On the Radiolysis of Silver Oxalate, Sulfate, and Thiocyanate

V. V. Sviridov, T. I. Pushkareva, G. A. Branitskaya

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

The radiolysis of silver oxalate has been studied previously. It was established that the main products of radiolysis are metallic silver and carbon dioxide. Electron-microscopic data indicate that radiolytic silver is released predominantly at defective regions of the surface of silver oxalate crystals in the form of particles whose dimensions do not exceed 0.1 μm [1].

In the present work we give the results of a comparative study of the radiolysis of oxalate and of two other silver salts—sulfate and thiocyanate—the action of ionizing radiation on which had not previously been investigated.

Silver oxalate was obtained by pouring 0.5 N AgNO₃ solution (“analytical grade”) into 0.5 N H₂C₂O₄ solution (“chemically pure”), taken in slight excess. Silver sulfate, “analytical grade,” was recrystallized twice from concentrated sulfuric acid before the experiments were carried out. Silver thiocyanate was obtained by the method described in [2].

The conditions of synthesis of the samples and of working with them excluded their possible exposure to visible light.

All samples were irradiated in air with γ-rays from Co⁶⁰ with an energy of 1.25 MeV. In all experiments the dose rate was 360 r/sec. Immediately after irradiation the samples were subjected to chemical analysis, as well as to X-ray diffraction and electron-microscopic studies. The X-ray patterns were taken with CuKα radiation on a URS-5M diffractometer. The electron-microscopic study was carried out on an EM-7 instrument. In this case the preparation of the specimens was performed by a method that had proved successful in similar studies: by making carbon or platinum–carbon replicas with extraction of the silver particles [3].

For carrying out chemical analysis, a weighed portion of the irradiated salt was dissolved in ammonia, after which the amount of silver that had passed into solution was determined by the Volhard method or with the aid of a membrane sulfide-selective electrode (P-S-711 [4]).

The results of the chemical analysis showed that the principal solid product of the radiolysis of all the silver salts studied is metallic silver. In order of increasing radiation stability, the samples are arranged in the sequence: $Ag_2C_2O_4 < Ag_2SO_4 < AgCNS$ (Table I).

Table I

Comparison of the radiation stability of silver salts

It should be noted that silver is detected by X-ray diffraction only when its content in the irradiated salt is considerable. When silver oxalate is irradiated, radiolytic silver is detected by X-ray diffraction at irradiation doses of not less than 160 Mrad; then its content is 24%. When this substance is irradiated with smaller doses, only a gradual decrease in the intensities of all interference lines belonging to the lattice of ox-

new substance. A largely similar picture was observed by us in the case of silver sulfate and thiocyanate: the most intense line of metallic silver with \(d = 2.36\ \text{Å}\) begins to appear on the X-ray diffraction patterns of irradiated \(Ag_2SO_4\) after irradiation with a dose of not less than 240 Mrad, which corresponds to 2% decomposition of the salt. In the case of silver thiocyanate, on the X-ray diffraction patterns of the irradiated salt, the lines corresponding to \(Ag_2S\), with the most characteristic interplanar spacings of 2.34 and 2.59 Å, appear first and more distinctly, and only subsequently do the lines corresponding to metallic silver appear.

A comparative study of the IR spectra of the initial and irradiated samples of \(Ag_2C_2O_4\), \(Ag_2SO_4\), and \(AgCNS\) showed that, during the radiolysis of \(Ag_2C_2O_4\), beginning with a dose of 1 Mrad, there is a decrease in the intensity of the main absorption bands belonging to the oxalate ion and the appearance of weak bands belonging to \(CO_2\) molecules. The presence of the latter in the IR spectra clearly indicates that the gaseous products of the radiolysis of \(Ag_2C_2O_4\), at least in part, are retained in the salt lattice. Upon irradiation of silver sulfate and thiocyanate, beginning with 5 Mrad, a decrease is also observed in the intensity of the absorption bands corresponding to the vibrational frequencies of \(SO_4^{2-}\) (\(\nu = 620\ \text{cm}^{-1}\) and \(1115\ \text{cm}^{-1}\)) and \(SCN^{-}\) (\(\nu = 2150\ \text{cm}^{-1}\)). Upon irradiation of the thiocyanate, moreover, new absorption bands appear with \(\nu = 620\ \text{cm}^{-1}\) and \(\nu = 1110\ \text{cm}^{-1}\), which may possibly belong to \(SO_4^{2-}\) ions formed as a result of oxidative processes involving atmospheric oxygen that accompany radiolysis. However, the question of the origin of these spectral bands requires additional investigation.

The results of the electron-microscopic study carried out on the formation of silver particles on the surface of samples of silver oxalate, sulfate, and thiocyanate irradiated with gamma rays made it possible—

made it possible to establish the following.

1. In the radiolysis of all the salts studied, even at comparatively small irradiation doses of the order of 1 Mrad, when the radiolysis products are not yet detected by methods of chemical analysis, minute silver particles in the form of grains with dimensions slightly exceeding the resolving power of the electron microscope (50–100 Å) are formed on the crystal surface. It is significant that these particles cover the surface of the samples under investigation very nonuniformly: on regions containing a large number of visible microscopic defects in the form of surface hillocks, etch pits, and growth-line ends, there are more of these particles; here they form entire clusters (Fig. 1a, b, c). The difference in the patterns observed in the radiolysis of the salts studied consists only in the rate at which the surface of the samples is covered by metallic silver particles, increasing in the series

\[ \mathrm{AgCNS} < \mathrm{Ag_2SO_4} < \mathrm{Ag_2C_2O_4}. \]

1. The metallic silver particles cease their growth, reaching, in the case of all three salts, dimensions of the order of 0.05–0.07 µm, and the average sizes of these particles practically do not change with increasing irradiation dose; only their total number on the surface of the irradiated crystals increases. Upon irradiation with doses of the order of 200 Mrad, the metallic silver particles merge with the formation of aggregates, and often of larger globules as well. Nevertheless, we did not observe the formation of a continuous metallic film.

