No. 251 　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1970

Determination of Hole Products of Radiolysis in Alkali-Halide Compounds

V. E. Tiliks, O. N. Vikhreva, K. N. Rubina

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

Under the action of ionizing radiation in alkali-halide compounds, hole products of radiolysis are formed—hole color centers ($V$-centers) and free halogen. At the first stage of irradiation, $V$-centers are formed, the concentration of which rapidly reaches saturation. Upon dissolution, the hole products disproportionate and form the ion $X_3^-$, which is a strong oxidizing agent. A number of acceptors have been proposed for its determination: potassium iodide, para-aminodimethylaniline (PADA), 3–3′-naphthidine, and others.

Until now, mainly the electron products of radiolysis have been studied (simple and complex color centers, colloidal metal). However, the study of hole products is of great interest for the radiation physics of solids in connection with such phenomena as the formation of a new phase, gas evolution, diffusion, etc.

For the study of hole products of radiolysis, physical methods are used: radiospectroscopy [1] and optical spectroscopy. Nevertheless, the question of the structure of hole centers, of the formation of a gas phase in the solid, and of the relationship of color centers with free halogen has still not been fully clarified. These questions can be resolved by means of a chemical method of analysis [2–6]. In work [2], for KClO₃ crystals the optical parameters of $V$-bands were determined. Studies of the structure of hole color centers ...

work in alkali-halide crystals was devoted to this [3]. In [4] an attempt was first made to determine free chlorine in irradiated $NaCl$ using the acceptor o-toluidine. The absence of stoichiometry between the yield of free chlorine and of localized electrons is explained by a parallel reaction occurring between the acceptor and electronic products. In [5;6], the acceptor $J^{-}$ was used to determine $Na^{0}$ in a number of alkali-halide crystals.

The examples cited characterize the need to apply chemical methods, but leave a number of questions unexplained. Thus, the chemical processes occurring with hole color centers during dissolution have not yet been studied, and no acceptors have been found for eliminating the influence of electronic products.

The present work is devoted to elucidating the nature of localized hole products during dissolution and to investigating a number of acceptors.

Experimental procedure

The objects of study were single crystals of $NaCl$, $KCl$, $KBr$, and $KJ$, grown by the Kyropoulos method from “chemically pure” grade starting material. Irradiation was carried out in a radiation $\gamma$-loop ($E_{\gamma}=1.4$ MeV; dose rate 2 Mrad/hour, $T=30^{\circ}C$). The $KJ$ crystals, in addition, were additively colored in $J_{2}$ vapor. Optical absorption spectra were recorded on an SF-4 at room temperature and at the temperature of liquid nitrogen. Dissolution was carried out in twice-distilled water; indicators ($J^{-}$, methyl orange, PADA, etc.) of “chemically pure” grade were used.

Experimental results

Work on determining the chemical forms of localized hole radiolysis products was carried out in the following order: optical absorption spectra were recorded for the crystals; some of them were analyzed chemically for $Me$ [7], and some for $X^{0}$, with PADA serving as the indicator.

It had first been established that stoichiometry is maintained between \(N_e\) and \(N_{x_2}\), after subtracting the \(X_2\) that has diffused into the gas phase [6]. In crystals at low doses, the amount of \(X_2\) in the gas phase is negligible (for \(KCl\) and \(NaCl\), 1–5%). The measured spectra of impurity bands at \(77^\circ K\) agreed well with the literature data [8] and showed that irradiation at \(-30^\circ C\) leads to the formation of \(V_2\)- and \(V_3\)-bands. In the spectra of \(KJ\), at doses greater than 60 Mrad, we observed optical absorption at 300 nm, characterized in [9] as the band of colloidal \(J_2\). For similar chlorides no such bands were observed. Measurement of the spectra at room temperatures gave a general, asymmetric band (Fig. 1), which is the sum of the \(V_2\)- and \(V_3\)-bands.

Fig. 1. Spectrum of an irradiated \(KCl\) crystal at different temperatures (Dose \(= 170\) Mrad)

Absorption at the maximum of this band \((\alpha_\nu)\) was used as a characteristic parameter for the increase in the concentration of \(V\)-centers with dose. Figure 2 shows the kinetic growth curves of \(N_e\), \(N_{Cl^\circ}\), and \(\alpha^{222\text{ nm}}\) for \(KCl\) under \(\gamma\)-irradiation. The stoichiometry of \(N_e\) and \(N_{Cl^\circ}\) is preserved over the entire interval; their growth in the second

the coloration stage is linear; \(\mathcal{E}_v\) rapidly reaches a maximum and remains constant.

Fig. 2. Kinetic curves for the formation of electronic and hole products in KCl (1—total number of localized electrons; 2—total number of localized hole products; 3—absorption coefficient of the hole band at room temperature).

The dissolution of such samples was carried out in water at various pH values and \(O_2\) contents. It was shown that complete recombination of electronic and hole products upon dissolution is observed only at small doses (up to 5 Mrad). At large doses the percentage of unrecombined electronic products (which form \(H_2\) and \(H_2O_2\) upon reaction with water) increases, and consequently so does the percentage of hole products in an unknown form of localization [10]. The unrecombined fraction of the radiolysis products increases linearly with dose; at a dose of \(\sim 400\) Mrad it reaches 50%. After dissolution of the crystals their spectra were recorded. Solutions of \(Cl_2\) in water with a known form of localization [11] were prepared beforehand. Figure 3 shows that, in a solution of an irradiated sample (\(pH = 7\)), bands appear that are characteristic of \(ClO^{-}\) and \(Cl_3^{-}\) ions. Such bands are also observed at \(pH = 5\) and \(pH = 7\), varying

only their ratio. In the presence of oxygen in the system, a weak band of \(ClO_2^-\) or \(ClO_2\) appears.

