RADIOLYSIS OF CRYSTALLINE ALKALINEEARTH BROMATES
1031
Radiolysis of the Crystalline Alkaline Earth Bromates by Cobalt-60 ?-Rays*
by J. W. Chase and G. E. Boyd Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
(Received August 9,1965)
The anhydrous crystalline alkaline earth bromates were radiolyzed by 'j0Co y-rays to decompositions exceeding l mole %. Bromate ion was decomposed to oxygen gas and bromite, hypobromite, and bromide ions, which were observed when the irradiated crystals were dissolved in alkaline aqueous solutions. Evidence was obtained suggesting that BrOz also was produced and stabilized in the crystal lattice. The bromate ion decomposition and the yields of oxidizing fragments increased nonlinearly with the dose. Bromide ion and oxygen gas yields increased linearly with dose for small doses, but a t larger doses the rate of formation of bromide increased and then became constant, while the rate of oxygen gas production decreased slightly. The bromite ion yield initially was strongly dose dependent, but subsequently it became essentially independent of the dose. The rate of formation of hypobromite ion increased from a vanishingly small value at low doses to a maximum and then approached zero a t large doses. The data were consistent with the hypothesis that bromide ion, bromite ion, oxygen gas, and possibly BrOz were formed directly by the dissociation of excited BrOa- ions, whereas hypobromite and some bromide ions were formed via precursors. Recombination reactions reconstituting bromate ion also occurred.
This study of the radiolysis of the anhydrous alkaline earth bromates was undertaken to elucidate further the effect of the crystalline environment on the decomposition of bromate ion by 6oCoy-rays. Significant differences in radiation stability have been shown to exist between the various alkali metal bromates,2 and it was expected that differences between the alkaline earth compounds would be observed. Differences in the stability of the molecular bromate ion in the alkaline earth salts from that in the alkali metal bromates also were expected based on observations previously made with the nitrates3 and perc h l o r a t e ~ . ~The alkaline earth bromates have been shown to be less stable thermally than the alkali metal salts,5 and their hydrates at room temperature to be more stable.
Experimental Section The hydrated alkaline earth bromates (City Chemical Corp.) were purified by several recrystallizations from ethanol-water mixtures. The anhydrous compounds were prepared by heating (ca. 125") the purified salts in a vacuum oven until a constant weight was reached. Thermogravimetric analysis confirmed
that all water was removed. The weighing bottles holding the dehydrated salts were capped immediately upon removal from the vacuum oven and were stored away from light over a desiccant. Each preparation was analyzed for alkali metal, alkaline earth, and bromide ion impurities (Table I). A satisfactory preparation of anhydrous Mg(BrO& could not be obtained because of its excessive thermal decomposition on drying. Heating the salt under vacuum for ca. 24 hr at 105" to obtain a completely anhydrous product yielded more than 800 ppm of bromide ion. Accordingly, the radiolysis for large doses only could be measured without a large blank correction. Approximately 3-5-g amounts of the purified crystalline salts were transferred in dry nitrogen to cylindrical (1) Presented before the Division of Physical Chemistry, 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965. Research sponsored by the U. 9. Atomic Energy Commission under contract with Union Carbide Corp. (2) G. E. Boyd, E. W. Graham, and Q. V. Larson, J. Phys. Chem., 66, 300 (1962). (3) J. Cunningham and H. G. Heal, Trans. Faraday Sac., 54, 1355 (1958). (4) L. A. Prince and E. R. Johnson, J. Phys. Chem., 69, 359 (1965). (5) J. W. Chase, to be published.
