KINETICS OF THE INTERACTION OF SOLID ACETATES WITH HYDROGEN SULFIDE AFTER PRELIMINARY IRRADIATION WITH ULTRAVIOLET LIGHT

E. I. KAPKOVA

Reactions involving solid substances find wide application in various branches of industry; for this reason, reactions of this kind have recently been attracting ever greater attention from researchers. The work of many Soviet scientists has been directed toward the study of various factors that exert an activating effect on the solid phase, and toward revealing a number of general regularities typical of the mechanism and kinetics of topochemical reactions. As a result of these investigations, definite successes have already been achieved in the field of the theory of the reactivity of solid substances and of the kinetics of reactions involving solids.

However, in this field there still remain a number of unsolved problems. This applies especially to topochemical reactions of the exchange type. Some of the factors on which the kinetics of these processes depend remain as yet unknown or little studied. Among the latter should be included, first of all, the action of light as a factor that can alter the reactivity of the initial solid phase and, consequently, the rate of the process as a whole.

The change in the reactivity of solid substances as a function of their preliminary illumination with ultraviolet light is of great interest. However, the data available in the literature on this question [1, 2, 3, 4] relate chiefly to topochemical decomposition reactions, which are only a special case of reactions involving solid substances. Moreover, these works have not yet revealed the general regularities relating to the acceleration or retardation of such reactions under the action of light. At the same time, it is known that in a whole series of technically important reactions the mechanism and kinetics of the process are often changed under the action of light.

The solution of a number of problems connected with the influence of light on the rate of topochemical reactions will, in time, make it possible to use solid reagents in various industrial processes under conditions of their illumination. It is not excluded that, under the influence of light, it may be possible to increase the activity of solid chemical absorbents, in particular those that may find application in the practice of gas-protection technology.

The aim of the present work was: a) to study the degree of change in the rate of interaction of solid acetates of silver, mercury, lead, copper, cadmium, and zinc with gaseous hydrogen sulfide as a function of the duration of irradiation of the initial preparations with ultraviolet light; b) to establish how long the activation of solid acetates is retained over time and whether their activation is retained in an atmosphere of oxygen.

In addition, it was of interest to establish a number of regularities concerning the influence of light on the kinetics of topochemical exchange-type reactions and

to outline a general classification according to the degree of activation of solid substances after their illumination with ultraviolet light.

Experimental Part

The experimental studies were carried out with anhydrous acetates of silver, mercury, lead, copper, cadmium, and zinc. Since the conditions of formation and the subsequent history of a crystal affect the reactivity of a solid substance, the starting preparations were prepared under definite conditions, in red light, and were stored in dark glass bottles over phosphorus pentoxide. For the experiments, powders of the above-mentioned acetates with particle diameters from 0.45 to 0.25 mm were used.

The source of ultraviolet radiation was a PRK-4 mercury–quartz lamp with a power of 220 watts. The principal circuit diagram for connecting the lamp to an alternating-current network is shown in Fig. 1. Data on the relative distribution of the energy emitted by PRK-type mercury–quartz lamps over the spectrum (relative to the energy of the 3650 Å line) are given in the table.

Lamp connection circuit

Fig. 1.

The method of carrying out the experiment was as follows. The starting substance was poured in a thin layer into a porcelain boat with a rectangular bottom, the area of which was 14 cm². The boat with the substance was located 7 cm from the mercury–quartz lamp. The voltage on the lamp remained 90 V throughout, and the current strength was 3 A.

Since the starting preparation, when irradiated with ultraviolet light, became appreciably heated, and since the heating temperature was different for different illumination exposures, it was necessary to achieve constancy of temperature. For this purpose, during the action of the light the boat with the starting acetates was placed in a specially made refrigerator. To study the kinetics of the interaction of the preliminarily illuminated solid acetates with hydrogen sulfide, the apparatus shown in Fig. 2 was assembled.

