Radiolysis of alkali metal chlorates
Abstract Generated abstract
This study examines the radiolysis of solid alkali metal chlorates under cobalt gamma irradiation, focusing on the composition, yields, and thermal stability of stable products formed from the chlorate ion. Irradiated sodium, potassium, rubidium, and cesium chlorates were analyzed for chloride, chlorite, chlorine dioxide, and perchlorate, with product accumulation found to be linear up to 16 Mrad and total decomposition yields increasing with the atomic number of the cation. The authors propose a model in which energy absorbed by the cationic lattice component is transferred to the complex anion, and show that the resulting yield relationships are approximately linear when normalized by electronic fractions and cation ionization potentials. Heating experiments indicate that chlorine dioxide decreases while chloride and perchlorate increase, with weak dependence on the chlorate matrix and an estimated activation energy of 13 kcal/mol for chlorine dioxide decomposition.
Full Text
Radiolysis of Alkali Metal Chlorates
L. T. Bugaenko, E. A. Zakharov, L. V. Serikov
(Presented by the scientific seminar of the Department of Radiation Chemistry)
The literature contains scattered information on the stable products of radiation-chemical transformations of certain chlorates [1, 2, 3]; however, no systematic studies of their radiolysis have been carried out, although these salts are of definite practical interest and may serve as convenient models for studying the processes of radiolysis of solid compounds.
The present work gives the results of studies of the composition, yields, and thermal stability of the final products of transformation of the chlorate ion in solid chlorates of alkali metals.
The study of radiation transformations in the group of alkali-metal chlorates also made it possible in this work to consider the question of changes in radiation stability within the series of chlorates studied. The explanation proposed in this work for the periodic dependences of the radiation stability of inorganic salts is based on the assumption that the energy of radiation absorbed by the cationic component of the lattice is transferred to the destruction of the complex anion, and it takes into account the change, along this series, in this constituent of the absorbed energy.
EXPERIMENTAL PROCEDURE
Preparations of alkali-metal chlorates of “chemically pure” grade were recrystallized three times from distilled water, dried in vacuum at a temperature of 25°C, and brought to a uniform dispersion.
Irradiation of the salts under study was carried out in a cobalt unit with an activity of 1600 g-eq. radium. The dose rate, according to a ferrous sulfate dosimeter, was 1170 rad/sec.
The irradiated chlorates were analyzed for the content of $Cl^-$, $ClO^-$, $ClO_2$, $ClO_2^-$, and $ClO_4^-$. Chlorine ions were determined nephelometrically with $AgNO_3$ [1]. $ClO_2$ and $ClO^-$ were determined spectrophotometrically by oxidation of $J^-$ ions to $J_2$ in a bicarbonate buffer solution. By the same method, at pH 1.5, $ClO_2^-$ was analyzed [5].
The method for determining the perchlorate ion is based on the color extraction reaction of $ClO_4^-$ with the dye malachite green [6]. Malachite green, reacting with $ClO_4^-$ ions, gives a green-colored compound that is readily extracted with benzene. The optical density of the extract is proportional to the concentration of $ClO_4^-$ ions; the absorption maximum, in spectrophotometric determination, is 640 nm.
The presence of the chlorate ion in a ratio of 5:1 in comparison with the concentration of the perchlorate ion leads to an error in the analysis. The presence of chloride ions, within a concentration ratio of 75:1, does not affect the accuracy of determining the perchlorate-ion concentration [6].
To a 20 mg sample weighed portion, add 2 ml of a saturated solution of sodium sulfite and 5 ml of 6 N $H_2SO_4$ (to reduce the chlorate ion to chloride), purge with air for 20 minutes, removing $SO_2$ from the solution, and neutralize the excess acid with alkali to pH 4.5–4.7. The solution is then diluted with water to 5 ml. 5 ml of the solution under study is placed in a separatory funnel and a 20-fold amount of dye, relative to the maximum permissible content of the perchlorate ion in the solution, is added, i.e., 1 ml of a $1 \cdot 10^{-2}$ M solution of malachite green.
Then 12 ml of benzene is added. The ratio of the organic and aqueous phases must be 2:1. The contents of the funnel are shaken and allowed to stand for 4–5 minutes. The optical density of the extract is measured relative to pure benzene. The molar extinction coefficient at $\lambda = 640$ nm is equal to
$23600 \pm 600$ L/(mol·cm).
