SEPARATION OF NITROGEN ISOTOPES DURING THE SYNTHESIS OF NITRIC OXIDE IN A HIGH-FREQUENCY TORCH DISCHARGE II

V. N. MELENEVSKII, I. A. TIKHOMIROV, V. V. MARUSIN

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

The present work is devoted to the further study of isotope effects in the electrogas synthesis of nitric oxide.

The high-frequency torch discharge was first discovered by S. I. Zilitinkevich in 1928 [1]. The torch is a luminous ionized volume of gas (Fig. 1), existing owing to the closing of the circuit by the capacitive current torch—ground. The torch discharge arises at the point of a high-frequency circuit that has the maximum voltage. If there is a point on the surface of the electrode, and also if a section of the electrode is preheated to red heat, the torch arises at this place by itself. The critical voltage at which the torch can burn is the smaller, the higher the frequency of the high-frequency oscillations supplied to the torch. On going from lower frequencies to higher ones, the minimum voltage of the torch decreases together with the decrease of the capacitive resistance torch—ground, and, beginning with the frequency at which the minimum voltage of the torch becomes equal to the initial voltage of the corona, a torch arises instead of a corona. The torch discharge exists stably at pressures from 0.2 mm Hg to 3 atm. To maintain stable burning of the torch, the expenditure of a certain power is required, necessary for creating an equilibrium minimum of charged particles in the discharge zone and an energy balance in the flame of the torch. The power is usually supplied to the torch from high-frequency generators. It should be noted that the operating mode of the high-frequency generator is closely connected with the burning mode of the torch, which is its load. The type, shape, and burning conditions of the torch, and the electrical and temperature conditions in it have been studied sufficiently well [1, 2, 3, 4, 5].

Fig. 1. Photograph of a torch discharge.

In a torch discharge burning in air, nitrogen oxides are formed. There are several works [6, 7] in which the dependence of the nitric oxide concentration on the discharge power is investigated. However, the nature of this process is still unclear. The question of what is the determining—

…ionization, excitation, dissociation, or the thermal action of the discharge on the nitrogen oxidation reaction, remains open. It therefore seemed all the more interesting to study the redistribution of nitrogen isotopes in the course of this reaction. It should be noted that such work is being carried out for the first time.

Experimental Part

For carrying out the experiments, an apparatus was constructed, the schematic diagram of which is shown in Fig. 2. Air, through a system of drying and purification columns 1, 2, 3 and a rheometer 4, entered reactor 5. The reactor, shown schematically in Fig. 3, consisted of a metal chamber 1, 500 mm high and 150 mm in diameter, an ignition electrode 2 attached to the chamber by means of a bellows, an electrode-nozzle 3 to which high-frequency oscillations from the generator were supplied, and an outlet tube 6. Electrode 3 was insulated from the chamber walls by a Teflon disk 4. For observing the process of burning of the torch, a viewing window 5 made of organic glass was provided.

Fig. 2. Schematic diagram of the apparatus.

Fig. 3. Schematic diagram of the reaction chamber.

Ignition of the torch was carried out as follows: when ignition electrode 2 was brought into contact with electrode-nozzle 3, a high-frequency arc discharge arose, which, when ignition electrode 2 was moved away, immediately changed into a torch discharge. The height of the torch reached 10 cm. The torch discharge burned steadily throughout the entire experiment. To excite the discharge, a high-frequency generator on a GU-80 tube was used, assembled according to a three-point circuit with self-excitation, with a power in the primary circuit of 800 W and an operating frequency of 25 MHz. The voltage in the secondary circuit was 5 kV. The generator circuit is shown in Fig. 4. The experiments were carried out at atmospheric pressure. The air-flow rate, by means of rheometer 4, was maintained at 4 liters/min. The nitric oxide formed in the discharge was oxidized to dioxide and frozen out in trap 6, cooled with liquid nitrogen. The nitrogen dioxide was then absorbed by a hydrogen peroxide solution with formation of nitric acid. Samples for mass—

