Abstract
Full Text
PHYSICAL CHEMISTRY
V. N. Shubin
ON THE NATURE OF THE REDUCING PARTICLE ARISING UNDER THE ACTION OF RADIATION ON WATER AND AQUEOUS SOLUTIONS
(Presented by Academician A. N. Frumkin, 26 III 1964)
A detailed study of the mechanism of radiation-chemical transformations and the determination of the kinetic characteristics of the active particles formed under the action of radiation are closely connected with the question of the nature of the particles themselves. Recently the view has been gaining ground that, if the oxidizing component can be identified with the already known radical OH, then with the reducing radical the matter is far from so simple (^1). It has been suggested that the reducing radical is not an H atom, as was originally believed, but is a free thermalized electron interacting with one or several water molecules \((e^- \cdot n\mathrm{H_2O})\) (^2, ^3). The properties of such a particle, called the polaron, should differ very substantially from those of ordinary atomic hydrogen formed, for example, by the following reaction
\[ \mathrm{H_2 + OH \to H_2O + H}. \tag{1} \]
By kinetic analysis of a number of experimental data it was found that the value of the ratio of the rate constants of the reactions
\[ \mathrm{H_2O_2 + (e^- \cdot nH_2O) \to OH + OH_{aq}^-}, \tag{2} \]
\[ \mathrm{O_2(e^- \cdot nH_2O) \to O_{2aq}^{-}}, \tag{3} \]
is equal to \(\sim 0.5\) (^1), whereas for analogous reactions involving atomic hydrogen (^4)
\[ \mathrm{H_2O_2 + H \to H_2O + OH}, \tag{2'} \]
\[ \mathrm{O_2 + H \to HO_2} \tag{3'} \]
it is substantially smaller and amounts to \(2.2 \cdot 10^{-3}\). If this is true, then in solutions of \(\mathrm{H_2O_2}\) containing high concentrations of \(\mathrm{H_2}\), where the reaction
\[ \mathrm{H_2O_2 + OH \to HO_2 + H_2O} \tag{4} \]
is completely suppressed and all OH radicals are transformed into atomic hydrogen by reaction (1), the following very characteristic dependence of the yield of hydrogen peroxide formation on the concentration of the introduced oxygen should be observed. At zero oxygen concentration \(G(\mathrm{H_2O_2})\) is minimal, since under these conditions \(\mathrm{H_2O_2}\) decomposes by a chain mechanism that includes reactions (1), (2), and (2′). The presence of traces of oxygen \(((\mathrm{O_2}) \sim 0.01(\mathrm{H_2O_2}))\) will inhibit the chain destruction of the peroxide owing to the effective capture by it of H atoms. However, destruction of \(\mathrm{H_2O_2}\) by the polaron formed under the action of radiation by reaction (2) will still take place under these conditions and, consequently,
\[ G_{(\mathrm{H_2O_2})}=G_{\mathrm{H_2O_2}}-\tfrac{1}{2}(G_{\text{pol}}-G_{\mathrm{OH}}). \]
The course of reaction (2) will be suppressed at substantially higher oxygen concentrations \(((\mathrm{O_2}) \geqslant 5(\mathrm{H_2O_2}))\), which will lead to an increase in the peroxide formation yield up to a value
\[ \sim \left[ G_{\mathrm{H_2O_2}}+\tfrac{1}{2}(G_{\text{pol}}+G_{\mathrm{OH}}) \right]. \]
With a further increase in the oxygen concentration the yield will remain unchanged.
In the case of formation during radiolysis of a particle identical to the radical arising by reaction (1), the presence of traces of oxygen will completely suppress the destruction of peroxide by H atoms. Then at \((\mathrm{O_2}) \geqslant 0.01(\mathrm{H_2O_2})\) no dependence of the yield of \(\mathrm{H_2O_2}\) formation on the oxygen concentration should be observed. Thus, knowledge of the experimental dependence makes it possible to draw a reliable conclusion about the difference,
or, conversely, a similarity of the reducing particles arising in two different reactions.
Below are reported the results of determining the dependence of the yield of hydrogen peroxide formation on the concentration of introduced oxygen under the action of Co\(^{60}\) \(\gamma\)-rays on neutral aqueous solutions of \(\sim 10^{-3}\,M\) H\(_2\)O\(_2\), saturated with H\(_2\) at 100 atm. The oxygen pressure above the solution was varied from 0.1 atm to \(\sim 10\) atm. The apparatus for working under pressure was described earlier \((^{5})\).
