SEPARATION OF NITROGEN AND OXYGEN ISOTOPES DURING THE SYNTHESIS OF NITRIC OXIDE IN AN ARC DISCHARGE

V. N. Melenevskii, I. A. Tikhomirov, Yu. G. Basov

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

Chemical reactions under conditions of electrical discharges proceed with different probabilities for isotopes of one and the same element, sufficient for this effect to be established. Thus, in works [1, 2, 3, 4, 5] it was shown that, during the synthesis of ozone in a silent discharge, a noticeable separation of oxygen isotopes occurs. It is also noted [6] that hydrogen and deuterium are fractionated in a glow discharge.

The aim of the present work is to establish and investigate isotope effects for nitrogen and oxygen during the synthesis of nitric oxide in an arc discharge.

Experimental Part

Description of the apparatus. Figure 1 presents a diagram of the apparatus on which the investigations were carried out. Nitrogen and oxygen from cylinders 1 and 2 entered mixer 3, then passed through a system of drying columns 4, 5, 6, filled respectively with silica gel, caustic potash, and phosphorus anhydride. The rate of the gas flow was measured with rheometer 7; the relative content of nitrogen and oxygen was determined with a gas analyzer of the GKhP-3M type. After drying, the gas mixture enter—

Fig. 1. Diagram of the apparatus.

columns 4, 5, 6, filled respectively with silica gel, caustic potash, and phosphorus anhydride. The rate of the gas flow was measured with rheometer 7; the relative content of nitrogen and oxygen was determined with a gas analyzer of the GKhP-3M type. After drying, the gas mixture enter-

... entered discharge tube 8; the nitrogen oxide formed was oxidized to dioxide and was frozen out in trap 10 at a temperature of \(-110^\circ\)C. The temperature was measured with a copper–constantan thermocouple. The unreacted nitrogen and oxygen passed through three-way stopcock 11 and escaped into the atmosphere.

To determine the concentrations of nitric oxide under various experimental conditions, the gas mixture under investigation was fed through three-way stopcock 11 and stopcock 17 into a previously evacuated removable gas pipette 18 (volume 1.5 l); in this case trap 10 was not cooled. The discharge tube was made of quartz glass with an inner tube diameter of 20 mm. The electrodes, in the form of stainless-steel cylinders, were attached to molybdenum leads; the distance between the electrodes was 6 mm. After leaving the discharge zone, the heated gas was cooled in a spiral coil. The discharge tube and the coil were cooled with running water. Power was supplied to the discharge tube from a DG-1 arc generator. The discharge was a low-voltage alternating-current arc. The discharge power was determined by a calorimetric method [7]. When the discharge current was varied from 1 to 4 A, the power varied correspondingly from 27 to 75 W. The NO concentration was determined by the usual method [8].

Figure 2 shows the curve of the dependence of the concentration of nitric oxide on the specific energy of the discharge at atmospheric pressure for an initial gas mixture of 50% \(O_2\) and 50% \(N_2\). The maximum concentration of nitric oxide \([\%NO]\) in the mixture attained in our experiments was 2.7 volume percent.

Fig. 2. Dependence of the concentration of nitric oxide on the specific energy of the discharge.

Procedure for carrying out the experiments and preparing samples for mass-spectrometric analysis. Before the start of an experiment, the required flow rates of nitrogen and oxygen were set. The discharge tube was switched on after the working gas had completely displaced the air from the system; at this time the three-way stopcock was open to the atmosphere. After the required amount of nitrogen dioxide had been frozen out in trap 10, stopcock 9 was closed. Next, by means of stopcock 11, trap 10 was connected with the rest of the system and evacuated to a pressure of \(2 \cdot 10^{-2}\) mm Hg. The pressure in the system was measured with thermocouple vacuum gauge 18 and mercury manometer 12. The purified nitrogen dioxide was thawed and converted to the gaseous state. Additional discharge tube 14 served to decompose nitrogen dioxide into nitrogen and oxygen. The electrodes of the discharge tube were connected to an IG-2 condensed-spark generator. Samples for mass-spectrometric analysis were prepared as follows: the gaseous nitrogen dioxide obtained in the arc-discharge process and purified as described above was fed into discharge tube 14 to a pressure of 150–200 mm Hg, where it decomposed into nitrogen and oxygen. The nitrogen and oxygen, through stopcock 17 and a ground joint (this joint also provided connection of removable gas pipettes for determining the nitric oxide concentration), were collected in a previously evacuated glass ampoule. The ampoule was then sealed off. A similar procedure for determining the isotopic composition of nitrogen and oxygen was used in works [9] and [10].

To verify that no additional isotopic fractionation occurred in this process, nitrogen samples were also prepared by another method [11]: nitrogen dioxide was absorbed by a hydrogen peroxide solution with formation of nitric acid. The nitric acid was reduced by Devarda’s alloy [12] to ammonia, which was then completely trapped by a 0.5 N hydrochloric acid solution. Samples of molecular nitrogen for mass-spectrometric analysis were obtained by the action of sodium hypobromite on an \(\mathrm{NH_4Cl}\) solution in the vacuum apparatus described in [13]. The results of isotopic analysis of samples prepared from the same specimen by different methods proved to be identical within the limits of measurement error. The separation coefficient of nitrogen and oxygen isotopes was calculated by the formula:

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

where \(c\) is the isotopic concentration of \(\mathrm{N}^{15}\) or \(\mathrm{O}^{18}\) in nitrogen dioxide, and \(c_0\) is the concentration of \(\mathrm{N}^{15}\) or \(\mathrm{O}^{18}\), respectively, in the initial mixture. In the course of the investigation it was established that nitrogen dioxide is depleted in heavy nitrogen and in heavy oxygen in comparison with the initial mixture. Some of the data on the study of the separation coefficients as a function of discharge power are given in Table 1.

Table 1

In addition, at a discharge power of 45 W the dependence of the separation coefficient at different flow rates through the discharge zone was recorded; it is presented in Fig. 3 as a function of the specific energy.

Fig. 3. Dependence of the separation coefficient on the specific energy of the discharge. 1 — separation coefficient for nitrogen; 2 — separation coefficient for oxygen.

On the basis of the fact that the separation coefficient is practically independent of time, one may conclude that this isotopic effect is kinetic in nature. The magnitude of the separation coefficient is not affected by the ratio between oxygen and nitrogen: experiments were also carried out with initial mixtures of 20% \(\mathrm{O_2}\)—80% \(\mathrm{N_2}\) and 80% \(\mathrm{O_2}\)—20% \(\mathrm{N_2}\).

The separation coefficient for nitrogen was found to be \(0.963 \pm 0.005\), and for oxygen \(0.955 \pm 0.005\).

Subsequently, having considered several probable mechanisms of formation of nitrogen oxide in an arc discharge and having calculated for them theoreti-

theoretical separation coefficient, it will apparently be possible, by comparison with experimental data, to determine the principal elementary acts through which this reaction proceeds, and to gain a deeper understanding of the nature of this process.

Conclusions

1. It has been established that, during the formation of nitric oxide in the discharge, it is depleted in \(N^{15}\) and \(O^{18}\) in comparison with the initial mixture; moreover, the isotopic separation does not depend on the specific power of the discharge or on the ratio of nitrogen to oxygen in the initial mixture.

2. Values of the single-pass separation coefficient have been obtained: for nitrogen \(0.965 \pm 0.005\), and for oxygen—\(0.955 \pm 0.005\).

3. An apparatus has been constructed that is convenient for determining the single-pass separation coefficient in the synthesis of nitric oxide in an arc discharge.

References

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