Experimental Methods for Determining Radiolysis Products of Alkali Metal Azides
Abstract Generated abstract
This paper describes experimental procedures for identifying and quantifying products formed during the radiolysis of alkali metal azides, with emphasis on sodium azide. The authors argue that dissolution of irradiated crystals can transform primary products and complicate interpretation, so they propose a sequential analytical approach using a single sample: direct measurement of nitrogen evolved from the crystal, determination of nitrogen retained in the lattice after dissolution, and solution analysis for hydroxide ions, ammonia, and undecomposed azide groups. The described vacuum, dissolution-cell, catalytic, and gas-evolution methods are intended to improve product balances over a range of irradiation doses and crystal dispersities. These methods support subsequent studies of sodium azide radiolysis kinetics under X-ray and proton irradiation and of impurity effects on radiation stability.
Full Text
EXPERIMENTAL METHODS FOR DETERMINING THE PRODUCTS OF RADIOLYSIS OF ALKALI-METAL AZIDES
E. P. Abakumov, Yu. A. Zakharov
(Presented by the scientific seminar of the Department of Radiation Chemistry, Chemical-Technological Faculty)
Early work on the radiolysis of azides was carried out within the general program of studies on the effect of radiation on various classes of solid chemical compounds, in order to elucidate the general regularities and particular features of radiolysis in each class of compounds. The radiolysis of sodium azide under the action of soft X-radiation was studied most fully in the works of Hill [1, 2].
The main goal of the work was to find, by chemical methods, the principal decomposition products in irradiated crystals. In this connection the following possible reactions were assumed:
\[ \mathrm{NaN_3} \rightarrow \frac{3}{2}\,\mathrm{N_2} + \mathrm{Na} \]
\[ \mathrm{N_3^-} \rightarrow \frac{3}{2}\,\mathrm{N_2} + e \]
A system of elementary stages in the radiolysis of sodium azide was developed by the author [1, 2] on the basis of determining secondary products, analyzed after destruction of the crystal lattice by dissolution, with the use of hypotheses concerning various interactions of the primary radiolysis products with the solvent. The products determined after dissolution of the irradiated salt NaN\(_3\) in water—nitrogen, hydroxyl ions, and ammonia—are present in the ratio 4:3:1. The composition of the gas after dissolution of the irradiated salt in liquid ammonia does not differ from the gas liberated upon dissolution in water. The principal conclusions concerning the accumulation of secondary radiolysis products are as follows:
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The accumulation of products is linear in the studied range of absorbed doses up to \(3 \cdot 10^{23}\) eV/g.
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The yield ratio \( \mathrm{N_2:OH:NH_3} \) depends little on the irradiation temperature of the azide crystals.
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All the nitrogen is retained by the sodium azide matrix.
In the present work we have attempted to develop methods for as complete an analysis as possible of the radiolysis products, as a result of the comprehensive determination of radiolysis products in the destroyed and undestroyed azide matrix.
The most convenient method of analysis is, naturally, imme—
direct determination of the primary products of radiolysis without destruction of the crystal lattice of the salt. When the irradiated crystal is dissolved, however, interaction of the radiolysis products may occur, as well as reactions of the substance itself with the solvent. In a number of cases this may lead to the formation of ionic products from radical ones, which considerably complicates the already quite complex picture of radiolysis. When irradiated sodium azide is dissolved, the following processes occur:
\[ \mathrm{Na}+\mathrm{H_2O}=\mathrm{Na^+}+\mathrm{OH^-}+\mathrm{H}, \tag{3} \]
\[ \overline{e}+\mathrm{H_2O}=\mathrm{OH^-}+\mathrm{H}, \tag{4} \]
\[ \mathrm{H}+\mathrm{H}=\mathrm{H_2}, \tag{5} \]
\[ \mathrm{H}+\mathrm{N_3^-}\rightarrow \mathrm{NH_3}, \tag{6} \]
\[ \mathrm{NaN_3}+\mathrm{H_2O}\rightleftarrows \mathrm{Na^+}+\mathrm{OH^-}+\mathrm{HN_3}. \tag{7} \]
In view of the foregoing, we have attempted the direct determination of gaseous products without destroying the azide matrix. The methods described below are designed for determining the principal products of azide radiolysis using a single weighed portion of the salt, applying the following sequence of operations: determination of nitrogen evolved from the crystal, determination of nitrogen retained by the crystal by dissolution, analysis in solution of the remaining products (\(\mathrm{OH^-}\), \(\mathrm{NH_3}\)) and of the undecomposed azide group.
Determination of \(\mathrm{N_2}\)
Nitrogen is probably formed by the following reactions:
\[ 2\mathrm{N_3^{-*}}\rightarrow 3\mathrm{N_2}+2e, \tag{8} \]
\[ 2\mathrm{N_3^0}\rightarrow 3\mathrm{N_2}, \tag{9} \]
\[ \mathrm{N_3^{-3}}+\mathrm{N_3^0}\rightarrow 3\mathrm{N_2}+e \tag{10} \]
and accumulates in the crystal lattice. Part of the nitrogen that is located near the surface or is formed at the surface is released from the crystal. The evolved nitrogen can be determined from the increase in pressure in a closed calibrated volume on the apparatus shown in Fig. 1.
Fig. 1. Vacuum apparatus for determining the gas evolved during radiolysis.
A weighed portion of azide (1) is placed in the irradiation cell (2). The system is evacuated to a pressure of \(10^{-4} \div 10^{-5}\) torr and conditioned under vacuum until complete degassing. The pressure is determined by means of an LT-2 thermocouple gauge (3) and an LM-2 ionization manometer (4), with the readings recorded on the tape of a PS-type or EPP-09 self-recording instrument (7).
