Oxidation of
Aqueous Br-
i447
by OH Radicals
On the Oxidation of Aqueous Br- by OH Radicals, Studied by Pulse Radiolysis Avner Mamou, Joseph Rabanl," Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 9 1 000, Israel
and David Behar Israel Atomic Energy Commission, Soreq Nuclear Research Center, Yavne 70600, Israel (Received September 27, 1976)
The equilibria Br- + OH P BrOH- and 2Br- + OH + Br- + BrOH- + Br; + OH- were studied in the pH range 9.5-10.7, in view of an apparent disagreement in the literature over the equilibrium constants. The apparently conflicting results are shown to be in agreement when the absorbance of the transient BrOH- is considered. The following equilibrium constants have been estimated: BrOH- + Br- is Brz- + OH-, K = 70 f 30; Br + Br- e Br2-,K = (1.1f 0.3) X lo5 M-l.
Introduction The oxidation of Br- by OH radicals has been a subject of several publi~ations.l-~ OH radicals react with Br- and produce the intermediate BrOH- which may react with Br-, H+, or dissociate spontaneously to form Br in equilibrium with Br2-. Reactions 1-5 are reversible. OH t Br- T-! BrOHBrOH- + Br- t Br; t OH-
(1) (2)
BrOH- + H + t B r + H,O BrOH- 3 Br t OH-
(4)
Br
+ Br- t Br2-
(3)
kinetics while its decay was second order. At sufficiently low pulse intensities, the formation and decay processes became well separated in time, and it was possible to determine D,,,, defined as the optical density when the buildup of absorption was just completed, before any of the transient absorbing species decayed away. With very low concentrations of Br-, corrections (up to about 10%) were carried out for the absorbance which disappeared during the building up processes. This was carried out by extrapolation of the decay traces to the middle time of the formation process. D,, is related to the optical absorption of Br2- and BrOH- according to
(5)
Of the rate and equilibrium constants, k1(2)= (1.06 f 0.08) X 1O'O M-l s-l, h-1(2)= (3.3 f 0.4) X lo7 s-l, kJ2)= (1.9 f 0.3) X lo8 M-l s-l, k3(') = (4.4 f 0.8) X 1O'O M-l s-', k4(2)= (4.2 f 0.6) X lo6 s-l, K1(3)= (2.9 f 1.4) X lo3 M-', K z ( 3 )= 3.7 f 1.5, and K6(4)= (2.2 f 1) X lo5 M-l were reported. Note that K4 = K 2 / K 6N 2 X M. The value reported for K1(3)from equilibrium optical absorbances data is about tenfold the ratio2 k l / k 1 ,as calculated from kinetic measurements. This difference called for additional experiments. The purpose of this work is the redetermination of K1 from optical density measurements in the pH range 7-11. We will also show that the experimental data are not in disagreement with kl/k-l = K 1 = 3202when the absorbance of BrOH- is taken into account.
Experimental Section The experimental procedure was precisely as reported before.' The pH values which are reported were measured with the aid of a pH meter, [OH-] calculated as equal to 10(pH-14).The temperature was (24 f 2) "C. Materials. NaOH and NaBr were Baker's (Analyzed grade). HC104 was Merck (pro analysi). N20 was Matheson's, and was bubbled through an acidic solution of ammonium metavanadate in contact with zinc amalgam to eliminate traces of 02.The water was triply distilled. Other materials were used as received. Results and Discussion Solutions saturated with N 2 0 were used in order to convert ea,- to OH radical^.^ The optical density was measured at 366 nm. During and after the electron pulse, OH radicals which were produced reacted with Br- leading to the reaction scheme 1-5. This resulted in a buildup of optical density, due to the absorptions of Br2-and BrOH-.2 Finally, the absorption decayed away as Br2- yielded Brand Brc. The buildup of absorbance obeyed first-order
where E X is the extinction coefficient of species X, [Br2-], and [BrOH-I,, represent the concentrations of Br2- and BrOH-, respectively, when equilibria 1-5 exist. If the same pulse intensity which gave a certain D,, is applied to a M) all the OH radicals neutral solution of Br- (>3 X eventually yield Br2-, because under such conditions equilibria 1-5 are shifted toward total formation of Br2-. (Note that equilibrium 3 is not important under our conditions, where only neutral and slightly alkaline solutions were used.) Under these conditions, the value of D,,, which will be referred to as Dmaptal,equals the right-hand side of eq 7, where the various [Y],, represent Drnaxtota'
= EE~;([B~; leq + [OHleq + [Brleq
+ [BrOH- l e q 1
(7)
the appropriate equilibrium concentrations of the type Y species, in slightly alkaline solutions. DmUtota1, however, is measured in neutral solution at sufficiently high [Br-] (>3 X M), so that equilibrium 5 is shifted strongly to the right, using as noted above equal pulse intensities (total radical concentrations) in both neutral and alkaline solutions. Let Dmaxneutral represent D,, in a neutral solution where the Br- concentration precisely equals its concentration in a parallel experiment carried out under the same Dmaxneutral = Drnaxtota1. conditions a t pH 9.5-10.7. [Br2-],,/([Br2-],, + [Brl,,). Dmaxneutral and D,, measured in parallel experiments, where the same dose per pulse was applied, were used for the calculation of K2 with
K? =
([BH-]/[Br-] )(1
+ KI-'[Br-]-'
U - 1 - K;'[Br-]-'
- UeJ
(8)
Equation 8 is obtained after inserting in (7) the various [Y],,, expressed by [Br2-], [OH-], [Br-I, and equilibrium The Journal of Physical Chemistry, Vol. 8 I, No. 15, 1977
A. Mamou, J. Rabani, and D. Behar
1440
TABLE I: Evaluation of K , in the pH Range 1 0 - 1 0 . 7 V
10.1 10.1 10.15 10.45 10.75 10.75 10.75
100 200 50 100 100 200 300
0.065 0.067 0.058 0.068 0.064 0.067 0.071
0.045 0.060 0.023 0.039 0.023 0.044 0.060
81 82 97 108 87 80 105 Av 9 0
a Solutions saturated N,O. 1 2 . 3 cm light path. Measured a t 3 6 6 nm. The differences in values of Dmaxneutrd reflect, in addition t o equilibria 2 and 5, changes in pulse intensities. Calculated with eq 8, using K, = 320 M-’s2 e, = 0.67,2 and K , = 1.1 x l o s M-’ (see text).