Fig. 1. Dependence of the H\(_2\)O\(_2\) concentration on dose
Fig. 2. Dependence of the yield of H\(_2\)O\(_2\) formation on the oxygen pressure above the solution. The dashed curve is the dependence expected in the case of the existence of the polaron \((k_2/k_3 \simeq 0.5)\)
The working solutions were prepared with twice-distilled water. Chemically pure H\(_2\)O\(_2\), twice distilled under vacuum, was used for preparing the solutions. The H\(_2\)O\(_2\) concentration was determined spectrophotometrically from the amount of oxidized ferrous sulfate. The dose rate was \(\sim 1.2 \cdot 10^{15}\) eV/cm\(^3\)\(\cdot\)sec. For each concentration of O\(_2\) above the solution, H\(_2\)O\(_2\) accumulation curves were obtained. A characteristic curve is shown in Fig. 1. The initial yields of hydrogen peroxide formation calculated from these data are plotted in Fig. 2 as a function of the oxygen pressure above the solution. As is seen from Fig. 2, the yield is almost constant throughout the entire region studied. The yield value, \(2.45 \pm 0.12\) mol/100 eV, is somewhat smaller than the value determined in the absence of H\(_2\)O\(_2\) and equal to \(G_{(\mathrm{H_2O_2})} = G_{\mathrm{H_2O_2}} + 1/2(G_{\mathrm{pol}} + G_{\mathrm{OH}}) = 3.15 \pm 0.15\) mol/100 eV. A certain increase in the yield at very high O\(_2\) concentrations is apparently due to capture of H atoms, which under ordinary conditions form H\(_2\). The dashed line shows the form of the dependence that should have been observed with the characteristics indicated for the polaron \((k_2/k_3 \simeq 0.5)\).
Analysis of the dependence obtained shows that the active particle formed under the action of radiation is, in its kinetic characteristics, evidently close to the radical arising in reaction (1).
The data of Fig. 2 make it possible to estimate the upper limit of the ratio of the rate constants of the reactions of the reducing radical arising under the action of radiation with oxygen and with peroxide. Indeed, it is obvious that the fraction of reaction (2) at the lowest oxygen concentration studied is smaller than the experimental error, i.e., the inequality holds
\[ 2\Delta G_{(\mathrm{H_2O_2})} = 0.37 > \frac{G_{\mathrm{H}}}{1 + \dfrac{k_3}{k_2}\dfrac{(\mathrm{O_2})}{(\mathrm{H_2O_2})}}, \tag{I} \]
whence \(k_2/k_3 < 2 \cdot 10^{-2}\). A similar estimate can also be made from the data of an experiment in which the dissolved oxygen was consumed by \(\sim 90\%\). Its concentration at the end of irradiation was \(\sim 10^{-5}\,M\)* (initial \(\sim 1.4 \cdot 10^{-4}\,M\)). Under these conditions no appreciable change in the instantaneous \(G_{(\mathrm{H_2O_2})}\) was observed. Since the accuracy of determining the instantaneous yield here is considerably lower, in the calculation the experimental error was taken to be \(\sim 20\%\).
* The estimate was made from the amount of H\(_2\)O\(_2\) formed.
Then from equation (I) we obtain \(k_2/k_3 < 5 \cdot 10^{-3}\). This value is in close agreement with the value found\(^{(4)}\) for atomic hydrogen arising by reaction (1). Thus, with a sufficient degree of reliability it may be concluded that the nature of the radicals formed in both processes is probably one and the same.
To explain some of the results obtained\(^{(6)}\) in aqueous solutions of organic substances, it was assumed that even under the action of radiation on water, both forms of the reducing radical are formed simultaneously: the polaron and the H atom, with yields \(G_{\text{pol}}\) and \(G'_{\mathrm H}\).
Fig. 3. Graphical solution of equation (III) from the data of work \(^{(6)}\)
In acidic solutions, in the absence of active acceptors of the polaron, the latter, by the reaction
\[ (e^- \cdot n\mathrm H_2\mathrm O) + \mathrm H^+ \to \mathrm H_{\mathrm{aq}} \tag{4} \]
is converted into atomic hydrogen, which reacts with the organic molecule RH with formation of \(\mathrm H_2\). Additions of organic acceptors specific for the polaron (such as acetone is considered to be, for example) decrease the yield of molecular hydrogen as a result of capture of the polaron by these acceptors. With increasing acetone concentration, \(G_{(\mathrm H_2)}\) decreases to \(G_{\mathrm H_2} + G'_{\mathrm H}\). The additional hydrogen yield is due to the reaction of “primary” atomic hydrogen with the basic acceptor, which is suppressed in the presence of inorganic acceptors (for example, ferricyanide) that capture both the polaron and the H atom.