The optical density obeys the Lambert–Beer law in the concentration range from $0$ to $1.25 \cdot 10^{-4}$ mol/L. The error of determination is 5%.
Heating of the irradiated chlorates was carried out in air at temperatures of $100^\circ$, $130^\circ$, and $150^\circ\mathrm{C}$.
EXPERIMENTAL DATA AND THEIR DISCUSSION
In chlorates irradiated at room temperature, the following stable products of transformation of the chlorate ion were observed: chloride, chlorite, perchlorate, and chlorine dioxide ions. In contrast to the data of work [1], hypochlorite ions were not found in an aqueous solution of irradiated chlorate. However, it is premature to draw from this a final conclusion about the complete absence of $\mathrm{ClO^-}$ ions in solid irradiated chlorates, until the nature and kinetics of hypochlorite-ion transformation in the dissolution solutions have been clarified.
The accumulation curves of all transformation products of the chlorate ion in the dose interval studied, up to 16 Mrad, were linear. The values of the yields of radiolysis products and the yield of decomposition of the chlorate ion are given in Table I. As can be seen, the yield increases with increasing atomic number of the alkali cation.
Table I
Yields of products of decomposition of the chlorate ion
| Salt | $G(\mathrm{Cl^-})$ | $G(\mathrm{ClO_2^-})$ | $G(\mathrm{ClO_2})$ | $G(\mathrm{ClO_4^-})$ | $G(-\mathrm{ClO_3^-})$ |
|---|---|---|---|---|---|
| $\mathrm{NaClO_3}$ | $1.06 \pm 0.04$ | $0.96 \pm 0.05$ | $0.36 \pm 0.04$ | $0.75 \pm 0.05$ | $3.1 \pm 0.2$ |
| $\mathrm{KClO_3}$ | $1.58 \pm 0.02$ | $1.07 \pm 0.12$ | $1.01 \pm 0.02$ | $1.10 \pm 0.12$ | $4.8 \pm 0.3$ |
| $\mathrm{RbClO_3}$ | $1.68 \pm 0.03$ | $1.54 \pm 0.12$ | $1.21 \pm 0.06$ | $1.18 \pm 0.15$ | $5.6 \pm 0.4$ |
| $\mathrm{CsClO_3}$ | $1.96 \pm 0.08$ | $1.81 \pm 0.05$ | $1.81 \pm 0.06$ | $1.38 \pm 0.15$ | $7.5 \pm 0.5$ |
Data only on the final stable products of radiolysis of the chlorate ion do not make it possible to consider the detailed mechanism of their transformations. However, the general scheme of these processes can be considered. Under the action of radiation on the chlorate ion, primary products arise:
\[ \mathrm{ClO_3^-} \;\leadsto\; \mathrm{ClO_3},\ \mathrm{ClO_3^+},\ (\mathrm{ClO_3^-})^*,\ \bar e, \]
which, as a result of secondary reactions, give the observed final products:
\[ \mathrm{ClO_3},\ \mathrm{ClO_3^+},\ (\mathrm{ClO_3^-})^* \;\to\; \mathrm{Cl^-},\ \mathrm{ClO_2^-},\ \mathrm{ClO_2},\ \mathrm{ClO_4^-}. \]
It was usually assumed that the increase in yield with increasing atomic number of the cation occurs at the expense of an increase in the free volume of the crystal in this series [7], which facilitates the decomposition of excited forms of the anion and hinders reverse reactions. However, an alternative explanation may also be proposed, namely that the destruction of the anion occurs to a considerable extent through absorption of energy by the cation, i.e., through direct action of the radiation on the cation:
\[ \mathrm{Me^+} \;\leadsto\; \mathrm{Me^{++}},\ (\mathrm{Me^{++}})^*,\ (\mathrm{Me^+})^*_{\mathrm{primary}},\ \bar e \]
and subsequent transfer of energy to the anion, accompanied by decomposition of the latter.