constants. er is the ratio C B ~ O H - / ~ Bat ~ ~ 366 nm, and was found to equal 0.67.2 U is defined by
U = (DmaxneUtral/Dmax)(l + K;’[Br-]-’)
(9)
A set of K2values, calculated with 8, at various pH values and [Br-1, are presented in Table I. Note that under our conditions only -0.3 pM radicals are formed, accompanied by roughly equivalent amounts of H+ and OH- so that the pH is not seriously affected. Another set of results in the pH range 9.5-10.7 gave an average K2 = 53 f 20 (maximum deviation). The larger scatter is probably due to several factors: the calculation of Kz is very sensitive to the measured values of Dm,neutrd, D,,,, pH, and probably temperature. In addition, the calculations are affected to various degrees by the choice of K1, K5, and era The errors in these parameters may introduce an additional systematic error in K2. We have determined K5 using various Br- concentrations at pH 3.7 (HCIOJ in N20 saturated or aerated solutions. When [Br-] was changed from 5 to 50 pM, considerable changes in D,, were observed. D, values at this pH will D m a P dchanged only little, in be referred to as DmaXacid. the Br- concentration range 1-10 mM, corresponding to the slight variation of the radical G values with the solutes’ concentration. The optical density values obtained by extrapolating from 1-10 mM Br- to the lower [Br-] solutions will be referred to as DmaxPIateau (extrapolation gives total initial yield of OH and therefore of ([Br2-] + [Br])). K5 = (1.1f 0.3) X lo5 M-l was calculated as an average of eight determinations using
The Journal of Physical Chemistry, Vol. 81, No. 15, 1977
pH 3.7 was chosen in order to enhance the formation of Br atoms2 so that better time separation between the formation and decay of Brz- was obtained. This was important since at the very low [Br-] used, the formation of Br at neutral pH is too slow for a good time separation, even with the low pulse intensities used in these experiments (0.26 pM radicals produced per pulse). The value K5 = 1.1X lo5 M-l is in reasonably good agreement with previous measurement^.^
Conclusions = 0.67 is taken for the calculations of When K2, it is shown that consistent values of K2 are obtained in the pH range 9.5-10.7 using K1 = 320 M-1.2 Rough calculations, assuming optimal conditions for the measurements of B in the previous work of Behaq3 indicate that his resulb agree within experimental error with the present data, and yield the same K2 values using the same parameters as here in eq 8. Exact calculations were not possible, since his data3 are presented in a manner that is not directly applicable to eq 8. At pH values lower than 10, the scatter of the results was greater (not shown in Table I). This is probably due to effects of impurities, as the [Br-] must be very low a t the lower pH’s to make it possible to observe the relevant equilibria. At pH >11,K2 values as defined by eq 8 decreased systematically with increasing pH. This became particularly evident at pH >11.5. This effect is perhaps due to the ionic dissociation of OH6 and/or BrOH-. (Nothing is known at the moment about the possible acid properties of BrOH-.) Despite the highly scattered data the constancy of K2as calculated by eq 8 in the pH range 9.5-10.7 may indicate that none of these dissociations are important in the above pH range. References and Notes (1) M. S. Matheson, W. A. Mulac, J. L. Weeks, and J. Rabani, J. Phys. Chem., 70, 2092 (1966). (2) D. Zehavi and J. Rabani, J . Phys. Chem., 76, 312 (1972). (3) D. Behar, J . Phys. Chem., 76, 1815 (1972). (4) M. S. Matheson and L. M. Dorfman, “Pulse Radiolysis”, The M.I.T. Press, Cambridge, Mass., 1969. (5) B. Cercek, M. Ebert, C. W. Gilbert, and A. J. Swallow, “Pulse Radwsis”, M. Ebert, J. P. Keene, A. J. Swallow, and J. H. Baxendele, Ed., Academic Press, New York, N.Y., 1965, p 83. (6) (a) J. Rabani and M. S. Matheson, J . Am. Chem. Soc., 66, 3175 (1964); (b) J. L. Weeks and J. Rabani, J . Phys. Chem., 70, 2100 (1966).