Volume YO, Number 4 April 1966
1032
J. W. CHASEAKD G. E. BOYD
Table I: Punty of the Anhydrous Alkaline Earth Bromates (All Values in ppm by Weight) Impurity
Li + Na
Mg(BrOa)ra
Ca(Br0a)z
Sr(Br0s)r
Ba(BrO&
Sr > Ca > blg. The rate of production of bromide ion became progressively larger with increasing dose after an initial nearly linear yield-dose dependence. This behavior suggested that part of the bromide found in the irradiated crystals was formed either through thermal or radiolytic decomposition of intermediates. The production of Oz(g) in Sr(Br03)zwas linear with dose within experimental error up to 15.5 X loz3ev mole-'; with Ca(Br03)zthere was a slight but definite nonlinear yield-dose dependence for doses as small as 0.12 X loz3ev mole-'. The dose dependence of the production of oxidizing fragments in Ca(BrO3)z and Sr(BrO& (Figure 3) and of bromite ion in irradiated CsBr03 (Figure 4)9 contrasts with the yields for Oz(g) and Br- ion in that saturation or "steady-state" concentrations of the former were approached at large doses. It, therefore, was necessary to assume that either a decomposition and/or a recombination reaction of a thermal or radiolytic nature acted to remove the oxidizing fragments. If Oz(g) were produced in the same decompositions which led to bromide ion, a similar dose dependence of the yields for these tlyo products would have been expected. The fact that the Oz(g) yield actually was less than expected on this hypothesis indicated that a reaction which consumed oxygen occurred in the crystal. The observation that the yield of bromite ion became constant at large doses (Figure 4) suggested that this reaction waslo BrOz-
+ 0 -+Br03-
(.4)
The virtually linear dependence of the OZ@) yield On dose mav be explained if the OXYWn _ _ atoms required in (A) were supplied predominantly by the decomposition of hypobromite BrO- +BrThe Journal of Physical Chemistry
+0
(B)
DOSE (e.v. mole" x 4023)
4. Yield of BrOz- and BrO- in t h e radiolysis of crystalline CsBrO, by 6oCo7-rays (hypobromite ion yield curve is convex to dose axis below 6 X loz3
Figure
ev mole-').
The effect of temperature upon the yields in crystalline CsBr03 as well as isothermal and isochronal annealing experiments with irradiated crystals of the cesium salt' have shown that thermal decomposition of the oxidizing fragments and thermally activated recombination reactions occur in the absence and in the presence of radiation. Hypobromite ion is known to be quite unstable and to break down to give Brand Oz(g); bromite is relatively stable and may be implicated as one of the fragments which reacted with oxygen to reconstitute bromate ion. Results from isothermal anneals of the oxidizing fragments produced in Sr(BrO3)z are given in Table IV. Heating the irradiated crystals, as in the case of irradiated CsBr03, caused the removal of some of the oxidizing fragments by a reaction which was quite rapid initially followed by a much slower process. In addition, the amount of Br03- ion decomposed decreased. If thermal decomposition to Br- ion only had occurred, the Br03ion decomposition would have remained constant. Consequently, a back-reaction converting oxidizing fragments to Br03- ions must have taken place. The changes observed on heating irradiated Sr(Br03)z may be explained by assuming a recombination reaction of BrOz- with oxygen (eq A) and a thermal decomposition of BrO- to yield Br- ion (eq B). The dependence of the yield of hypobromite ion on dose (Figure 4) indicates that this species was not a (9) BrOz- and BrO- yields also were obtained arith Sr(BrOs)a, but not to a dose sufficiently large to give a "steady-state'' concentration. However. the shams of the vield-dose curves were quite similar to those for CsBrOs. (10) The oxygen atoms consumed in eq A are assumed to have been formed by the thermal decomposition of oxidizing fragments. Reaction with radiolytically produced 0 atoms may have occurred t o a minor extent, however.
RADIOLYSIS O F CRYSTALLIKE ALKALrivE EARTH BROMATES
1035
Table JV : Isothermal Annealing (125') of Oxidizing Fragments Produced in Crystalline Sr(BrO,)* by 6oCo?-Rays (Dose = 5.5 X loz3ev mole-') Time of heating, hr
Oxidizing power, mequiv/mole of Br&-
0 0.5 1 2 3 5 8 16 30 46
24.5 21.4 20.2 (21.1) (21.0) 19.2 18.8 18.8 18.5 18.9
OXYGEN- BROMINE RATIO)
Bromate decomposition, mmoles,"ole of BrOa-
10.2 9.92 (8.83) 9.91 9.73 9.51 9.90 9.52 9.47 9.76
3.0
t.50
3'0k7-
2.0
primary radiolysis product of Br03- ion. At small doses the BrO- yield was zero within experimental error, but subsequently it increased in a manner similar to that observed with bromide ion. However, in contrast to the Br- ion yield, at large doses (ie., >IO X ev mole-'), the yield of hypobromite became independent of dose. The yield of BrO- also showed a distinct temperature dependence: at -196" and a dose of 1.73 X l o z 3ev mole-', the amount produced in CsBr03 was 40% smaller than at room temperature. I n contrast, tho yield of BrO2- mas changed by only ca. ;%, as expected for a species formed by a direct process. Analogous observations have been made in the radiolysis of KC104, where hypochlorite was found but not as a primary radiolytic product." The lack of a dependence of the BrO- yield on dose at large doses (ie., "steady-state" BrO- ion concentration) may be understood if this ion decomposed thermally at the same rate as it was formed from a precursor.12 Information as to the nature of the precursor of hypobromite ion may be inferred from the average oxidation number, Z,of bromine in the oxidizing fragments. The ratio of the measured "total oxidizing power" (mequiv g-*) of the radiolyzed crystals (Figure 3) to the nimoles g-' of bromine contained in the fragments as estimated from the difference in the bromide ion and bromate decomposition yields (Figure 1) gives the average change in oxidation number, 2 1, involved in the complete reduction of the fragments. (mequiv "ole-') on the extent The dependence of of bromate ion radiolysis is shown in Figure 5 . Initially, 2 was greater than 3.5, but this decreased rapidly, followed by a gradual decrease to an apparent limiting value of ca. 2.0. Although the latter value corresponds to an equimolar mixture of BrOn- and BrO- ions, a
+
z
DECOMPOSITION ("ole
Br-/mole BrO;
)
Figure 5. Oxidation number and oxygen-bromine ratio of an average oxidizing fragment as B function of bromate decomposition.
