Abstract
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PHYSICAL CHEMISTRY
Corresponding Member of the Academy of Sciences of the USSR VIKT. I. SPITSYN and I. E. MIKHAILENKO
EFFECT OF THE INTENSITY OF RADIOACTIVE RADIATION OF S³⁵ ON THE RATE OF ISOTOPIC EXCHANGE OF SULFUR IN THE SYSTEM
K₂SO₄—SO₃
In studying the isotopic exchange of sulfur between solid sulfates of alkali elements and gaseous sulfuric anhydride under high-temperature conditions (¹), the authors observed that the level of radioactivity of the preparations under investigation affects the rate of isotopic-exchange processes. This phenomenon was subjected to more detailed investigation.
Fig. 1. Diagram of the apparatus for studying isotopic exchange in the K₂SO₄—SO₃ system.
The action of the beta radiation of S³⁵ on the degree of isotopic exchange of sulfur between solid potassium sulfate labeled with S³⁵ and gaseous sulfuric anhydride was studied. The investigation was carried out under identical conditions at a temperature of 840° with preparations of K₂SO₄ differing in the magnitude of their specific activity. A diagram of the apparatus used is shown in Fig. 1. The round-bottom flask (1), condenser (2), test tube (4) with a ground joint and gas-inlet tubes, and the wash bottles (7), (8), (9) were made of molybdenum glass; tube (6) and the boat in it were made of quartz glass. A 60% oleum of “chemically pure” grade was poured into the flask; distillation of SO₃ was carried out at a temperature of 60–70°. Through the condenser and the three-way stopcock (3), with the two-way stopcock (5) closed, sulfuric anhydride entered a graduated test tube (4) with a scale division of 0.05 ml. A measured amount of SO₃ was carried by a stream of dry air through stopcock (5) into the reaction space of the quartz tube (6). After passing over the substance, the sulfuric anhydride was trapped in wash bottles with concentrated H₂SO₄ (7) and 0.1 N NaOH solution (8) and (9). A definite weighed portion of the radioactive potassium sulfate preparation was placed in the quartz boat. Before the beginning of each experiment and after it, dry air, entering through the side arm of the three-way stopcock (3), was passed through the entire apparatus. The temperature of the furnace (10) was measured with a platinum—platinum-rhodium thermocouple. During the experiments the temperature, equal to 840°, was maintained with an accuracy of ±5°. In each experiment the weighed portion of sulfate was 0.3–0.4 g; the amount of SO₃ was 0.3 ml, or 0.6 g. The rate of the stream of dry air carrying along the sulfuric anhydride reached 37 l/h. The time of passage of SO₃ over the preparation under study was 10 min. The weighed portion of sulfate remained in the heated zone of the furnace for 20 min. The sulfur-labeled K₂SO₄ preparations were prepared by introducing into their solutions a small amount of
amount of active sodium sulfate. The solutions were then evaporated to dryness and the precipitate was calcined at 800°. The same specific surface of the \(K_2SO_4\) samples was achieved by subjecting the carefully ground preparations to sieving into fractions. The particle size ranged from 0.17 to 0.10 mm.
After the experiment, the weighed portion of active sulfate was dissolved in a volumetric flask. A definite volume of the solution was evaporated on a sheet of filter paper placed in a round aluminum dish. The activity of the preparation was always measured under identical geometrical conditions with respect to an end-window counter. The solutions of highly active preparations obtained after the experiments were diluted in such a way that the pulse-counting rate for all preparations was of approximately the same order (1500–2000 pulses/min). With each \(K_2SO_4\) preparation of different specific activity, 4–6 isotope-exchange experiments were carried out. Examples of the changes in the activity of potassium sulfate that occurred are given in Table 1, and the mean results of the isotope-exchange measurements—in Table 2 and in Fig. 2. The degree of exchange was calculated by the formula
Fig. 2. Dependence of the degree of isotopic exchange on the specific activity of \(K_2SO_4\)
\[ W=(A_0-A_1)/B, \]
where \(W\) is the degree of exchange in percent, \(A_0\) is the activity of the initial salt, taken as 100%, \(A_1\) is the activity of the reaction product in percent of the activity of the initial compound; \(B=\dfrac{N_1}{N_1+N_2}\); \(N_1\) is the atomic concentration of the element under study (sulfur) in the passed \(SO_3\); \(N_2\) is the atomic concentration of sulfur in the radioactive preparation.
It follows from the results presented that the rate of isotopic exchange at a specific radioactivity of \(K_2SO_4\) of the order of 0.02–0.03 µc/g is practically constant and is about 12%. It then rises to 26.7% when the activity of potassium sulfate reaches 0.35 µc/g. The maximum degree of exchange, 66.9%, is observed at a specific activity of about 2.3 µc/g. A further increase in the specific activity of potassium sulfate to 8–16 µc/g then leads to a decrease in the degree of exchange to 33–37%. The region of still higher specific activities has not yet been investigated by us.
It may be assumed that the beta particles emitted by sulfur-35, at sufficient intensity of their radiation, cause excitation of the ions forming the \(K_2SO_4\) crystal lattice and, in particular, of the \(SO_4^{2-}\) ions. Apparently, in this state the \(SO_4^{--}\) ions from the solid phase are more readily able to exchange sulfur atoms with gaseous sulfur anhydride, \(SO_3\) radicals, \(SO_2\), etc. This leads to an increase in the exchange rate by several times in comparison with weakly active preparations.
