Reactions of the Electron Products of Radiolysis of Alkali-Halide Compounds in Aqueous Solutions

D. I. Tilik, R. A. Kan, I. I. Shvarts, L. T. Rugaenko

(Submitted by the scientific seminar of the Department of Radiation Chemistry)

The dissolution of irradiated alkali-halide compounds leads to partial recombination of the electron and hole products of radiolysis. Prevention of recombination, which is necessary for determining the concentrations of these products, is achieved by introducing into the solution various electron acceptors (for example, $H^+$, $NO_3^-$) and hole acceptors (for example, $J^-$). It has been found that the directions of the reactions of simple ($F$) and colloidal centers differ. This must be used for their separate determination.

Along with physical methods for studying the composition and structure of the products of radiation-chemical transformation of alkali-halide compounds (AHC), some physicochemical methods based on the measurement of chemical or energetic effects occurring upon dissolution have recently begun to be used [1–7]. By using differentiated solvents, it is possible, in principle, to separate the various chemical forms of electron and hole products that arise during irradiation of AHC. At the same time, the dissolution method has its own difficulties, consisting in the fact that the chemical form of a particle may change during the transition from an AHC into solution; and in each individual case it is necessary to investigate what process occurs upon dissolution.

Earlier [1], on the basis of the results of a study of the composition of the products formed upon dissolution of irradiated and additively colored AHC, the assumption was put forward that $F$-centers in this process generate hydrated electrons in aqueous solution. If this conclusion is correct, then by dissolving irradiated and additively colored

EPR can be used to create solvated electrons in any solvent [8].

The present work is devoted to further study of the mechanism of dissolution of irradiated and additively colored alkali halide crystals and to the development of chemical methods for analyzing products that arise from electron centers during dissolution in aqueous solutions of various composition (pure water, neutral sodium nitrate solutions, acidified ethanol solutions).

Experimental procedure.

Crystals of \(NaCl\), \(KBr\), grown from “chemically pure” grade starting material, were used in the work. Irradiation was carried out with a cobalt-60 source. Crystals additively colored in sodium vapor were also used. The chemical processes during dissolution of the crystals were studied on a chromatographic setup (sensitivity for \(NO_2^-\): \(3\cdot 10^{-10}\) g/cm\(^2\), measurement error \(\pm 8\%\)), equipped with an electronic integrator of the areas of chromatographic peaks. Halogen products of radiolysis were determined spectrophotometrically. All reagents used were of “chemically pure” and “special purity” grades; the water was deionized and twice distilled. Absorption spectra of the colored crystals were measured on an SF-4A.

Experimental results.

The yields of molecular hydrogen and chlorine during dissolution in pure deaerated water of irradiated \(NaCl\) samples that had received different doses were investigated. As shown in Fig. 1, the amount of products formed (\(N_{H_2}\) and \(N_{Cl_2}\)) is linearly related to the number of electron centers, but at low concentrations of the latter stable products are not observed. This indicates a decrease in the contribution of recombination reactions as the dose increases.

Figure 2 presents the dependence of the amount of hydrogen formed during dissolution of additively colored metallic sodium alkali halide crystals in neutral water containing various amounts of sodium nitrate. In the case when dissolved

For NaCl containing only colloidal metal \((\lambda_{\max}\approx 600\ \text{nm})\), the amount of hydrogen formed did not depend on the concen-

Fig. 1. Yield of products upon dissolution of irradiated NaCl in water: 1, 2—the amounts of hydrogen and chlorine, respectively.

Fig. 2. Effect of the \(\mathrm{NO_3^-}\) ion on the formation of \(\mathrm{H_2}\): 1—NaCl, \(\lambda_{\max}\approx 600\ \text{nm}\) (colloid), 2—KBr, \(\lambda_{\max}\approx 630\ \text{nm}\) (\(F\)-centers).

tration of nitrate ions in the solution (curve 1). If, however, KBr containing only \(F\)-centers \((\lambda_{\max}\approx 630\ \text{nm})\) was dissolved, then with increasing concentration of nitrate ions the amount of hydrogen formed decreased (curve 2).

