H₂SO₄ solution; 2—H₂O₂ is formed during irradiation of a nitrogen-saturated H₂SO₄ solution

1. Addition to a solution of H₂SO₄ or HClO₄ of an active radical acceptor (4·10⁻³ mole/l KBr), which lowers the concentration of H atoms by ~10³ times, does not change the dependence of the potential on dose in comparison with a solution of pure acid.

2. When the solution flows through the cell, i.e., under conditions in which, during irradiation, the stationary concentration of the molecular products of radiolysis is significantly reduced, while the stationary concentration of radicals changes little in comparison with a quiescent solution, irradiation does not cause a shift of the potential in the negative direction. When the flow is stopped, the potential under irradiation changes according to the same law as in cell II (Fig. 3).

3. In cell II the rise of the potential to values of 0.85 V occurs at a higher dose than in cell I, in which the stoichiometric ratio of the concentrations of H₂ and H₂O₂ in the solution is disturbed in the direction of increasing H₂O₂.

All this indicates that atomic hydrogen does not play an essential role in establishing the hydrogen potential under irradiation. S. D. Levina and T. V. Kalish arrived at the same conclusion regarding the behavior of a Ni electrode under irradiation (7).

A potential of 0.85 V is the stable state of the Pt electrode in irradiated H₂SO₄ solution; moreover, the dose at which this value of the potential is reached depends somewhat on the state of the electrode surface and on how rapidly the gaseous products of radiolysis are exchanged between the solution and the gas phase. Attainment of a potential of 0.85 V in apparatus I occurs upon absorption of 3·10¹⁹ eV/cm³, and in apparatus II upon 2—5·10²⁰ eV/cm³, independently of dose rate*. After it has reached—

* The same dose as in cell II is required also in a cell of volume 6 cm³, completely filled with nitrogen-saturated solution, in which the escape of hydrogen from the solution was eliminated.

... min, irradiation for an hour or more (dose rate \(1 \cdot 10^{17}\) eV/cm\(^3\)·sec) does not change its value. Termination of irradiation in this state also has almost no effect on the magnitude of the potential. Upon repeated irradiation of the same electrode in the same solution after a steady potential has been attained, the potential shifts in the negative direction by only 20–50 mV, and then the potential again returns to the steady value (Fig. 2, curve 2, start of irradiation—point \(A\)). If the solution is replaced by a fresh one, then under irradiation the potential again shifts toward the equilibrium hydrogen value (Fig. 2, curve 2, replacement of the solution—point \(B\)), and then to 0.85 V. Such phenomena are repeated many times on the same electrode when the solution is replaced.

The shift of the potential from the hydrogen value in the positive direction is associated with the accumulation of H\(_2\)O\(_2\) in the solution during irradiation. In Fig. 4, for comparison, curves are juxtaposed for the dependence of the Pt potential in H\(_2\)SO\(_4\) solution on the concentration of H\(_2\)O\(_2\). Curve 1 shows the change in the Pt potential when a solution of H\(_2\)O\(_2\), saturated with nitrogen, is added to an H\(_2\)SO\(_4\) solution saturated with hydrogen. The potential at first changes almost not at all, and at an H\(_2\)O\(_2\) concentration of \(\sim 2.4 \cdot 10^{-3}\) mol/l it shifts sharply in the positive direction to values of \(\sim 0.8\) V. Curve 2 represents the dependence of the potential on dose at a dose rate of \(7 \cdot 10^{16}\) eV/cm\(^3\)·sec, recorded in cell II and recalculated as a dependence of the potential on the concentration of the H\(_2\)O\(_2\) formed (taking the yield of H\(_2\)O\(_2\) to be 0.8 molecule/100 eV). As is seen from Fig. 4, H\(_2\)O\(_2\), irrespective of the manner in which it is introduced into the solution, causes a shift of the potential to values of \(\sim 0.8\) V. The accumulation of H\(_2\)O\(_2\) to a stationary concentration also explains the following results. The hydrogen potential at atmospheric hydrogen pressure does not change under prolonged irradiation. However, the Pt potential in H\(_2\)SO\(_4\) solution at a hydrogen pressure of 0.1 atmosphere, under prolonged irradiation (dose \(\sim 3 \cdot 10^{20}\) eV/cm\(^3\)), shifts to 0.85 V. In the first case, owing to the large exchange current of the hydrogen evolution–ionization reaction, accumulation of H\(_2\)O\(_2\) to the stationary concentration is unable to shift the potential in the positive direction. When the hydrogen concentration is decreased by a factor of 10 and the hydrogen exchange current is correspondingly reduced, reduction of the H\(_2\)O\(_2\) present in the solution at the stationary concentration shifts the potential in the positive direction.

Thus, the potential of a Pt electrode in 0.8 \(N\) H\(_2\)SO\(_4\) solution under the action of radiation is determined by the molecular products of the radiolysis of water accumulating in the solution—hydrogen and hydrogen peroxide. Radical products play no appreciable role in establishing the potential on Pt; the greater part of them evidently recombines in the bulk of the solution and on the electrode surface.

Institute of Electrochemistry
Academy of Sciences of the USSR

Received
26 II 1960

CITED LITERATURE

1. V. I. Veselovskii, Report of the Soviet delegation at the International Conference on the Peaceful Uses of Atomic Energy, Investigations in the Field of Geology, Chemistry, and Metallurgy, Publishing House of the Academy of Sciences of the USSR, 1955, p. 320; Ts. I. Zalkind, V. I. Veselovskii, Collected Works on Radiation Chemistry, Publishing House of the Academy of Sciences of the USSR, 1955, p. 66; Ts. I. Zalkind, V. I. Veselovskii, G. Z. Gochaliev, Proceedings of the First All-Union Conference on Radiation Chemistry, Publishing House of the Academy of Sciences of the USSR, 1958, p. 123; Ts. I. Zalkind, V. I. Veselovskii, Collected Works: The Effect of Ionizing Radiation on Inorganic and Organic Systems, Publishing House of the Academy of Sciences of the USSR, 1958, p. 66.
2. W. E. Clark, J. Electrochem. Soc., 105, 483 (1958).
3. F. S. Dainton, E. Collinson, Ann. Rev. Phys. Chem., 2, 99 (1951).
4. J. H. S. Henderson, E. G. Lovering et al., Canad. J. Chem., 37, 164 (1959).
5. P. I. Dolin, Collected Works on Radiation Chemistry, Publishing House of the Academy of Sciences of the USSR, 1955, p. 20.
6. N. A. Bakh, V. D. Bityukov, Publishing House of the Academy of Sciences of the USSR, Department of Chemical Sciences, in press.
7. S. D. Levina, T. V. Kalish, DAN, 130, 573 (1960).