Use of the Nitrate Ion as an Electron Acceptor in Frozen Solutions

I. A. Latok, V. P. Beletsky, T. T. Bugaenko

(Presented by the scientific seminar of the Department of Radiation Chemistry)

Determination of the yield of electrons arising under the action of ionizing radiation on solids is a very important experimental problem. In liquid solutions, the yield of electrons that have escaped recombination in spurs is usually determined from the degree of conversion of some acceptor. One effective acceptor is the nitrate ion, whose reaction rate constant with the hydrated electron is of the order of \(1 \cdot 10^{10}\ \mathrm{l/(mol\cdot s)}\) /1/. In frozen aqueous solutions the nitrate ion is also an electron acceptor, although it is less effective than in the liquid phase. Thus, if in the liquid phase all electrons are captured by nitrate ions at a concentration of the latter of \(\sim 10^{-3}\ \mathrm{M}\), then in solutions frozen at \(-196^\circ\mathrm{C}\) this occurs only at concentrations of \(10^{-1}\ \mathrm{M}\) and even higher /2/. The ion-radicals \(\mathrm{NO_3^{2-}}\), formed as a result of electron capture by nitrate ions, upon warming of the matrix are converted into nitrite ions, from whose yield the electron yield can be found, as was done, for example, in work /3/. However, in this case it must be shown that the nitrite ion arises only through the reaction of the electron with the nitrate ion. In the present work we attempt to determine whether this condition is fulfilled in the case of alkaline and perchlorate matrices.

Experimental Method

Aqueous solutions of \(\mathrm{NaClO_4}\) (8 M) and \(\mathrm{NaOH}\) (9 M), frozen at \(-196^\circ\mathrm{C}\), were irradiated with \(\gamma\)-rays from \(\mathrm{Co}^{60}\) and with X-rays of maximum energy 60 keV, as described in works /4,5/. All solutions were prepared with distilled water; before irradiation, air was displaced with nitrogen. Reagents of “chemically pure” grade were additionally recrystallized. The nitrite ion was determined spectrophotometrically with the Griess reagent after thaw—

of the irradiated samples. Samples for recording EPR spectra were irradiated in ampoules made of Luch-2 glass. Measurements were carried out on an RE-1301 radiospectrometer /5/.

Results and Discussion

We investigated the dependence of the yield of formation of ion-radicals \(\mathrm{NO_3^{2-}}\) and nitrite ions on the concentration of sodium nitrate in an 8 M aqueous solution of sodium perchlorate and a 9 M aqueous solution of sodium hydroxide, irradiated at the temperature of liquid nitrogen. Figure 1 shows the accumulation curves of the nitrite ion in both matrices. In the case of the perchlorate matrix, at a low concentration of nitrate ions \((<0.05\,\mathrm{M})\) the accumulation curve of nitrite ions is nonlinear and, with increasing dose, a stationary nitrite concentration is rapidly reached, the value of which is the lower, the lower the concentration of nitrate ions; moreover, the dependence of the concentration of nitrite ions on dose is satisfactorily described by the exponential formula:

\[ C = C_{\infty}(1-e^{-kD}), \tag{1} \]

where \(C\) and \(C_{\infty}\) are the concentrations of nitrite ions at a given and at an infinitely large dose \(D\), and \(k\) is a constant. The initial yield of nitrite ions in these solutions was calculated from formula (1). For high concentrations of nitrate ions, the accumulation curves of nitrite ions are practically linear. In the alkaline matrix, the accumulation curves of nitrite ions are linear at all concentrations of nitrate ions in the interval \(10^{-3}\)–2 M.

The accumulation curves of the ion-radical \(\mathrm{NO_3^{2-}}\) were recorded in the perchlorate matrix. At high doses the accumulation curves of \(\mathrm{NO_3^{2-}}\) are also nonlinear.

The nonlinearity of the accumulation curves of nitrite ions and of the ion-radicals \(\mathrm{NO_3^{2-}}\) in the case of the perchlorate matrix indicates the presence of reverse reactions directly during irradiation and also, possibly, during melting of the matrix. Such a reverse process may be, for example, the reaction of the ion-radical \(\mathrm{NO_3^{2-}}\) with the \(\mathrm{OH}\) radical:

\[ \mathrm{NO_3^{2-}} + \mathrm{OH} \longrightarrow \mathrm{NO_3^-} + \mathrm{OH^-}. \tag{2} \]

Fig. 1. Accumulation curves of nitrite ions in perchlorate (a) and alkaline matrices (b).
Axis labels: \(C, 10^{-3}\ \mathrm{M}\); \(D^{\mathrm{abs}}, 10^{18}\ \mathrm{eV/g}\). Curve labels: (a) \(2\ \mathrm{M}\ \mathrm{NaNO_3}\), \(0.1\ \mathrm{M}\ \mathrm{NaNO_3}\), \(0.01\ \mathrm{M}\ \mathrm{NaNO_3}\); (b) \(0.25\ \mathrm{M}\ \mathrm{NaNO_3}\), \(0.001\ \mathrm{M}\ \mathrm{NaNO_3}\).

The nitrite ion can be oxidized, for example, by oxygen atoms that arise under the direct action of radiation on the perchlorate ion:

\[ \mathrm{NO_2^- + O \longrightarrow NO_3^-}, \tag{3} \]

or by OH radicals:

\[ \mathrm{NO_2^- + OH \longrightarrow NO_2 + OH^-} \tag{4} \]

As is known for liquid solutions, the ion-radical \(\mathrm{O^-}\) is a considerably weaker oxidizing agent. Evidently, therefore, in an alkaline matrix the accumulation curve of the nitrite ion is, over the dose interval investigated, linear.