A probable scheme of the process includes capture of a thermalized electron by the ionized cation and destruction of neighboring anions at the expense of the energy thereby released:
\[ \mathrm{Me^{++}} + \bar e \;\to\; (\mathrm{Me^+})^* \]
\[ (\mathrm{Me^+})^* + \mathrm{ClO_3^-} \;\to\; \mathrm{Me^+} + \mathrm{Cl^-},\ \mathrm{ClO_2^-},\ \mathrm{ClO_2},\ \mathrm{ClO_4^-}. \]
In this case, naturally, it is assumed that the cation affects neither the character nor the rate of the secondary reactions leading to the formation of the final products. Then, if this scheme is adopted, for an individual salt the observed yield of chlorate will be the sum of the following two components:
\[ G(-\mathrm{ClO_3^-}) = G_{\mathrm{an}}\cdot \varepsilon_{\mathrm{an}} + G_{\mathrm{kat}}\cdot \varepsilon_{\mathrm{kat}}, \tag{I} \]
where \(G_{\mathrm{an}}\) is the yield of destruction of the chlorate ion due to the energy absorbed by it itself, \(G_{\mathrm{kat}}\) is that due to the energy absorbed by the cation, and \(\varepsilon_{\mathrm{an}}\) and \(\varepsilon_{\mathrm{kat}}\) are the electronic fractions of the anion and cation,
Fig. 1. Dependence of the yield of radiolysis products of irradiated chlorates and bromates on the electronic fractions of the anion and cation.
respectively. In order to extend this formula to a series of salts, it is necessary to introduce into the configuration a coefficient for the variable constituent—the cation—taking into account the change in its properties within the series of salts. As such a normalizing factor one may choose the ionization potential of the cation, assuming that the excitation potentials form, to a first approximation, the same series as the ionization potentials. Normalization may be carried out with respect to any cation; we choose the first member of the series under study—the sodium ion. Then equation (1) takes the following form:
\[ G(-ClO_3^-)=G_{\mathrm{an}}\cdot \varepsilon_{\mathrm{an}} +G_{\mathrm{cat}}\cdot \varepsilon_{\mathrm{cat}}\cdot \frac{J(Na^+)}{J_{\mathrm{cat}}}, \tag{2} \]
where \(J_{\mathrm{cat}}\) is the ionization potential of the salt cation, and \(J(Na^+)\) is the ionization potential of the sodium ion. For a graphical solution of equation (2), it is more convenient to present it in the form
\[ \frac{G(-ClO_3^-)}{\varepsilon_{\mathrm{an}}} = G_{\mathrm{an}} + G_{\mathrm{cat}}\cdot \frac{\varepsilon_{\mathrm{cat}}}{\varepsilon_{\mathrm{an}}} \cdot \frac{J(Na^+)}{J_{\mathrm{cat}}}. \tag{3} \]
As follows from Fig. 1, the yields of all products, in the coordinates of equation (3), fall on straight lines, which makes it possible to regard the proposed model as quite probable as a first approximation.
Let us examine in more detail the basic assumptions adopted by us in deriving expression (3). First of all, as the normalizing factor that takes into account changes in the series of chlorates in the conditions of ionization and excitation of the cation, we introduced the ratio of the second ionization potentials. The scheme of elementary processes given above assumes the capture of an electron by \(Me^{++}\) situated in the lowest free band (the conduction band). It is usually assumed that the upper filled band is an anion band (i.e., it is obtained by splitting the upper filled electronic term of the anion), while the lower, free band is a cation band and is obtained by splitting the lower free (i.e., first excited) level of the cation.
The introduction, as the normalizing coefficient, of the ratio \(\mathcal{J}_{Me}^{+}/\mathcal{J}_{\mathrm{cat}}\), instead of the physically more comprehensible ratio of the first excitation potentials of the cations, was made on the assumption that \(\mathcal{J}_{\mathrm{cat}}\) changes along the series in proportion to the change in the excitation potential.
Within the framework of the first approximation, the work takes into account only double ionization of the alkali metal and the reverse reaction
\(Me^{++} + e \rightarrow Me^{+} + E\)
as a source of energy going to the destruction of the complex anion. Thus, multiple ionization of the cations and the role in the destruction of the anion of that part of the radiation energy absorbed by the cation component, which is released upon recombination of such multiply charged cations with an electron, remain unaccounted for. However, when considering the phenomenon in a first approximation, this assumption may be justified by the fact that, as is known, the cross section of multiple ionization \((\sigma n)\) rapidly decreases with increasing multiplicity \((n)\), being approximately described by the following expression [9]:
\[ \sigma n = \sigma_1 n^{-(n-1)} . \]
Hence it becomes understandable why multiple ionizations are disregarded, since their cross section is much smaller than that of the most massive double ionization.
