Chemistry

Corresponding Member of the Academy of Sciences of the USSR I. A. Kazarnovskii,
N. P. Lipikhin and M. V. Tikhomirov

Isotopic Exchange of Oxygen Between the Free Hydroxyl Radical and Water

In radiation chemistry and in the theory of oxidative processes, the free hydroxyl radical plays a major role as an intermediate product.

Despite numerous investigations, only scant and contradictory data are available on the reactivity of this radical (1–4). This situation is apparently due to the absence of a method for obtaining free hydroxyl in sufficient concentrations and without admixtures of atomic hydrogen or ions of heavy metals.

We investigated the isotopic-exchange reaction

\[ \mathrm{O^{16}H + H_2O^{18} = H_2O^{16} + O^{18}H}, \]

using potassium ozonide as a new source of the free hydroxyl radical (5, 6).

The decomposition of potassium ozonide by water occurs instantaneously at room temperature and at 0°, with vigorous evolution of oxygen. When water enriched in \(\mathrm{H_2O^{18}}\) is used, the reaction proceeds according to the equations:

\[ \mathrm{KO_3 + H_2^{*}O = K^{*}OH + OH + O_2} \tag{1} \]

\[ \mathrm{OH + H_2^{*}O = ^{*}OH + H_2O} \tag{2} \]

\[ \mathrm{^{*}OH + ^{*}OH = H_2^{*}O + \frac{1}{2}\,^{*}O_2} \tag{3} \]

\[ \mathrm{^{*}OH + ^{*}OH = H_2^{*}O_2} \tag{4} \]

As investigations in our laboratory have shown, the rate of the disproportionation reaction of hydroxyls is 4–5 times greater than the rate of their dimerization.

If the rate of isotopic exchange of oxygen between free hydroxyl and water (equation (2)) is not negligibly small in comparison with the rates of the disproportionation and dimerization reactions of hydroxyl radicals, then the gaseous oxygen evolved during the decomposition of potassium ozonide by water should be enriched in the isotope \(\mathrm{O^{18}}\). The experiments carried out confirmed this.

The potassium ozonide preparation used had the following composition: \(\mathrm{KO_3}\) 89.3%; \(\mathrm{KOH}\) 8.4%; \(\mathrm{H_2O}\) (in the form of \(\mathrm{KOH \cdot H_2O}\)) 2.3%.

The decomposition of a weighed portion of potassium ozonide by water containing 1.38% \(\mathrm{H_2O^{18}}\) was carried out in the apparatus shown in Fig. 1A.

Reaction vessel 1, with a capacity of 35 cm\(^3\), was equipped with a magnetic stirrer 2 rotating at a speed of 500 rpm. A thin-walled glass capillary 4, 2.2 mm in diameter, to one end of which a glass rod was fused, was filled beforehand in a dry chamber with potassium ozonide powder, weighed, and sealed.

Before the experiment, the capillary with the weighed portion was fused to a glass tube with a lead weight 6 suspended on a thin silk thread from the plug of ground joint 8. The initial solution was introduced into vessel 1, after which the apparatus was connected, by means of ground joint 9, to the part of the setup (Fig. 1B) consisting of gas burette 11 and ampoule 12 for taking oxygen samples. The apparatus made it possible to carry out the decomposition of potassium ozonide in a nitrogen atmosphere or in vacuum.

When the ground-glass stopper 8 was rotated (Fig. 1), the weight and the capillary with the KO₃ powder were lowered, and the capillary, reaching the rough surface of the rotating quartz shell of the stirrer, was gradually destroyed. Introduction of potassium ozonide powder into the solution usually continued for 3–5 min.

Fig. 1. Diagram of the experimental apparatus. A—apparatus for decomposing potassium ozonide with water: 1—reaction vessel, 2—quartz stirrer, 3—magnet, 4—capillary with a weighed portion of KO₃, 5—guide tube, 6—lead weight, 7—silk thread, 8—ground-glass stopper for lowering the capillary, 9—ground joint. B—apparatus for investigating the isotopic composition of oxygen during decomposition of KO₃ with water: 10—apparatus for decomposing potassium ozonide with water; 11 and 16—gas burettes, 12 and 17—ampoules for collecting oxygen samples, 13—rotating magnet, 14 and 15—apparatus for dehydration and decomposition of hydrogen peroxide, 18—trap, 19—McLeod gauge, 20—Toepler pump.

The oxygen liberated (in a mixture with nitrogen) was collected in gas burette 11 and then transferred into evacuated glass ampoules 12 for mass-spectrometric analysis. The analysis was carried out on a Nier-type mass spectrometer, similar to that described previously (⁶). The error in the determinations of O¹⁸ did not exceed ±2% of the measured value. Cylinder oxygen was used as the standard; its O¹⁸ content was taken to be 0.204%. The degree of exchange was referred to the total amount of hydroxyl radicals, subtracting the amount used for formation of hydrogen peroxide (which was determined with 0.1 N KMnO₄ solution). In the experiments described, no more than 15% of the hydroxyl radicals went to formation of H₂O₂.

Table 1 contains the results of determinations of the isotopic composition of oxygen liberated during decomposition of potassium ozonide with heavy water, and the degree of exchange found. As is seen from these data, the degree of exchange of the free hydroxyl radical with water is about 10% at +20 and 0° and does not depend on the pH of the solution. This independence from the pH of the solution confirms that exchange actually occurs between free hydroxyl and water, and not between hydroxyl and hydroxyl ions.