A weighed portion of the solid substance, irradiated for a definite interval of time, in cup 8 (Fig. 2) was placed in reaction vessel 6 on quartz spring balance 7. The initial position of the quartz spiral was fixed with cathetometer 17. The hydrogen sulfide used for the experiments, obtained in the usual way by the interaction of ferrous sulfide with hydrochloric acid, was previously dried in absorption tubes 1, 2, 3 with the aid of P₂O₅ and admitted into the reaction vessel at a rate of 10.8–11.3 l/hr. Such a fluctuation in the rate of the gas stream ...

is entirely permissible, since in the reaction vessel there was always a large excess of hydrogen sulfide compared with the amount required for the reaction to proceed. The excess hydrogen sulfide was absorbed in absorption columns 15 filled with copper carbonate. After 20 minutes the admission of hydrogen sulfide was stopped and, by means of a two-stage rotary vacuum pump, a vacuum was created in the reaction vessel

Fig. 2.

to \(1 \cdot 10\) mm Hg. This was quite sufficient to remove from the reaction vessel the excess gas and the adsorbed acetic acid. Pumping was carried out until the sample in the cup reached constant weight. The extension of the spring was measured with a cathetometer from the displacement of mark 0. To continue the reaction, hydrogen sulfide was again admitted into the reaction vessel for

for a definite time; then the gas and vapors were pumped out, the position of the quartz spiral was recorded, etc.

The temperature at which the experiments were carried out was \(23^\circ\text{C}\). Temperature regulation was performed with a toluene thermoregulator with an accuracy of up to \(\pm 0.2^\circ\text{C}\).

The first series of experiments was designed to determine the effect of the exposure to illumination on the rate of interaction of solid acetates with hydrogen sulfide. The results of these experiments are reflected in the curves presented in Figs. 3, 4, 5, 6, 7, and 8, where the reaction rate in \(g/\text{min}\) is plotted on the ordinate axis, and time in minutes on the abscissa axis. For comparison, the kinetic curves obtained for the interaction of unilluminated acetates with hydrogen sulfide are also given here.

Fig. 3.

Visible labels in the figure:

• Ordinate: rate \(v\), \(g/\text{min}\)
• Abscissa: \(t\) in minutes
• \( \mathrm{Pb(C_2H_3O_2)_2} \rightarrow \mathrm{PbS}\)
• Legend:
• not illuminated, \(\mathrm{Pb(C_2H_3O_2)_2}\)
• after illumination, 5 min
• 15 min
• 30 min
• 60 min
• 180 min

The interaction of lead acetate with hydrogen sulfide was studied at the following exposures of preliminary irradiation of the initial preparation: 5, 15, 30, 60, and 180 minutes. The interaction of solid acetates of mercury, copper, cadmium, and zinc with gaseous hydrogen sulfide was studied at two exposures of illumination of the acetates: 60 and 180 minutes. Finally, silver acetate was studied only at one irradiation exposure—60 minutes.

Analysis of the curves makes it possible to say that preliminary irradiation with ultraviolet light under the conditions indicated above significantly activates solid acetates, as a result of which the rates of their conversion into sulfides increase. However, it should be noted that the above-listed

acetates react with hydrogen sulfide at different rates, and the effect of ultraviolet light is manifested to different degrees. The latter

Fig. 4.

Fig. 5.

can be clearly seen from Fig. 9, where the rate constants for the interaction of various acetates with hydrogen sulfide are presented as a function of

Fig. 6.

Fig. 7.

exposure to illumination. From Fig. 9 it is evident that ultraviolet light has the greatest effect on lead acetate, then on the acetates of copper and mercury. The effect of light is smaller on cadmium acetate, and still smaller on zinc acetate. It is important to note that the duration

Fig. 8.

Fig. 9.

of irradiation of the initial preparation has little effect on changes in the reaction rate. This can be seen from Fig. 10, where the order of change in the rate of interaction of lead acetate with hydrogen sulfide is presented as a function of the exposure to illumination over one and the same interval of time. On this basis we come to the conclusion that ultraviolet light acts mainly only on the surface layer of the solid phase.

Fig. 10.

In addition to the acetates considered, the acetates of sodium, magnesium, barium, and aluminum were also taken for investigation; under our experimental conditions they proved incapable of reacting with hydrogen sulfide, i.e., they do not react with hydrogen sulfide either without illumination or after preliminary irradiation with ultraviolet light.