Fig. 3. Spectra of aqueous solutions of irradiated \(KCl\) and various chemical forms of chlorine \((D_{\text{abs.}}\sim100\ \text{Mrad})\).

The corresponding bands of \(X_3^-\) and \(XO^-\) ions also appear upon dissolution of bromides and iodides; in this case, the \(XO^-\) band is less pronounced.

Determination of the possibility of using various acceptors of hole products was carried out as follows: the sensitivity of the acceptor was checked in artificially prepared halogen solutions; the influence of oxygen and of various acceptors of electron products \((NO_3^-,\) acetone) was examined; the influence of localized electrons was studied by dissolving additively colored crystals; and the change in coloration with standing was determined. The results are shown in Table I.

Table I

Properties of free-halogen acceptors

* \(+\) — weakly affects
** — strongly affects

Discussion of the Results

The difference in the kinetic growth of \(Na^0\) and \(X_v\) indicates the localization of free halogen in a form that cannot be determined photometrically. According to [12], this may be gaseous \(Cl_2\), trapped in micropores. Since these micropores are surrounded by a layer of colloidal metal, in optical spectra they should appear as colloidal bands. If \(Cl_2\) is uniformly distributed over the lattice, then it also cannot be optically determined, since \(\varepsilon_{330}^{Cl_2}\sim150\) [13]. Upon dissolution, \(V\)-centers may dissociate with the formation of both \(Cl_2\) and \(Cl^\circ\), which in water can partially hydrolyze:

\[ Cl_2+H_2O \rightarrow Cl^-+H^+ + HClO . \tag{1} \]

In an acidic medium hydrolysis does not occur, and [[unclear: it]] is stabilized in the form of the ion \(Cl_3^-\):

\[ Cl_2+Cl^- \rightleftharpoons Cl_3^- . \tag{2} \]

Accordingly, the rate constants of hydrolysis are: \(K_{\text{hydr}}=3.68\cdot10^{-4}\) and of formation of \(Cl_3^-\): \(K_2=0.18\) [14].

The appearance in an acidic medium of \(ClJ^{-}\) indicates the occurrence of rapid specific reactions of hole color centers with \(H_2O\), since at \(pH=1\) hydrolysis should be completely suppressed. In the presence of \(O_2\) in the solution, \(ClO_2\) can form,

\[ Cl_2 + 2O_2 \longrightarrow ClO_2, \tag{9} \]

which exhibits a broad absorption band with a maximum at 350 nm, and in an alkaline medium is transformed into the ion \(ClO_2^{-}\). The electron products, as shown in [10], react rapidly in the diffusion layer and in the bulk appear in the form of stable compounds \((H_2,\ H_2O_2)\). It has also been shown that the hole products in this layer do not react, but diffuse into the bulk. Thus, it may be considered that in an acidic medium the principal part of the hole products upon dissolution is transformed into the \(X_3^{-}\) ion, which reacts with acceptors in the bulk of the solution. On the basis of these data, the requirements for an acceptor of hole products can be formulated:

1/ the acceptor must react with \(X_3^{-}\) at a sufficient rate \((k > 10^6\ \text{l/mol·sec})\), and must not react with \(e^{-}_{aq}\) or \(H^0\) (in an acidic medium);

2/ the reaction of the acceptor with \(X_3^{-}\) must be irreversible and stoichiometric (this excludes the use of oxidation–reduction indicators of the o-tolidine type);

3/ the acceptor must not react with the acceptor of electron products or with the products of these reactions. (In [15] the effect of the electron acceptor \(NO_3^{-}\) on the determination of hole products is shown.)

Comparing the indicators used, one may say that the best proved to be \(J^{-}\) for a neutral medium (\(O_2\) as an acceptor of \(e^{-}_{aq}\), for suppression of recombination) and aromatic amines—3,3′-naphthidine and p-aminodimethylaniline. By the method of competing

reactions it has been established that introducing them into the solution at concentrations up to \(10^{-9}\) M suppresses the recombination of electron and hole products (in some cases, to suppress recombination it is necessary to introduce into the solution a second acceptor for the electron products). The analytical characteristics of the indicators provide sufficient accuracy (up to \(\pm 5\%\)) and rapidity of analysis.

The method we have developed for determining hole products includes dissolving the irradiated specimen in a solution of the corresponding acceptor and spectrophotometric determination of the reaction product. The method has been tested in studies of the radiolysis of a number of alkali-halide crystals, geological samples, etc.

Conclusions

1. It has been shown that, when irradiated samples are dissolved in aqueous solutions, the hole products of radiolysis pass into solution and, in a neutral medium, take the form of \(XCl^{-}\) and \(X_3^{-}\) ions, while in an acidic medium they are predominantly in the form of \(X_3^{-}\).

2. A procedure is proposed for the chemical determination of hole products using the indicators \(J^{-}\), p-aminodimethylaniline, and 3,3′-naphthidine.

In conclusion, the authors express their gratitude to Candidate of Physical and Mathematical Sciences K. K. Shvarts for consultations and assistance in the work.

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Institute of Physics, Academy of Sciences of the Latvian SSR.