It was shown earlier [1, 2] that dissolution of additively colored crystals in water and in a \(\mathrm{NaNO_3}\) solution is accompanied by the formation of hydrogen and \(\mathrm{NO_3^{2-}}\) according to the reactions:

\[ e_{aq}^{-}+\mathrm{H_2O}\longrightarrow \mathrm{H}+\mathrm{OH}^{-} \qquad K_1^{*}=16 \tag{1} \]

\[ \mathrm{H}+\mathrm{H}\longrightarrow \mathrm{H_2} \qquad K_2=1.5\cdot 10^{10} \tag{2} \]

\[ e_{aq}^{-}+\mathrm{NO_3}^{-}\longrightarrow \mathrm{NO_3}^{2-} \qquad K_3=1.0\cdot 10^{10} \tag{3} \]

\[ 2\mathrm{NO_3}^{2-}+\mathrm{H_2O}\longrightarrow \mathrm{NO_2}^{-}+\mathrm{NO_3}^{-}+2\mathrm{OH}^{-} \tag{4} \]

* The reaction rate constants given here and below, in \(M/\text{mol}\cdot\text{s}\), are taken from [10].

In addition, the following reaction is possible:

\[ e^-_{aq} + e^0_{aq} \longrightarrow H_2 + 2OH^- \qquad K_5 = 0.5 \cdot 10^{10}, \tag{5} \]

In the case of an irradiated crystal, a recombination reaction also proceeds [1, 4]:

\[ e^-_{aq} + Cl^0 \longrightarrow Cl^- \qquad K_6 = 10^{10} [4] \tag{6} \]

The curve 2 presented in Fig. 2 can be treated by the method of stationary concentrations under the assumption that all processes occur in a thin layer directly adjacent to the crystal. The rate of dissolution of the crystal and, consequently, the concentrations of the products in this layer may, as a first approximation, be taken as constant. Then, for the competing pair of reactions (3) and (1), or (3) and (5), we obtain the following expression for the relationship of the amount of hydrogen formed (it is proportional to the average rate of the hydrogen-formation reaction):

\[ 1 + \frac{K_1[NO_3^-]}{K_{1,5}[\alpha]} = -\frac{N_H^0}{N_H}, \tag{A} \]

where \([\alpha]\) is the stationary concentration of hydrated electrons in the case of competition between reactions (5) and (3), or the concentration of water in the case of competition between reactions (1) and (3); \(N_H^0\) and \(N_H\) are the amounts of hydrogen formed at zero and at the given concentration of nitrate ions in the solvent. Solution of this equation gives the value \(K_{1,5}[\alpha] = 4 \cdot 10^4\). This value agrees with the occurrence of both reaction (1) and reaction (5).

Table I presents data on the yield of hydrogen during dissolution of additively colored crystals and metallic sodium in acidic solutions containing or not containing ethanol.

Figure 3 gives data on the hydrogen yield in acidified alcohol solutions during dissolution of irradiated \(NaCl\) (which mainly contains \(F\)-centers); Fig. 3a gives the dependence of the amount of hydrogen formed on the acidity of the medium, and on

Table I. Hydrogen yield \((\mathrm{H}_2/\mathrm{cm}^3,\ 10^{-16})\)
during dissolution of additively colored crystals and sodium.

Fig. 3b — as a function of alcohol concentration at constant acidity.

Fig. 3. \(Y_{\mathrm{H}_2}\) during dissolution of irradiated \(NaCl\):
a — \(3\,M\ \mathrm{C}_2\mathrm{H}_5\mathrm{OH}\) solution; b — \(0.1\,N\ \mathrm{H}_2\mathrm{SO}_4\) solution. Crystal 1 was irradiated with a dose of \(\approx 300\) Mrad, crystal 2 with a dose of \(\approx 100\) Mrad.