value greater than 3.5 requires that species of higher oxidation number also be present in small amounts. These species appeared to be quite unstable thermally; it seemed logical to suppose that they might have been the free radicals, BrO2 and Br03. Indirect evidence that the species BrOz was an intermediate in the radiolysis may be found by estimating the average amount of oxygen in the oxidizing fragments. The stoichiometric composition of an "average fragment'' was computed from the difference between the amount of Oz(g) expected (assuming 1.5 moles of gas released per Br03- ion decomposed) and the amount of Oz(g) measured experimentally divided by the amount of bromine in the fragments. As may be seen (Figure 5 ) as the decomposition increased, the composition of an "average fragment" in Ca(Br03)z decreased from BrOz o0 to Br01.6, and in Sr(Br03)2 from BrOzol to BrO163. If only BrOz- and BrOhad been formed initially, the composition would have been less than the BrO2.o observed; if only BrOzwere produced, the observed stoichiometry would have obtained, but 2 would have been too small; if Br03 were formed, both 2 and the observed average ratio of oxygen to bromine would have been too small. Accordingly, it was assumed that the oxidizing (11) L. A. Prince and E. R. Johnson, J . Phys. Chem., 6 9 , 3 7 7 (1965). (12) N o evidence was found for the thermal or the radiolytic decomposition of BrOz - to form BrO
-.
Volume 70,Number 4
April 1966
J. w. CHASE
1036
1
1
1
’1
/
1
S T R O N T I U M BROMATE
1 I
3.0
1
I
I
I
I
G. E. BOYD
I
I
STRONTIUM BROMATE
I
0
I
AND
4
I
I
I
1
2 3 4 5 6 DOSE ie.v. mole‘ x 1 0 ) ~ ~
I
7
Figure 7. Dose dependence of 100-ev yields for (values for BrOz, Br02-, and BrO- were calculated).
-
/rO2 0
4
2
I 3
I 4
I 5
I 6
I 7
DOSE (e.v. m o d x ioz3)
Figure 6. Calculated oxidizing fragment yields as a function of dose (curve for total oxidizing power from experimental measurements in Figure 3; note similarity of dose dependence of BrO- and BrOz- yields to those observed in Figure 4).