Having established that preliminary irradiation with ultraviolet light activates certain acetates, especially the acetates of lead, mercury, and copper, as a result of which the reaction of their interaction with hydrogen sulfide is clearly accelerated, it was of interest to trace whether the activation of the indicated acetates is preserved with time. For this purpose, a second series of experiments was carried out. The acetates of lead, mercury, and copper, after illumination with ultraviolet light for a definite interval of time, were transferred from the boat into small test tubes, which were well stoppered and kept in a dark place. After strictly defined intervals of time from the moment of illumination had elapsed, separate portions of the acetates were subjected to the action of hydrogen sulfide.

The experiments showed that the activation of the indicated acetates is completely retained after a fairly considerable interval of time from the moment of their illumination (up to thirty days). This gave some grounds for assuming that, under the action of ultraviolet light on acetates, an irreversible photochemical reaction occurs in the latter, with the liberation of certain products that play the role of catalysts in the interaction of the acetates with hydrogen sulfide. It may be assumed that such products are metals in the atomic state.

Fig. 11.

Fig. 12.

If it is assumed that, upon illumination of acetates with ultraviolet light, metal is liberated in the atomic state, then under the action of oxygen on the illuminated preparations their passivation could occur. Experiments directed toward clarifying this question showed that the activation of the acetates of copper and cadmium in an oxygen atmosphere is retained, whereas in the acetates of lead and mercury a slight decrease in activity is observed in an oxygen atmosphere. The latter can be seen from Figs. 11 and 12. But since passivation by oxygen appears only to an insignificant degree, it is difficult, on the basis only of the experiments carried out, to explain its nature. Nevertheless, it must be thought that oxygen somewhat passivates the active centers of the solid phase, as a result of which the rate of the reaction of interaction of lead and mercury acetates with hydrogen sulfide decreases.

Discussion of the Results

When solid substances are irradiated with ultraviolet light, two cases are possible: as a result of absorption of active light by a molecule, the latter may either become excited, or it will dissociate into atoms. In the latter case, the transition to the atomic state may be either a reversible process, when deactivation of the active products occurs, or

an irreversible process, when aggregation of the products being formed takes place.

The act of energy absorption is closely connected with the nature of the absorbing substance and with the character of the electronic levels. Since we investigated identical salts of different metals, it must be assumed that the nonidentical action of light on different acetates is connected with the nature and properties of the cations entering into the composition of the acetates. Obviously, the influence of light will be manifested first of all in a change in the state of the cation. The transition of a cation liberated under the action of light into the atomic state depends on its tendency to accept electrons, i.e., on its oxidizing ability.

Fig. 13.

If, in the initial solid substance, under the influence of light, an oxidation–reduction process takes place, in which light promotes this process, then in the case of the occurrence of certain exchange topochemical reactions it is possible for products to be liberated that will play the role of activators; that is, many topochemical reactions of the exchange type may be regarded as reactions proceeding in several stages, one of which is an oxidation–reduction process. The oxidation–reduction process itself predetermines to a large extent the rate of one reaction or another. A. P. Buntin [5] points to the oxidation–reduction character of certain exchange topochemical reactions.

The cations of the acetates studied by us, according to the magnitudes of the normal oxidation–reduction potentials, are arranged in the following series:

\[ \begin{array}{c} \text{reducing potential}\ \longrightarrow\\[2mm] \mathrm{Hg|Hg} > \mathrm{Ag|Ag} > \mathrm{Cu|Cu} > \mathrm{Pb|Pb} > \mathrm{Cd|Cd} > \mathrm{Zn|Z} > \mathrm{Al|Al} > \mathrm{Mg|Mg} > \mathrm{Na|Na} > \mathrm{Ba|Ba}\\[2mm] \longleftarrow\ \text{oxidizing potential} \end{array} \]

If the acetates studied by us are compared with respect to their reactivity toward hydrogen sulfide and the degree of action of ultraviolet light on them, an analogous series is obtained, although there are certain deviations, which will be considered below.