In an acidified alcohol solution, as was assumed in [7], the following reactions occur:

\[ e_{aq}^{-} + H^{+} \longrightarrow H \qquad k_7 = 2.3 \cdot 10^{10} \tag{7} \]

\[ H + C_2H_5OH \longrightarrow H_2 + CH_3CHOH \qquad k_8 = 2.0 \cdot 10^{7} \tag{8} \]

\[ \mathrm{Cl}^{0} + \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH} \longrightarrow \mathrm{H}^{+} + \mathrm{Cl}^{-} + \mathrm{CH}_{3}\mathrm{CHOH} \tag{9} \]

\[ 2\mathrm{CH}_{3}\mathrm{CHOH} \longrightarrow \begin{gathered} \text{aldehyde}\\ \text{ethylene glycol} \end{gathered} \tag{10} \]

In the presence of alcohol, owing to reactions (7) and (8), one hydrated electron produces not one hydrogen atom, but a hydrogen molecule. A doubling of the hydrogen yield (see Table I) was observed only in the case of dissolution of an NaCl crystal containing \(F\)-centers. In the dissolution of HTC with colloidal metal and in the dissolution of metallic sodium, alcohol has no effect on the amount of hydrogen formed. This permits the conclusion that reaction (7) is not characteristic of colloidal metal.

The increase in the hydrogen yield with decreasing pH (Fig. 3a) can easily be explained by suppression of the recombination reaction (5) owing to the rapid occurrence of reaction (7). At the same time, as Fig. 3b shows, only at high alcohol concentration do all hydrogen atoms participate in reaction (8), whereas at low alcohol concentration, probably, some of the \(H^{0}\) atoms participate in the reaction

\[ \mathrm{H}^{0} + \mathrm{Cl}^{0} \longrightarrow \mathrm{H}^{+} + \mathrm{Cl}^{-} \tag{11} \]

The data presented in Fig. 3b, under the assumptions stated above

Fig. 4. Graphical solution of the equations.
1, 2 — solutions of equations (B) and (11), respectively.

assumptions, can be treated by the method of stationary concentrations for a scheme including the competing reactions (6), (8), and (9). The equation obtained is:

\[ K'=-\frac{K_6}{K_8\cdot K_9} =\frac{N_{H_2}^{\infty}+N_{H_2}}{N_{H_2}^{2}}\,[C_2H_5OH]^2 =\gamma\,[C_2H_5OH]^2, \tag{Б} \]

where \(N_{H_2}^{\infty}\) and \(N_{H_2}\) are the amount of hydrogen formed at the limiting and at the given concentration of styrene; after taking logarithms, it has the form

\[ \lg K'=\lg \gamma+2\lg [C_2H_5OH]. \tag{В} \]

The graphical solution of this equation is presented in Fig. 4(I). But the slope of the straight line in these coordinates proved to be not two, but unity, i.e., the reaction scheme used does not agree with the experiment.

Discussion of the Results

When the irradiated crystal is dissolved in pure water, the amount of hydrogen evolved is smaller than the number of electrons in the electron centers. The observed difference is explained by the presence of the recombination reaction (6), which apparently proceeds in part in the form

\[ e_{aq}^{-}+Cl_2^{-}\longrightarrow 2Cl^{-} \tag{12} \]

since in this case chlorine very effectively combines with a chlorine ion according to the reaction

\[ Cl+Cl^{-}\longrightarrow Cl_2^{-}. \tag{13} \]

The decrease in the contribution of recombination reactions (6) and (12) as the number of electron centers increases may be associated with an increase in the fraction of complex electron centers, which are less

effectively enter into reactions of the type (1), (5), (6), (12).

In works [5, 6], when determining the stored energy by the calorimetric method, it was assumed that dissolution of the alkali-halide crystal is accompanied only by reactions of the type (5) and (14):

\[ 2Cl(Cl_2,\,2Cl_2^-)+H_2O \longrightarrow H^+ + Cl^- + HClO \tag{14} \]

From the results presented in the present work it follows that the chemical correction determined in [5, 6] is valid for complex electron centers and only for a part of the \(F\)-centers, since for the latter reaction (1) and (5) also occur. For example, upon dissolution of alkali-halide crystals with colloidal metal, the electron acceptors (ions \(H^+\) and \(Ag^+\)) do not change \(N_H\), which is possible only when reaction (5) occurs. Since reaction (5) can be effective only at a high specific concentration of electrons, in this way we obtain some confirmation that such color centers actually are particles of colloidal metal or of some quasimettalic formation.