The radiolysis patterns considered for the silver salts studied are, to a considerable extent, similar to the previously described patterns of photolysis and radiolysis of \(\mathrm{Ag_2C_2O_4}\) [1, 5]. This similarity is manifested first of all in the structural sensitivity of the process, since the probability of the onset of the reaction on different regions of the surface proves to be unequal. Decomposition processes are localized primarily on unsta—

Fig. 1.

The most typical electron-microscopic images of the surface of silver salts irradiated with $\gamma$-rays.

Irradiation in air: a — $\mathrm{Ag_2C_2O_4}$, b — $\mathrm{AgCNS}$, c — $\mathrm{Ag_2SO_4}$.

Irradiation in the presence of water: d — $\mathrm{AgCNS}$. Dose: 10 Mrad.

on the surface areas of crystals possessing excess free energy. This is evidently associated with the lower probability that, at defects as compared with regular regions of the crystal lattice, recombination processes will occur for the primary products formed in the solid under irradiation. The cessation of growth of metallic silver particles upon reaching a certain critical size is characteristic of all the silver salts studied by us, and was observed not only during radiolysis but also during photolysis [5]. It is possible that the silver particles cease to grow as a result of recrystallization processes at the metal–salt boundary, although in the work of V. V. Boldyrev and co-workers [6] this phenomenon is explained as the result of a decrease, in a certain region of the lattice around the growing silver nuclei, in the concentration of interstitial \(Ag^+\) ions participating in the process of growth of the metal particles.

Radiolysis of moist samples of \(Ag_2C_2O_4\), \(Ag_2SO_4\), \(AgCNS\), as well as samples of these salts located under a layer of water, proceeds more effectively than in dry samples (Table I). The effect of water on the radiolysis of silver oxalate is especially pronounced.

In all cases, the influence of water on the character of reaction localization and on the features of its course is manifested in the same way. During radiolysis in the presence of water, as during radiolysis of dry salts, patterns of decoration by radiolytic silver of the surface microrelief are observed (Fig. 1d). However, the particles of metallic silver formed have larger dimensions (0.1 \(\mu\)m) than in the radiolysis of dry salts.

The efficiency of radiolysis of the salts studied in an aqueous \(AgNO_3\) solution remained still more significant than upon irradiation of these salts in water, and the growth of silver particles ceases when they reach sizes on the order of 0.2 \(\mu\)m.

The high efficiency of the radiolysis of silver salts in the presence of water gives grounds to assume that under these conditions a substantial role is played by silver ions that pass into solution or are adsorbed on the surface of the crystals. Possessing high mobility, these ions can readily be discharged on silver particles that are in close contact with the salt lattice. Consistent with this is the fact that the presence of silver ions in water promotes an acceleration of radiolysis. The reason for the cessation of growth of the silver particles in this case is probably the possibility of detachment of silver particles from the salt crystals, due to the fact that the most favorable conditions for crystal dissolution exist at the salt–metal–water boundary, where the \(Ag^+\) ions passing into solution can immediately attach to the growing metal particle.

Conclusions

A study has been carried out of the radiolysis of silver oxalate, sulfate, and thiocyanate under the action of gamma rays. It has been established that the radiation sensitivity of these salts decreases in the series \(Ag_2C_2O_4 > Ag_2SO_4 > AgCNS\). Water causes a noticeable acceleration of radiolysis.

Electron-microscopic examination showed that during the radiolysis of all the salts studied, both dry and under a layer of water, silver particles are deposited nonuniformly on the surface of the crystals, predominantly on the most defective regions, and cease growing upon reaching certain sizes (of the order of 0.05–0.07 μm in the case of radiolysis of dry salts and of the order of 0.1 μm during radiolysis of salts located under a layer of water).

Literature

1. G. A. Branitskii, V. V. Sviridov. In the collection Heterogeneous Reactions and Reactivity. “Vysshaya Shkola” Publishing House, Minsk, 1964, 166.

2. G. Brauer. Handbook of Preparative Inorganic Chemistry, IL Publishing House, Moscow, 1956, 326.

3. G. S. Gritsenko, B. B. Zvyagin, R. V. Borskaia, A. I. Gorshkov, N. D. Samotoin, I. É. Fronsova. Methods of Electron Microscopy of Minerals. “Nauka” Publishing House, Moscow, 1969.

4. E. E. Khavan. Areas of Application of Ion-Selective Membrane Electrodes. Budapest, 1968.

5. G. A. Branitskii, V. V. Sviridov, A. I. Lesnikovich. In the collection Heterogeneous Reactions and Reactivity. “Vysshaya Shkola” Publishing House, Minsk, 1964, 149.

6. V. V. Boldyrev, V. I. Eroshkin, V. T. Pismenenko, I. A. Ryzhak, A. A. Medvinskii, I. V. Shmidt, L. I. Kefeli. Kinetics and Catalysis, 1968, 9, issue 2, 260.