Further, in the model under consideration, for generality it would be necessary to take into account changes in the series of similar chlorates not
not only the probabilities of ionization of the cations, but also the probabilities of ionization and excitation of the anionic component of the lattice. A change in the probability of ionization of the anion would occur when, in the series of chlorates, the width of the forbidden band changes.
Our experiments show, however, that the edge of the optical absorption band changes only very slightly in the series of chlorates.
Together with these data, the absence of noticeable changes in the probability of ionization of $\mathrm{ClO_3^-}$ in the series of chlorates under consideration is also indicated by the constancy, established in the work, of the radiation yield of the product of anion ionization—the radical $\mathrm{ClO_3}$—calculated per 100 eV of energy absorbed by the anion, upon irradiation of $\mathrm{NaClO_3}$ and $\mathrm{KClO_3}$.
And, finally, the work contains the assumptions that the secondary decomposition reactions of the anion due to energy absorbed by the cationic component are analogous to the reaction in the direct radiolysis of the anion; in this case the series dependences of the rates of such secondary reactions are weakly expressed or absent.
The first of these assumptions is difficult to test experimentally. Some confirmation of the second assumption was obtained by us in studying the secondary thermal reactions of interconversion of the products of radiolysis of chlorates, and is reported below, in the second part of the present work.
The values of the yields of formation of each of the products and of the yield of decomposition of the chlorate ion due to the direct action of radiation on the anion and cation, obtained within the framework of all these approximations from expression (3), are presented in Table 2. The values thus obtained show that the contribution of the alkali-metal cation to the decomposition of the anion is decisive in the radiolysis of solid chlorates.
Equation (3) was checked for the case of radiolysis of bromates. In Fig. 1 a straight line is given, constructed in the coordinates of equation (3) from the data of Boyd’s work [3]. As is evident from the figure, the dependence remains similar to the radiation-chemical decomposition of alkali-metal chlorates.
Table 2
Yields of chlorate radiolysis products due to the direct action of radiation on the anion and cation
| $\mathrm{Cl}^-$ | $\mathrm{ClO_2^-}$ | $\mathrm{ClO_2}$ | $\mathrm{ClO_4^-}$ | $-\mathrm{ClO_3^-}$ | |
|---|---|---|---|---|---|
| $G_{\text{an}}$ | 1.10 | 0.40 | 0.25 | 0.8 | 2.55 |
| $G_{\text{cat}}$ | 1.45 | 1.95 | 1.60 | 0.95 | 5.95 |
The data in the table show that, despite the considerable energy released in reactions of the type
$M^{++} + e \rightarrow M^{+} + \varepsilon$,
destruction of the $\mathrm{ClO_3^-}$ anion proceeds mainly not to the products of its deep decomposition.
The thermal stability of the radiolysis products of the chlorate ion was studied at 100, 130, and 150°C. It was found that the concentration of chlorine dioxide decreases, the concentration of chloride and perchlorate ions increases, and the concentration of chlorite ions does not change as the heating time is increased. Typical curves for the change in chlorine dioxide concentration are given in Fig. 2, and those for chloride and perchlorate ions in Fig. 3. In all cases, upon prolonged holding, a certain stationary concentration of the products, characteristic of the given temperature, is reached. The magnitude of the change in concentration is the greater, the higher the temperature.