As for the maximum in the magnitude of exchange at a specific activity of 2 µc/g, the reason for its occurrence is still not clear. It is possible that, under excessively intense radioactive radiation, ionization of \(SO_3\) molecules begins to occur to a noticeable extent near the surface of the solid potassium sulfate. This may hinder their adsorption and, correspondingly, exchange. On the other hand, charges may arise on the surface of potassium sulfate under intense radiation, producing the same consequences.
It should be noted that the addition of sodium sulfate by itself does not affect the rate of isotopic exchange of sulfur in potassium sulfate. For a \(K_2SO_4\) preparation with a specific activity of \(1.7\cdot10^{-2}\) µc/g and a \(Na_2SO_4\) content of 0.4%, the rate of isotopic exchange proved to be 11.9%, which practically does not differ from the exchange value at the same specific activity but with a lower \(Na_2SO_4\) content (0.04%).
In the experiments described, we also do not have a simple radiation-chemical
chemical decomposition of potassium sulfate, which would be accompanied, for example, by the elimination of SO₂ and the loss of a corresponding amount of activity. Calcination of the active K₂SO₄ preparation in a stream of air under the same conditions as in the exchange experiments with SO₃ showed a complete absence of change in weight, as well as in the content of S³⁵.
Table 1
Examples of changes in the activity of potassium sulfate in isotope-exchange experiments with SO₃
Temperature 840°. SO₃ charge 0.58 g.
| K₂SO₄ preparation | K₂SO₄ charge, g | Weight of K₂SO₄ after experiment, g | Change in weight, g | Change in weight, % | Initial activity, imp/min | Activity of K₂SO₄ after experiment, imp/min | Activity of K₂SO₄ after experiment, % | Degree of exchange, % |
|---|---|---|---|---|---|---|---|---|
| 1 | 0,3362 | 0,3362 | — | — | 2164·10² | 1942·10² | 89,8 | 11,7 |
| 2 | 0,4034 | 0,4039 | +0,0005 | +0,1 | 4625·10² | 4160·10² | 90,0 | 12,0 |
| 3 | 0,3780 | 0,3785 | +0,0005 | +0,1 | 4713·10³ | 3719·10³ | 78,9 | 27,5 |
| 4 | 0,3756 | 0,3761 | +0,0005 | +0,1 | 2726·10⁴ | 1344·10⁴ | 49,3 | 65,9 |
| 5 | 0,3129 | 0,3129 | — | — | 2514·10⁴ | 1187·10⁴ | 47,2 | 66,1 |
| 6 | 0,3512 | 0,3512 | — | — | 9897·10⁴ | 7310·10⁴ | 73,9 | 33,4 |
| 7 | 0,3563 | 0,3561 | −0,0002 | −0,1 | 1669·10⁵ | 1212·10⁵ | 72,4 | 35,4 |
A phenomenon close to that described above was observed by Gordon and Hart (²): treatment with Co⁶⁰ gamma rays of a solution of gaseous deuterium in light water led to isotopic exchange of deuterium and protium, which by itself does not proceed at a noticeable rate under the same conditions. These authors believe that in this case exchange takes place through products of radiolysis of water.
| Initial activity of the K₂SO₄ charge, imp/min | Activity after experiment, imp/min | |
|---|---|---|
| 1 | 1440·10⁵ | 1439·10⁵ |
| 2 | 1186·10⁵ | 1186·10⁵ |
Table 2
Isotopic exchange of sulfur between potassium sulfate and SO₃ at 840°
| K₂SO₄ preparation | Introduced Na₂SO₄ impurity, % | Observed specific activity, imp/min, g | Absolute activity, μc/g | Number of experiments | Degree of exchange, % |
|---|---|---|---|---|---|
| 1 | 0,04 | 6,44·10⁵ | 1,7·10⁻² | 5 | 11,7 |
| 2 | 0,1 | 9,37·10⁵ | 2,6·10⁻² | 6 | 11,5 |
| 3 | 0,1 | 12,5·10⁶ | 3,5·10⁻¹ | 4 | 26,7 |
| 4 | 0,4 | 72,6·10⁶ | 2,0 | 5 | 65,5 |
| 5 | 0,4 | 80,4·10⁶ | 2,3 | 5 | 66,9 |
| 6 | 2,6 | 28,2·10⁷ | 7,8 | 5 | 33,3 |
| 7 | 3,0 | 58,5·10⁷ | 16,2 | 4 | 36,6 |
In heterogeneous systems similar to the one we studied, an important role must be played by the energetic action of radioactive radiation on the adsorption layer of gaseous or liquid products formed on the surface of the solid phase. The processes of isotopic and chemical exchange occurring on the surface of a solid, and the adsorption, catalytic, and other properties of solids, may depend on this action. At the same time, a radiation-chemical action of the radiation on the gaseous or liquid phase in contact with the radioactive solid is not excluded. In addition, within the volume of the solid and on its surface, recoil nuclei may create defects in the crystal lattice, which play the role of additional active centers for the phenomena of adsorption and isotopic exchange.
Institute of Physical Chemistry
Academy of Sciences of the USSR
Moscow State University
named after M. V. Lomonosov
Received
27 IV 1958
CITED LITERATURE
- Vikt. I. Spitsyn, I. E. Mikhailenko, Zhurn. neorg. khim., 3, 1254 (1958).
- S. Gordon, E. J. Hart, J. Am. Chem. Soc., 77, 3981 (1955).