The data characterizing the oxidation–reduction properties of the cations may serve, to a certain extent, as a prerequisite for explaining the fact that acetates of mercury, silver, copper, lead, cadmium, and zinc react with hydrogen sulfide, whereas acetates of aluminum, magnesium, sodium, and barium do not interact with hydrogen sulfide. With an increase in the oxidizing potential, that is, with an increase in the ability of cations to acquire electrons and be reduced to free metals, the reactivity of acetates toward gaseous hydrogen sulfide increases and the effect of illumination is intensified. The higher the oxidizing potential, the more easily the cation acquires electrons, and the greater will be the probability of formation, under the action of light, of the atomic metal.

Nevertheless, certain deviations from this regularity are observed. The latter can be seen from Fig. 13, where the magnitude of the oxidizing potential of the cations is plotted on the ordinate axis, and on the abscissa axis—the con-

the rate constant for the interaction of the given acetates with hydrogen sulfide at different irradiation exposures. From Fig. 13 it is evident that the mercury and lead acetates deviate from the indicated regularity. These deviations may presumably be explained by the following factors: 1. The oxidizing character of the metal. As already noted, in mercury and lead acetates partial passivation occurs under the action of oxygen. 2. The possibility cannot be excluded that mercury and lead acetates, interacting with sulfides or with adsorbed acetic acid, i.e., with reaction products, form rather unstable complexes which will react slowly with hydrogen sulfide. It is known, for example, that mercury acetate readily gives complexes with acetic acid and with mercury sulfide. 3. The crystal structure, the type of packing of the molecule in the solid substance, may perhaps have an effect. Unfortunately, however, the structure of acetates remains poorly studied.

Conclusions

1. The effect of ultraviolet light on the rate of interaction of solid acetates of silver, mercury, copper, lead, cadmium, zinc, sodium, magnesium, barium, and aluminum with gaseous hydrogen sulfide has been studied. It has been established that these acetates may be classified, with respect to ultraviolet light, into three groups:

a) acetates whose reactivity increases sharply after illumination; this group includes the acetates of silver, mercury, copper, and lead, and, in terms of activity, they are arranged in the following series:

\[ \mathrm{Pb(C_2H_3O_2)_2} > \mathrm{AgC_2H_3O_2} > \mathrm{Cu(C_2H_3O_2)_2} > \mathrm{HgC_2H_3O_2}; \]

b) acetates whose reactivity toward hydrogen sulfide changes only slightly after their illumination; this group includes the acetates of cadmium and zinc;

c) acetates of sodium, magnesium, barium, and aluminum, which do not react with hydrogen sulfide either before irradiation with ultraviolet light or after irradiation with ultraviolet light.

1. It has been proved that one of the principal factors responsible for the activation of solid acetates under the influence of illumination with ultraviolet light is the oxidation–reduction potential of the cations entering into the composition of the acetates.

2. It has been proved that preliminary illumination with ultraviolet light causes activation of the acetates, and the reaction rate changes especially strongly in the first stage of the process. At the same time, no directly proportional dependence on the illumination exposure is observed.

3. It has been shown that, when certain storage conditions are observed, the activity of lead, mercury, and copper acetates activated by light is retained for a long time.

I express my deep gratitude to Professor-Doctor A. P. Buntin for the guidance and assistance that he provided in carrying out this work.

Literature

1. Rotinskii S. Z. and Shul’ts E. On the question of catalysis of a solid by a solid. Ukr. Chem. Zh., Vol. III, Book 2, 186, 1928.

2. Erofeev B. V. and Trusova K. I. Study of the kinetics of reactions involving solid substances. ZhFKh, 12, 4, 346, 1938.

3. Larner and Moon. Influence of Ra-rays on solids, J. Chem. Soc., p. 1398, London, 1933.

4. Macdonald I. V., The Thermal Decomposition of Silver Oxalate, J. Chem. Soc., p. 832, 1934.

5. Buntin A. P. and Aivazian N. A. Interaction of hydrogen halides with solid acetates of silver and mercury. Proceedings of Tomsk University, 101, 1–9, 1949.