It turns out, however, that the results mentioned can easily be explained by assuming that the atomic hydrogen formed under the action of radiation interacts with acetone by two reactions:
\[ \mathrm{CH_3COCH_3} + \mathrm H \begin{cases} \to \mathrm{CH_3COHCH_3}, & \tag{5}\\ \to \mathrm{CH_3COCH_2} + \mathrm H_2, & \tag{5'} \end{cases} \]
Then, assuming that the radiolysis mechanism includes reactions (5), \((5')\), as well as the reactions
\[ \mathrm H + \mathrm H^+ \to \mathrm H_2^+, \tag{6} \]
\[ \mathrm{RH} + \mathrm H_2^+ \to \mathrm R + \mathrm H_2 + \mathrm H^+, \tag{7} \]
\[ \mathrm{RH} + \mathrm{OH} \to \mathrm H_2\mathrm O + \mathrm R, \tag{8} \]
we obtain the following expression for the yield of molecular hydrogen in acidic solutions:
\[ G_{(\mathrm{H}_2)}=G_{\mathrm{H}_2}+ \frac{G_{\mathrm{H}}}{1+\dfrac{k'_5(\mathrm{Ac})}{k_5(\mathrm{Ac})+k_6(\mathrm{H}^+)}} . \tag{II} \]
Transforming it, we finally have:
\[ \frac{k'_5}{k_5}+\frac{k_6(\mathrm{H}^+)}{k_5(\mathrm{Ac})} = \frac{G_{(\mathrm{H}_2)}-G_{\mathrm{H}_2}} {G_{\mathrm{H}}+G_{\mathrm{H}_2}-G_{(\mathrm{H}_2)}} =\varphi(G). \tag{III} \]
Figure 3 presents a graphical solution of this equation using data on the dependence of the hydrogen yield in isopropanol solutions on the acetone concentration and the acidity of the solution, as well as when both are varied simultaneously. The results of all the experiments fall well on a single straight line. This gives grounds to assume the existence of one type of primary reducing radical. The slope of the straight line is \(k_6/k_5 \simeq 3.3\), and the intercept on the ordinate axis is \(k'_5/k_5 \simeq 0.2\).
The idea that one type of reducing radical is formed under the action of radiation on water is apparently also supported by considering data on the dependence of the hydrogen yield on the concentration of added ferricyanide during irradiation of solutions of three different substances—isopropanol, glucose, and glycerol (7). In this case ferricyanide captures H atoms by the reaction
\[ \Phi+\mathrm{H}\to(\Phi)^-+\mathrm{H}^+, \tag{9} \]
which competes with reaction (6). Then, for the hydrogen yield, the following expression is valid:
\[ G_{(\mathrm{H}_2)}=G_{\mathrm{H}_2}+ \frac{G_{\mathrm{H}}}{1+k_9(\Phi)/k_6(\mathrm{H}^+)} . \tag{IV} \]
Fig. 4. Graphical solution of equation (V) using the data of work (7)
A graphical solution of this equation, transformed to the form
\[ 1+\frac{k_9(\Phi)}{k_6(\mathrm{H}^+)} = \frac{G_{\mathrm{H}}}{G_{(\mathrm{H}_2)}-G_{\mathrm{H}_2}}, \tag{V} \]
is presented in Fig. 4. The slope of the straight line obtained is \(k_9/k_6 \simeq 6.2\cdot 10^{-2}\).
Thus, the data presented in this work can be fully explained on the assumption that the reducing radicals arising under the action of radiation on water and through reaction (1) have the same nature, and that under the action of radiation reducing particles of only one kind are formed.*
In conclusion, I consider it my duty to express gratitude to Prof. P. I. Dolin for participating in the discussion of the results and for valuable advice.
Institute of Electrochemistry
Academy of Sciences of the USSR
Received
16 XII 1963
CITED LITERATURE
- A. O. Allen, Radiation Chemistry of Water and Aqueous Solutions, Moscow, 1963.
- J. Weiss, Nature, 186, 751 (1960).
- J. T. Allan, G. Scholes, Nature, 187, 218 (1960).
- C. J. Hochanadel, Radiation Res., 17, 286 (1962).
- V. N. Shubin, P. I. Dolin, ZhFKh, 34, 2480 (1960).
- J. Rabani, G. Stein, J. Chem. Phys., 37, 1865 (1962).
- J. Rabani, G. Stein, Trans. Farad. Soc., 58, 2150 (1962).
* However, it is apparently impossible to assert with complete certainty that the primary radical is a hydrogen atom, since the oxidation reaction of molecular hydrogen by the OH radical in an aqueous medium may proceed according to the equation:
\[ \mathrm{H}_2+\mathrm{OH}\to e^-_{\mathrm{hydr}}+\mathrm{H_3O^+}_{\mathrm{hydr}} \tag{1′} \]