The curves for the change in the concentration of $\mathrm{ClO_2}$ and of the $\mathrm{Cl^-}$ and $\mathrm{ClO_4^-}$ ions cannot be described by first- or second-order equations. Similarly to how the curves for changes in radical concentration during hindered recombination are described [8], the curves can be treated kinetically while allowing for some portion of the compound whose concentration is being changed. Thus, the curve for the decrease in the concentration of $\mathrm{ClO_2}$ is satisfactorily described by the equation:
\[ C = (C_0 - C_{\infty}) e^{-kt} + C_{\infty}, \tag{IV} \]
where $-C_0$ is the initial concentration, and $C_{\infty}$ is the limiting concentration at the given temperature, $k$ is the rate constant of decomposition. Values of $k$ are given in Table 3. The curve for the change in the concentration of $\mathrm{Cl^-}$ and $\mathrm{ClO_4^-}$ ions is satisfactorily described by the equa-
solution:
\[ C = C_0 + (C_\infty - C_0)(1 - e^{-kt}) \tag{У} \]
The rate constants for the accumulation of these ions for 150°C are also given in Table 3.
Table 3
Values of the constants for decomposition of \(\mathrm{ClO_2}\) and formation of \(\mathrm{Cl^-}\) and \(\mathrm{ClO_4^-}\) ions.
| Salt | 150°C, \(-\mathrm{ClO_2}\) | 150°C, \(\mathrm{Cl^-}\) | 150°C, \(\mathrm{ClO_4^-}\) | 130°C, \(-\mathrm{ClO_2}\) | 100°C, \(-\mathrm{ClO_2}\) |
|---|---|---|---|---|---|
| \(\mathrm{NaClO_3}\) | \(0.18 \pm 0.03\) | \(0.10 \pm 0.01\) | \(0.10 \pm 0.01\) | \(0.08 \pm 0.01\) | \(0.02 \pm 0.01\) |
| \(\mathrm{KClO_3}\) | \(0.19 \pm 0.03\) | \(0.08 \pm 0.01\) | \(0.11 \pm 0.01\) | \(0.06 \pm 0.01\) | \(0.02 \pm 0.01\) |
| \(\mathrm{RbClO_3}\) | \(0.14 \pm 0.02\) | \(0.12 \pm 0.04\) | \(0.11 \pm 0.04\) | \(0.05 \pm 0.01\) | \(0.05 \pm 0.01\) |
| \(\mathrm{CsClO_3}\) | — | \(0.08 \pm 0.03\) | \(0.10 \pm 0.02\) | \(0.07 \pm 0.01\) | \(0.05 \pm 0.01\) |
Although the data obtained should be regarded as preliminary, some conclusions can be drawn. The constants for decomposition of chlorine dioxide and the constants for formation of chloride and perchlorate ions do not depend, or depend only weakly, on the type of salt; i.e., the thermal reaction, to a first approximation, does not depend on the type of chlorate matrix. With increasing temperature, the rate of decomposition of chlorine dioxide and the rates of formation of the products of its conversion increase. The activation energy for the decomposition of \(\mathrm{ClO_3}\) is \(13 \pm 3\) kcal/mol. It should also be noted that the rate constants for the appearance of chloride and perchlorate ions are lower than the rate constant for decomposition of chlorine dioxide. This makes it possible to consider that chloride and perchlorate ions appear as a result of decomposition of chlorine dioxide.
For 100 and 130°C, the amount of chloride and perchlorate ions formed coincides with the amount of chlorine dioxide decomposed, whereas for 150°C, i.e., in experiments in which the accuracy of the experiment is higher, the amount of chlorine dioxide decomposed
Fig. 2. Change in the concentrations of \(\mathrm{Cl^-}\) and \(\mathrm{ClO_4^-}\) ions in irradiated chlorates during heating \((t^\circ = 150^\circ\mathrm{C})\).
Fig. 3. Change in the concentration of chlorine dioxide in chlorates during heating. 1 — \(100^\circ\mathrm{C}\); 2 — \(130^\circ\mathrm{C}\); 3 — \(150^\circ\mathrm{C}\).
less than the amount of products formed from it. At present it is not yet possible to give a complete scheme of the reactions of thermal transformation of the radiolysis products of the chlorate ion; however, in principle, one may consider the decomposition of chlorine dioxide either into a chlorine atom and oxygen, or into chlorine oxide and atomic oxygen. Atomic oxygen can oxidize the chlorite ion to the perchlorate ion. Atomic chlorine, evidently, is converted into chloride ions by reaction with the electron centers formed during radiolysis at the moment of dissolution. To construct a scheme for the thermolysis of the radiolysis products of chlorate ions, additional experiments are needed not only on the products investigated in the present work, but also on the radical intermediate products and electron centers.
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