power of the irradiated crystals could be attributed to three species, BrOz, BrOz-, and BrO-. The data of Figures 1 and 3 for Ca(BrO3)z and Sr(BrO& were “smoothed” with a computer program, and values of Z and 0 : B r were calculated for interpolated doses. These values served to establish two equations relating the fraction of each oxidizing species present, and, with a “material-balance” equation, algebraic solutions for the percentages of BrOz, Br02-, and BrO- were obtained. The concentrations of these species then were found from the known concentrations of bromine in all the oxidizing species. The dependence of the calculated yields expressed as mequiv mole-’ on the radiation dose in Sr(Br0a)z is shown in Figure 6, and the variation of the 100-ev yields estimated from Figure 6 are shown in Figure 7. A check on the calculations was afforded by stoichiometry
estimated oxidizing power was 0.821 and the experimental value was 0.820 mequiv mole-’. The foregoing treatment shows that the assumption of one additional radiolytic species, BrOz, is sufficient to account for all of the observed radiolytic yields. It also must be assumed that BrOz was stabilized within the crystal and that when the crystal was dissolved, Br02 reacted with arsenite to produce Br- ion. The presence of species such as Br03 or Br04- was not required. The necessary yields of BrOz appear to be quite small and relatively unimportant except for small doses. A direct identification of this species in the irradiated crystal, however, remains to be obt ained. l 3 The following radiolytic mechanism summarizes the state of our knowledge of the radiolysis of bromate ion at this time
0
+
r(
hr--BrO-+
+
0
30
I n this mechanism BrOz is assumed to capture a secondary electron and dissociatively deexcite to give BrOion and an oxygen atom, or, alternatively, BrOz-. The species BrOz-, BrO-, and BrOz all reach “steadyG(Oz) = ‘/,G(BrOd ‘/~G(Br02-) state” concentrations at large absorbed doses, and the G(Br0-) 3/2G(Br-) (C) radiolysis of Br03- ion then goes effectively to BrThus, for a dose of 0.1 X ev mole-’, G ( O Z ) = ~~~~~ 1.96 compared with the experimental value of 1.95. (13) The assumption of BrOz does not seem unreasonable as, in the radiolysis of KCIOS, ClOa is reported to be formed (cf. ref 14) and The oxidizing power yield also may be estimated from also in the radiolysis of the alkali metal and alkaline earth perthe calculated concentrations of BrO2, BrO2-, and chlorates (cf. ref 4). ev mole-’, the BrO-; thus, for a dose of 0.1 X (14) H. G. Heal, Can. J. Chem.,37, 979 (1959).
+
The Journal of Physical Chemistry
+
+
PHOTOIONIZATION OF THE LEUCOCARBINOLS OF MALACHITE GREEN
ion and oxygen gas. A mechanism analogous to the above will describe the radiolytic decomposition of
1037
KC103 by X-rays14 to form On(g), CIOz-, CIOz, C10-, and C1-.
The Effect of the Properties of Solvents of Various Dielectric Constants and Structures on the Photoionization of the Leucocarbinols and Leucocyanides
of Malachite Green, Crystal Violet, and Sunset Orange and Related Phenomena
by Edward 0. Holmes, Jr. Hughes Research Laboratories, Malibu, California (Received August 12, 1966)
Photoionization occurs only in solvents whose dielectric constant is greater than 4.7. Malachite green leucocyanide shows 100% photoionization in three straight-chain alcohols but less in chlorinated hydrocarbons; hence, a “chemical factor” is present. The photoionization of the leucocyanides of crystal violet and sunset orange in absolute alcohol are and absorbance ratios of malachite green leucocyanide were reported. The average A, determined in various solvents after various treatments. The anomalous photochemical behavior of malachite green carbinol in ethyl alcohol is interpreted on the basis of two forms, one of which is nonphototropic. The absorption bands of crystal violet, malachite green, and sunset orange ions are related to structure.
I. Historical The reason for the choice of this problem is that in 1957 the author1 published a paper on the degree of photo:onization of malachite green leucocyanide (MGLC) in solvents consisting of mixtures of two of the following : (1) n-hexane, (2) 1,2-dichloroethane, and (3) 1,l-dichloroethane. It was discovered that photoionization increased with increasing dielectric eonstant of the solvent. Very little photoionization occurred at values below 4.5, but above this value photoionization increased rapidly. Although the dipole moment of the solvent increased a t the same time, it was felt that the effect was due largely to increasing dielectric constant ( E ) . Spore? states that “the dielectric constant is not the sole criterion for photoionization. . . .” Hence, it seemed both interesting and worthwhile to undertake
an investigation to determine whether or not a “chemical factor,” also, might not be influential, and to learn something about its nature.
11. Qualitative Results To begin with, solutions of malachite green leucocyanide (MGLC) in solvents of various dielectric constants were made up and irradiated with ultraviolet light transmitted through a Pyrex filter. It was noted whether or not photoionization occurred by visually observing the appearance of the characteristic color of the carbonium (MG+) ion. It was found that photoionization began with solvents whose dielectric constants were in the range E = 4.5-4.7, except for diethyl ether where no photoioniza(1) E. 0. Holmes, Jr., J. Phys. Chem., 61,434 (1957). (2) A. H.Sporer, Trans. Faraday SOC.,57, 983 (1961).
Volume 70,Number 4
April 1966
