Thermal Decomposition of Silver Sulfite at Low Temperatures

G. G. Savel'ev, V. A. Budkov, A. A. Kabanov, Yu. A. Zakharov

(Presented by the Department of Radiation Chemistry)

In a number of works [1, 2, 3], the thermal decomposition of silver sulfite at temperatures above 150°C was studied. It was found that in this region the reaction is structurally sensitive and that there are definite correlations between the influence of homophase additives on thermal decomposition and the electrical conductivity of the solid solutions formed by the additives.

Continuing these investigations, we studied the thermal decomposition of silver sulfite at temperatures of 100–150°C, and also measured its electrical conductivity in the range 20–200°C.

The investigations were carried out on aged preparations obtained by the procedure described in work [3], from starting preparations of chemically pure grade.

Fig. 1. Curves of the thermal decomposition of silver sulfite. \(P\)—pressure in the system.

For thermal decomposition we used a volumetric high-vacuum apparatus [4]. The rate of thermal decomposition was monitored by the change in pressure in a closed system. For thermal decomposition, samples of \(4 \pm 0.2\) mg were taken; during decomposition the pressure varied from \(10^{-3}\) to \(3 \cdot 10^{-1}\) mm Hg. Fig. 1 presents several typical

curves of thermal decomposition of pure silver sulfite at several temperatures. It is evident from the figure that, in a certain temperature interval, the decomposition rate decreases as the temperature increases. We investigated this interval in detail (at intervals of 2–3°C). The results are presented in Fig. 2 in the coordinates “decomposition rate, in relative units—experiment temperature.”

It follows from the figure presented that the curve of the temperature dependence of the decomposition rate has a characteristic bend with a maximum and a minimum at 120 and 145°C, respectively. Similar results were also obtained for an Ag₂SO₃ preparation containing 1% Pb++ (Fig. 2).

To measure the specific electrical conductivity (σ), we pressed from Ag₂SO₃ specimens measuring 25 mm × 5.92 mm × 2 mm at a pressure of 2000 kg/cm², 20°C, with the pressure maintained for 3 minutes. The density

Fig. 2. Change in the 110–150°C temperature interval of the thermolysis rate of pure Ag₂SO₃ (1) and containing 1 mol.% Pb++ (2).

of the specimens, without correction for porosity, was 4.1 g/cm³. The pressing pressure was selected from the condition that tribochemical decomposition of Ag₂SO₃ be absent. Aquadag electrodes were applied to the faces 5.92 × 2 mm. The specimens with electrodes were dried before the measurements at 50°C for 14 hours. Before pressing, the powder was stored over P₂O₅ for 40 days. The resistance was measured in air, at atmospheric pressure, with an E6-3 teraohmmeter; the maximum measurement error was ±5%. The temperature was measured with a copper–constantan thermocouple, whose junction was placed in an additional control specimen, and with a thermometer. Three specimens were studied; the discrepancy in the values of σ between them was no more than 15%.

Figure 3 gives the obtained dependence of the electrical conductivity of Ag₂SO₃ on temperature in the coordinates \(\lg \sigma — \frac{1}{T^\circ K}\). During heating, the activation energy changes twice and is 0.86, 1.7, and 0.56 eV, with points of inflection (B and C) at 110 and 147°C.

From the literature, we know of only one example of anomalies in the temperature dependence of the rate of thermal decomposition similar to those established by us and described in the present communication. Such anomalies are observed in the region of a modification transition (about 240°C) during the thermal decomposition of ammonium perchlorate and are explained by the authors of work [5] by the endothermic nature of the transition.

Silver azide also has a polymorphic transition at the decomposition temperature (180–190°C); however, in the temperature dependence of the decomposition rate, at the point of the polymorphic transition only a change in the activation energy from 35 to 42 kcal/mole is observed, i.e., only one bend is found [6]. This question, apparently, has not been specially investigated. In [7] it is shown that at the point of the polymorphic transition of $\mathrm{AgN_3}$ there is a bend in the temperature dependence of the electrical conductivity; moreover, the temperature course of the conductivity is outwardly almost completely analogous to that shown in Fig. 3 for $\mathrm{Ag_2SO_3}$, i.e., as the temperature is raised the apparent activation energy first (in the transition region) increases and then decreases (Fig. 7 in [6]). Thus, there is an almost complete analogy in the behavior of $\mathrm{Ag_2SO_3}$ in the temperature interval 100–150°C and in the behavior of $\mathrm{AgN_3}$ and $\mathrm{NH_4ClO_4}$ during polymorphic transitions. On the basis of these analogies one may assume the presence of a polymorphic transition in $\mathrm{Ag_2SO_3}$ at 110–145°C.

The rearrangement of the crystal lattice of $\mathrm{Ag_2SO_3}$ and the presence of a thermal effect (probably endothermic) accompanying this process, as well as the change in the conditions of contact between individual crystals in pressed $\mathrm{Ag_2SO_3}$, lead to a sharp increase in the conductivity of the sulfite in the interval 117–142°C. These processes, apparently, are completely completed at $142 \pm 2^\circ\mathrm{C}$, and at higher temperatures we are dealing with a new modification of $\mathrm{Ag_2SO_3}$, the thermolysis of which has been studied in [1–3].

Fig. 3. Temperature dependence of the electrical conductivity of silver sulfite.

It is interesting to note that the temperature region of anomalous thermolysis of $\mathrm{Ag_2SO_3}$—the interval BB (120–145°C) in Fig. 2 with negative activation energy of decomposition—is somewhat narrower than the temperature segment BB (Fig. 3), for which an increase in conductivity and an increase in the activation energy of the process are observed.

In discussing the possible causes of the special behavior of $\mathrm{Ag_2SO_3}$ in the interval 120–145°C, the presence of two bends on the curve decomposition rate—decomposition temperature remains unclear. In connection with the latter, it should be noted that a second possible reason for the appearance of anomalies in the temperature dependence of the rate of thermal decomposition of $\mathrm{Ag_2SO_3}$ may be associated with the formation, at temperatures of 120–145°C, of silver dithionate by the reaction of the ion-radical $\mathrm{SO_3}$ with silver sulfite, as was shown in a number of older works [8, 9, 10].

Further investigation of the causes of the observed anomalies in the temperature dependence of the rate of thermolysis of $\mathrm{Ag_2SO_3}$ will be continued by us.

References

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