Figure 2 presents the dependence of the yields of nitrite ions and of the ion-radicals \(\mathrm{NO_3^{2-}}\) on the concentration of nitrate ions in perchlorate and alkaline matrices. For a \(0.1\ \mathrm{M}\) solution of \(\mathrm{NaNO_3}\) in \(8\ \mathrm{M}\ \mathrm{NaClO_4}\), practically the same value of \(G(\mathrm{NO_2^-})\) was obtained as, according to the data of work /3/, in a \(0.1\ \mathrm{M}\) solution of \(\mathrm{NaNO_3}\) in \(10\ \mathrm{M}\ \mathrm{NaClO_4}\). For the alkaline matrix, the values of \(G(\mathrm{NO_2^-})\) obtained by us for the corresponding nitrate concentrations are lower than those reported in work /3/. A constant value of the yields of nitrite ions and ion-radicals \(\mathrm{NO_3^{2-}}\) is reached at a concentration of nitrate ions

equal to 0.7. In an alkaline matrix \(G(\mathrm{NO_3^{2-}})=1.7\pm0.1\); in a perchlorate matrix \(G(\mathrm{NO_2^-})=2.3\), and \(G(\mathrm{NO_3^{2-}})=2.8\) particles per 100 eV.

Fig. 2. Dependence of the yields of nitrate ions and \(\mathrm{NO_3^{2-}}\) ion-radicals on nitrate concentration.

The nitrite ion is formed upon warming as a result of the reactions

\[ \mathrm{NO_3^{2-}}+\mathrm{H_2O}\longrightarrow \mathrm{NO_2}+2\mathrm{OH^-}, \tag{5} \]

\[ 2\mathrm{NO_2}+\mathrm{H_2O}\longrightarrow \mathrm{NO_2^-}+\mathrm{NO_3^-}+2\mathrm{H^+}. \tag{6} \]

By stoichiometry, one nitrate ion corresponds to two \(\mathrm{NO_3^{2-}}\) ion-radicals. According to the data of [3], in an alkaline matrix \(2G(\mathrm{NO_2^-})=G(\mathrm{NO_3^{2-}})\), but in a perchlorate matrix this ratio is not fulfilled, and \(2G(\mathrm{NO_2^-})>G(\mathrm{NO_3^{2-}})\).

Several causes are possible as a result of which the above equality will be violated.

1. The \(\mathrm{NO_3^{2-}}\) ion-radical is not the only precursor of the nitrite ion. For example, a reaction of atomic oxygen with the nitrate ion is possible:

\[ \mathrm{NO_3^-}+\mathrm{O}\longrightarrow \mathrm{NO_2^-}+\mathrm{O_2}, \tag{7} \]

which also leads to formation of the nitrite ion.

1. The nitrate ion removes excitation from perchlorate ions and

it itself decomposes into nitrite ion and oxygen:

\[ \mathrm{NO}_3^{-*} \longrightarrow \mathrm{NO}_2^- + \mathrm{O}. \tag{8} \]

1. Simultaneously with the ion-radical \(\mathrm{NO}_3^{2-}\), nitric oxide is formed; it is not observed in the EPR spectrum, but it reacts with the Griss reagent like nitrite ion. Since in an alkaline matrix \(G(\mathrm{NO}_3^{2-}) = 2G(\mathrm{NO}_2^-)\), one might suppose that \(\mathrm{NO}\) is formed as a result of the following sequence of reactions:

\[ \mathrm{NO}_3^- + \mathrm{H}_2\mathrm{O}^{+} \longrightarrow \mathrm{NO}_3^{*} + \mathrm{H}_2\mathrm{O} \tag{9} \]

\[ \mathrm{NO}_3^{*} \longrightarrow \mathrm{NO} + \mathrm{O}_2. \tag{10} \]

In an alkaline matrix the \(\mathrm{H}_2\mathrm{O}^{+}\) ions rapidly react with \(\mathrm{OH}^-\) ions, thereby protecting the nitrate ion from destruction.

1. Addition of an electron to the nitrate ion with dissociative ionization:

\[ \mathrm{NO}_3^- + \bar{e} \longrightarrow \mathrm{NO}_2^- + \mathrm{O}^-. \tag{11} \]

This reaction would have to proceed in both matrices, but since the difference in the yields of nitrite and of ion-radicals is observed only in the perchlorate matrix, reaction (11) is unlikely.

The most probable reasons for the nonfulfillment in the perchlorate matrix of the equality \(G(\mathrm{NO}_3^{2-}) = 2G(\mathrm{NO}_2^-)\) appear to us to be the first and second of those listed above.

Thus, if in an alkaline matrix the nitrate ion is a specific electron acceptor /3/, then in a perchlorate matrix the nitrate ion is an acceptor not only of electrons; the yield of the product of its transformation—the nitrite ion—exceeds the yield of electrons formed under the action of radiation. Determination of the electron yield in the matrix from the yield of nitrite ions can be used only in the case where it has been proved that the equality \(G(\mathrm{NO}_3^{2-}) = 2G(\mathrm{NO}_2^-)\) is fulfilled.

Moscow State University

Faculty of Chemistry

References

1. A. K. Pikaev. The Solvated Electron in Radiation Chemistry. Moscow, Nauka, 1969, p. 151.

2. L. T. Bugaenko, O. S. Povolozkaya. High Energy Chemistry, 1, 480 (1967).

3. B. G. Ershov, E. L., A. K. Pikaev. High Energy Chemistry, 1, 544 (1967).

4. V. N. Belevskii, L. T. Bugaenko. High Energy Chemistry, 1, 253 (1967).

5. V. N. Belevskii, L. T. Bugaenko. ZhFKh, 39, 2958 (1965).