In interpreting the relatively low degree of exchange, it must be taken into account that this reaction competes with the very rapid reactions of disproportionation and dimerization of free hydroxyl radicals.

Table 1

Isotopic exchange of free hydroxyl with water containing 1.38% H\(_2\)O\(^{18}\)

\(^{1}\) Here and below, 0.35 \(N\) H\(_2\)SO\(_4\).

\(^{2}\) This control experiment showed that, upon decomposition of potassium ozonide with twice-distilled water, the evolved oxygen has a normal isotopic composition.

It was also of interest to determine the isotopic composition of the oxygen in the hydrogen peroxide formed. In this case one could expect a several-fold greater enrichment of the hydrogen peroxide by the isotope O\(^{18}\) than in the oxygen evolved directly upon decomposition of KO\(_3\) with heavy water, since the molecular oxygen, which constitutes more than \(4/5\) of all the oxygen (equation (1)), does not exchange with water.

The experiments performed fully confirmed this prediction. The complicating circumstance in this case was the small amount of hydrogen peroxide formed under the conditions of our experiments. In these experiments, decomposition of KO\(_3\) with water containing 1.57–3.00% H\(_2\)O\(^{18}\) was carried out in an apparatus with a magnetic stirrer, similar to that described above, but with a capacity of 10 cm\(^3\).

After decomposition of the potassium ozonide with water, the hydrogen peroxide formed was determined in a portion of the solution by titration with 0.1 \(N\) KMnO\(_4\) solution. Another portion of the reaction solution (7 cm\(^3\)) was transferred from apparatus 10 into flask 14, and 3 cm\(^3\) of acidified 0.16 \(N\) cerium sulfate solution was placed in ampoule 15. Preliminary degassing of the solutions was carried out by repeated freezing and thawing under vacuum (0.01 mm Hg). Then, with the aid of a Toepler pump 20, a vacuum of \(1–3 \cdot 10^{-3}\) mm Hg was established in the system, after which the hydrogen peroxide was distilled at room temperature from the solution into ampoule 15, placed in a Dewar vessel with solid carbon dioxide.

After completion of the distillation, the system was again evacuated to \(10^{-3}\) mm Hg, the contents of ampoule 15 were thawed, and the hydrogen peroxide was decomposed by cerium sulfate; the reaction proceeds according to the equation:

\[ 2\mathrm{Ce}^{4+} + \mathrm{H_2O_2^*} = 2\mathrm{Ce}^{3+} + 2\mathrm{H}^+ + \mathrm{O_2^*}. \tag{5}^{1} \]

The evolved oxygen was transferred into ampoules 17 for mass-spectrometric analysis.

Table 2 gives data on the isotopic composition of the oxygen of hydrogen peroxide. From these data it follows that, in the oxygen of hydrogen peroxide

\(^{1}\) It has been established that in this reaction oxygen is evolved quantitatively from hydrogen peroxide and its isotopic composition does not depend on the isotopic composition of the water (\(^{4,7,8}\)).

Table 2

Isotopic composition of oxygen in hydrogen peroxide formed during decomposition of potassium ozonide by water enriched with $\mathrm{H_2O^{18}}$ at 0°1

the enrichment with the isotope $\mathrm{O^{18}}$ was 2.2–4.7 times (or, on average, 3 times) greater than that of the oxygen evolved directly during decomposition of potassium ozonide by water.

The deviation of the observed degree of enrichment from the calculated value (the calculation required an increase not by 3 times, but by 5–5.5 times) should be attributed to errors due to the very small amounts of oxygen (1–2 cm³) evolved from hydrogen peroxide under the conditions of our experiments, the difficulty of complete degassing of the $\mathrm{H_2O_2}$ solutions, and other factors.

The results obtained confirm that, during the decomposition of potassium ozonide by water, hydrogen peroxide is indeed formed from free hydroxyl radicals.

Considering the question of the character of diffusion of free hydroxyl in the radiolysis of water, Dainton[^3], on the basis of unpublished experiments that showed the absence of rapid exchange between the free hydroxyl radical and water, came to the conclusion that this radical diffuses by the normal mechanism and not by the Grotthuss mechanism.

Our experiments, which have revealed very rapid exchange, lead to the opposite conclusion and indicate the presence of interaction of free hydroxyl radicals with water in the spirit of Grotthuss’s views.

This result appears to be important for understanding the reactivity of the free hydroxyl radical.

Scientific Research Physicochemical Institute
named after L. Ya. Karpov

Received
18 II 1958

REFERENCES

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6. I. A. Kazarnovskii, Z. A. Bagdasar’yan, N. P. Lipikhin, “A new source of free hydroxyl radical.” Communication at the All-Union Conference on Chemical Kinetics, July 23, 1955 (in preparation for publication).
7. B. V. Ottesen, L. M. Gol’din, M. I. Men’shikov, T. M. Kukavadze, Vopr. med. khim., 2, 79 (1950).
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1. In experiment No. 32 water containing 1.57% $\mathrm{H_2O^{18}}$ was used; in experiments Nos. 34, 35, 56, and 57—2.78% $\mathrm{H_2O^{18}}$; in experiments Nos. 51 and 52—3.00% $\mathrm{H_2O^{18}}$. ↩