Pulse-radiolysis of aqueous KBrO4 solutions

Volume 19, number 2

CHEMICAL PHYSICS LETTERS

PULSE-~AD~OLYSIS

OF AQUEOUS

KBr04

15 March 1373

SOLUTLONS

K.J. OLSEN Deparrmerrt ofi”lrgsics,

iJtliversit_v of Aarlw,

8000 Aarhus CT,Denmark

K. SEHESTED Acc.d?epartrzerq

Danish A.E.K. Risd, 4000 Roskilde,

Denmark

Received 8 January 1973

Pulse-radiolysis of aqueous KBrO;l solutions show that BrO, reacts with e,iq by the reaction BcO;+e& -* Bra; + o-. x-e f BrOz= (7.0 * 0.7) X f09 M-l set-‘. The reactions between BrO, and ti, OH and O-are The rate constan~z%or these reactions are less than 10’ hl-* set-*.

The pulse-radiolysis and flash-photolysis of aqueous ox~Ilalo~en anions have been the subject of thorough investigations in recent years, revealing a number of differences between the chlorine and bromine analogues [ I, 21. Since the preparation [ 3,3] of BrOT a few papers on the radiolysis of KBrO, in solid state [S,6] and the role of BrOT in the radiolysis of solid bromates [7] have been published, showing close resemblance [S] between KCIO, and KBrO+.. Studies of the Rash-photolysis [8] of KBrOi in aqueous solutions and the radiolysis of frozen aqueous glasses [P] , however, show more resemblance between BrO, and i0,. Here we wish to report the results of a pulse-radioiytic study of KBrO, solutions. The i(Br04 used was prepared by one af us (E.H. A.). Recrystalfization removed traces oi KBr03. The content of KBrO, was less than 0,003 mole 5%[3]. The pulse-rad~olysis equipI~lent has been described by Christensen et al. [lo]. 10 MeV electron pulses of 0.25 to I ttsec duration were obtained from a Varian Linear Accelerator. The dose was 6-8 krad/puise in a 1 psec pulse. The dosimetry was carried out with a 1W3 M K,Fe(CN), solution saturated with N,O. e of Fe(CN)z- was taken to be 1000 M-J cm-1 at 420 nm and G = 5.3, .’

sIow.

In neutral, argon saturated 101~ and IO-7 M KBrO, solutions the only transient detected in the wavelength region 250-700 run was OH. The yield of OH was higher and the decay slower than in pure argon saturated H,O. In neutral argon saturated 4 X iW3 M KB r0 4 solutions G(@H) was identicaf to G(OH) in neutral N20 saturated Hz0 both sarutions having @OH) = 5.3, taking [ 111 I$,$) = 380 M-1 cm- *, No difference be tweet1 the decay of OH in the two solutions was detected_ When the KBrO, concentration was below 2 X IO-” M e,q could be observed in neutral argon saturated solutions. In the con~entra~on range 2 X 10-4-.5 X 10-S M KBrO, the decay of e& Followed first-order kinetics with a first-order rate constant proportional to the KBrO4 concentration. ke,q+BaG was found to be (7.02 0.7) X IO9 M-I set- 1. From the yield of OH it must be concluded, rhat the reaction of e& is

ei’ + BrO, + BrOi f O- ,

(1)

0- +H+-,OH..

(2).

In argon s&mated acid solution, IO-” M HCIO, 213

Vplume 19, number 2

CHEMICAL PHYSICS LETTERS

+ 1 O- .3 M KBr04, the yield and the decay of OH was the same as in acid solution without KBr04. The O- radicals produced in reaction (1) were detected as 05 in alkaline (pH = 13) oxygenated solutions. Measurements of the yield of 0, as function of the ratio between the concentration of BrCI, and 0; showed

good agreement

the following

e-aq + BrOq + BrO; e-aq+0*+07

o-+o,+o- i

with calculations

based

on

scheme

+ O-

~

>

(1)

(3 (4)

3 ’

with k3 = 2 X lOlo M-l set-l [12], k, = 2.6 X IO9 M-I set-I [13] and the [OZ] = 2.4 X 10m4 M. At pH = 13, G(O-) was taken as 2.6, G(e,4) = 3.3, and 1141 +c = 1900 M-l set-I . The measured yie!ds are compared with the calculated values in table 1. The good agreement between the experimental and calculated yieids shows that O- does not react with BrOz or at least very slowly. From the yields and decay of OH in neutral and acid solutions it can be concluded that neither OH nor H reacts with BrO, with rate constants > IO7 Ivl-1 set-l. The BrOF produced in reaction (I) could be detected In a IO-4 M KBrO4 solution after a series of pulses with a total dose of 200 krad. At the end of the series a transient absorption with decay and spec-

Table 1 W’;) [Broil

0 IO+ M 3 X 1O-3 hf 8X lo-JM

exp.

CdC.

2.6 4.4 5.2 5.8

2.6 4.5 5.3 5.9

15 March 1973

tral characteristics eaq + Bfi3

+ BrO,

of BrO2 appeared + 02-

produced

by

.

(5)

Reaction (1) is in good agreement with the observations in frozen aqueous solutions [8] where it was found that BrO, reacted with electrons without formation of any absorption in the visible. The reaction between the electron and BrO, in the frozen aqueous solutions did not produce any radical detectable by ESR besides possibly O-. The present results show a marked difference between Cl07 and BrO, in the reactivity towards e& in agreement with the results from aqueous glasses [9]. BrO, must be considered as an intermediate between CIO, and 10, since IO, reacts [ 151 with eTq in a reaction analogous with reaction (1) but reacts with I-1 and OH as well.

References 111 G.V. Buxton and MS. Subhani, J. Chem. Sot. Faraday T:n~s. I68 (1972) 947. A. Trenin, Israel J. Chem. 8 (1970) 103. I:; E.H. Appelman, J. Am. Chem. Sot. 9G (1969) 1900. 141 E.H. Appelman. InorE. Chem. 8 (1969) 223.

J.R. Bpberg, J. Chem. Phys. 55 (1971) 4867. I:; G.E. Boyd and L.C. Brown, J. Phys. Chem. 74 (1970) 3490. [71 L.C. Brown, G.hI. Beym and G.E. Boyd, J. Am. Chem. Sot. 91 (1969) 2250. [Sl U.K. Klaning, K.J. Olsen and E.H. Appelman, to be published. 191 K.J. Olsen, Trans. Faraday Sot. 67 (1971) 1041. 1101 H.C. Christensen, G. Nilsson, P. Pugsberg and S.O. Nielsen, Rev. Sci. Instr. 40 (1969) 786. [Ill J.K. Thomas, Trans. Faraday Sot. 61 (1965) 702. [I21 E.J. Hart and Ehi. Fielden, Advan. Chem. Ser. 50 (1965) 253. [ 131 G.E. Adams, J.W. Boag and B.D. Michael, Nature 205 (1965) 898. [ 141 G. Czapski and L.M. Dorfman, J. Phys. Chem. 68 (1964)

1169.

[ 151 S.N. Bixrttacharyya and D.K. Bardhan, Bull. Chem. Sot. Japan 43 (1970) 2808.