After closing valve (5), the leakage curve is recorded. Irradiation is begun if leakage into the system is either absent or insignificant. The decomposition curve is recorded and the pressure increase \(\Delta P\) is determined from the calibration curve. Knowing the volume of the system section isolated by valve (5), it is easy to determine the volume of evolved gas, \(V_{\mathrm{evol}}\),
\[ V_{\mathrm{evol}} = \frac{\Delta P \cdot V}{760}. \]
The greater the surface-to-volume ratio, the greater the amount of nitrogen evolved from the crystal. Experiments carried out by us with NaN\(_3\) crystals of various dispersities showed that, for azide crystallite sizes smaller than 50 microns, all the nitrogen is released from the azide matrix. At dispersities from 100 to 1000 microns, only \(40 \div 10\%\) of the total amount of nitrogen is released. The accuracy of this method exceeds the accuracy of determining nitrogen retained in the crystal lattice after dissolution. This provides the only possibility for determining radiolytic nitrogen at doses below \(10^{19}\) eV/g. At higher irradiation doses, the evolved nitrogen can be determined from the weight loss of a sample weighed before and after irradiation with an accuracy of up to \(10^{-5}\) g.
Determination of Nitrogen Retained by the Crystal Lattice
The method described in [3], which consists in determining under a microscope the gas bubble evolved upon dissolution of irradiated crystals, has a number of shortcomings, for example, the need to work with single crystals irradiated with a dose not lower than \(10^{19}\) eV/g. We have developed cells for determining retained nitrogen in polycrystalline preparations. One type of these cells is shown in Fig. 2. The use of the developed cells is convenient in that it makes it possible to compare the amount of nitrogen evolved into the vacuum and retained by the matrix. By increasing the weighed portion of sodium azide, one can obtain a quantitative estimate of radiolytic nitrogen in the region of low doses.
Fig. 2. Cell for determining small amounts of gas evolved upon dissolution of a crystal.
When working with the cell, the sample under investigation is placed in the boat (1), and it is lowered onto the water poured into the inner beaker of the cell (2). With the stopcocks open [3, 6, 8], the inner beaker is inserted into the outer one, ground to fit it. The stopcocks (3, 8) are closed and the system is thermostated (9); when the drop of liquid (4) in the capillary (5) becomes fixed in one place, the boat with the sample is overturned by a magnet or by any other method. The gas evolved during dissolution moves the liquid seal in the capillary to a new position. Knowing the diameter of the capillary and the magnitude of the displacement, it is easy to determine the volume of nitrogen evolved. The Forbalon (7) protects the system from fluctuations of atmospheric pressure. The duration of the determination depends on the rate of dissolution and is on average 1–2 hours. In designing and operating the apparatus it is necessary to observe the following recommendations: the volume of the free space above the floating boat should be reduced to a minimum; when the capillary is brought out of the horizontal position, the drop of liquid must move freely along the capillary without stopping. Using a cathetometer, changes in volume of less than \(1\ \mathrm{mm}^3\) can be determined with a capillary diameter of \(1\ \mathrm{mm}\). The accuracy can also be increased by carrying out the measurements under reduced pressure in the system. To measure large amounts of gas it is necessary to increase the diameter of the capillary and its length.
In cells of the type shown in Fig. 2, analytical determinations can be carried out provided that the reaction is accompanied by the evolution or absorption of gas. The limits of applicability are determined by the amount of gas evolved and by its composition. In the case of simultaneous evolution of hydrogen and nitrogen, the volume of hydrogen can be determined by combustion, passing discharges between platinum electrodes sealed into the space above the boat.
Determination of \(\mathrm{OH}^-\)
The determination is based on the catalysis of certain reactions by hydroxide ions. In this case it is necessary that the catalysis be specifically basic and that the catalyzed reaction proceed with the evolution of gas. This condition is satisfied by the reaction of decomposition of nitrosoacetones under the action of alkalis [4].
\[ \mathrm{CO} \begin{gathered} \nearrow \mathrm{CH_2\!\cdot\!(CH_3)_2}\\ \searrow \mathrm{CH_2\!\cdot\!C(CH_3)_2} \end{gathered} \begin{gathered} \nearrow\\[-0.6em] \searrow \end{gathered} \mathrm{N\!\cdot\!NO} \xrightarrow{\mathrm{OH^-}} \mathrm{CO} \begin{gathered} \nearrow \mathrm{CH\!:\!C(CH_3)_2}\\ \searrow \mathrm{CH\!:\!C(CH_3)_2} \end{gathered} +\mathrm{N_2}+\mathrm{H_2O}. \tag{11} \]
The decomposition follows a first-order reaction up to an \(\mathrm{OH}\) concentration of about \(0.05\ \mathrm{N}\); the rate constant does not depend on the nature of the alkali. The determination of alkali is based on the direct dependence of the rate of decomposition (in our case, the volume of gas evolved) on the concentration of the hydroxide ion.
Determination of the undecomposed azide group
The determination is based on the reaction
\[ 3\mathrm{N_3^-}+\mathrm{NO_2^-}+4\mathrm{H^+}\to 5\mathrm{N_2}+2\mathrm{H_2O}, \tag{12} \]
the method is applicable at a decomposition of the order of 10% for compiling a balance over all products.
Using the sequence of operations described above, we study the kinetics of radiolysis of \(\mathrm{NaN_3}\) in X-ray and proton fields.
radiation and certain questions concerning the effect of impurities on the radiation stability of NaN₃. The results obtained constitute the content of work [5].
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