Fig. 4. Circuit diagram of the high-frequency generator.

were obtained from nitric acid for mass-spectrometric analysis by the known method [8]:

\[ \mathrm{NO_3^-} + \text{Devarda’s alloy }(50\%\,\mathrm{Cu},\ 45\%\,\mathrm{Al},\ 5\%\,\mathrm{Zn}) \xrightarrow{\mathrm{NaOH}} \mathrm{NH_3}\ (2) \tag{1} \]

\[ \mathrm{NH_3} \xrightarrow{\mathrm{HCl}} \mathrm{NH_4Cl} + \mathrm{NaOBr} \longrightarrow \mathrm{N_2}. \]

The separation coefficient was calculated by the formula [9]

\[ \alpha=\frac{c\cdot(1-c_0)}{c_0\cdot(1-c)}, \tag{2} \]

where \(c\) is the isotopic concentration of \(\mathrm{N}^{15}\) in nitrogen monoxide, and \(c_0\) is the isotopic concentration of \(\mathrm{N}^{15}\) in atmospheric nitrogen.

As a result of the investigations it was found that, in the torch discharge, nitrogen monoxide is depleted in the nitrogen isotope \(\mathrm{N}^{15}\) in comparison with the initial atmospheric nitrogen. The separation coefficient was found to be \(0.970 \pm 0.008\). The separation of nitrogen isotopes during the synthesis of nitric oxide in a high-frequency torch discharge is, in all probability, due to kinetic isotope effects of the individual stages of formation and decomposition of nitric oxide (since this reaction is reversible). The question of which stage (electron impact, bond rupture and formation of new bonds, etc.) is determining in the process of nitrogen isotope separation will be considered in one of the following communications.

Work to elucidate the influence of discharge conditions on the separation coefficient is continuing.

Conclusions

1. A study has been made of the redistribution of nitrogen isotopes during the synthesis of nitric oxide in a high-frequency torch discharge.

2. It has been established that, during the synthesis of nitric oxide in the discharge, it is depleted in the nitrogen isotope \(\mathrm{N}^{15}\) in comparison with the initial isotopic composition.

3. An experimental value of the separation coefficient equal to \(0.970 \pm 0.008\) was obtained.

REFERENCES

1. S. I. Zilitinkevich. Discovery of the torch discharge. Telegraphy and Telephony without Wires, 9, No. 6, 652, 1928.

2. N. A. Kaptsov. Electronics, 385, Moscow, GITTL, 1954.

3. H. Margenau. Kondition and dispersion of ionized gases at hight frequencies, Phys. Rev., 69, No. 9/10, 508, 1946.

4. Ya. N. Matveev. Development of an electric torch with change of pressure. ZhTF, 11, 1054, 1941.

5. A. S. Bamberg, S. V. Dresvin. On the temperature of a torch discharge stabilized by an air flow. ZhTF, 32, 6, 1962.

6. K. N. Mochalov. On the relation of the kinetic constants of the nitrogen oxidation reaction in a torch discharge to certain discharge parameters. ZhFKh, 21, 1299, 1937; Synthesis of nitric oxide in a high-frequency torch discharge, DAN SSSR, 18, 329, 1938; DAN SSSR, 20, 297, 1938; Kinetics of nitrogen oxidation in a high-frequency torch discharge. ZhFKh, 13, 1224, 1939.

7. P. A. Serebryakov. On chemical reactions in a torch discharge. ZhFKh, 14, 175, 1940.

8. I. I. Brown and G. M. Begun. J. Chem. Phys. Fractionation between Nitric Acid and the Oxydes of Nitrogen, 30, 1206, 1959.

9. A. I. Brodskii. Chemistry of Isotopes, p. 56, Moscow, Publishing House of the Academy of Sciences of the USSR, 1957.