The different behavior of \(Cl_2^-\), arising from simple and complex electron centers, can be used for separate determination of the amount of \(F\)-centers (\(N_F\)) and colloidal metal (\(N_{\text{col.}}\)) in additively colored crystals. The determination can be carried out in two ways:

1. Dissolution of the crystal in 0.1 N \(H_2SO_4\) gives \(N_e\) [9]—the total number of electrons. The yield of hydrogen in a \(MeNO_3\) solution makes it possible to find \(N_{\text{col.}}\), and then

\[ N_F = N_e - N_{\text{col.}}. \]

1. Dissolution of the sample in a neutral \(NO_3\) solution with measurement of both \(H_2\) and \(NO_2\) makes it possible to find at once \(N_F\) as \(\tfrac{1}{2}N_{NO_2}\) and \(N_{\text{col.}}\) as \(N_H\).

According to preliminary results, in the case of irradiated crystals an analogous procedure can also be used to separate \(N_F\) and \(N_{\text{col.}}\).

Since chlorine atoms in chloride solutions are present in the form of the dichloride ion \(Cl_2^-\), which is evidently less reactive with respect to organic compounds than chlorine atoms, the dependence observed in Fig. 3b ...

the alcohol concentration can be represented as competition of the following reactions

\[ H^{+}Cl^{-} \longrightarrow H^{+}+Cl^{-} \tag{11} \]

\[ H^{+}C_{2}H_{5}OH \longrightarrow H_{2}^{+}C_{2}H_{4}OH \tag{8} \]

\[ Cl+Cl^{-} \longrightarrow Cl_{2}^{-} \tag{13} \]

This scheme leads to the equation:

\[ K''=-\frac{k_{11}}{k_{8}k_{13}}= \frac{N_{H_2}^{0}-N_{da}}{N_{H_2}^{2}}\,[C_{2}H_{5}OH] =\psi [C_{2}H_{5}OH] \tag{г} \]

and in logarithmic form:

\[ \lg K''=\lg \psi+\lg [C_{2}H_{5}OH], \tag{д} \]

which agrees well with the experimental data (see Fig. 4, line 2).

The fact that the hole products do not react rapidly with alcohol makes it possible to determine \(N_e\) and \(N_{rad}\) in one solution in the presence of a suitable halogen acceptor. It should be noted that this method gives the correct answer, evidently, only when analyzing samples with \(F\)-centers (Table 1).

The methods for determining \(N_e\) considered in this work are used for studying various questions of radiolysis of alkali-halide crystals and for a more accurate determination of \(N_e\) in additively colored crystals [9].

The experimental results considered again confirm the possibility of formation of \(e_{aq}^{-}\) upon dissolution of irradiated and additively colored crystals.

The authors express their gratitude to Doctor of Chemical Sciences A. K. Pikaev for discussion of the results and to Candidate of Physical and Mathematical Sciences A. P. Arympov for providing the crystals.

Conclusions

1. Dissolution of irradiated crystals in water leads to partial recombination, the extent of which depends on the concentration and structure of the defects.

2. It has been shown that the reactions of simple ($F^-$) and colloidal electron centers differ. On the basis of this, a method is proposed for the separate determination of $N_F$ and $N_{\text{col}}$.

3. A mechanism is proposed for the processes in an acidic alcoholic solution. Dissolution in it leads to reactions with acceptors in the boundary layer only of the electronic products.

Literature

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5. Phelps T., Pearerstein E. Phys. Rev. 128, 1575 (1962).

6. E. K. Zavadovskaya et al. In the collection “Radiation Physics of Nonmetallic Crystals,” Kiev, 1967, p. 300.

7. Rade G. L. Berichte der Bunsengesellsch. 71, 108 (1967).

8. A. K. Pikaev, “The Solvated Electron in Radiation Chemistry,” Moscow, 1969.

9. K. K. Shvarts, D. E. Ekmanis, Udod, A. F. Lyudina, D. E. Tiliks, R. A. Kan, FTT, No. 3 (1970).

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Institute of Physics, Academy of Sciences of the Latvian SSR

Moscow State University named after M. V. Lomonosov