2088
J. Phys. Chem. 1980, 84,2088-2089
cyanide by OH (eq 4) has been determined at times suf-
Yield for the Scavenging of OH Radicals in the Radiolysis of N,O-Saturated Aqueous Solutions'
Sir: In radiation chemical studies of aqueous systems nitrous oxide is frequently used to convert hydrated electrons into OH radicals via reaction 1. For solutions eaq-+ N 2 0 + HzO
-
N2 + OH-
+ .OH
(1)
saturated with N20 the radiation chemical yield for oxidation of solutes by OH radicals has frequently been taken to be in the range 5.4-5.6, based on the assumption that the yields of 2.7-2.8 for reactions of OH radicals and hydrated electrons observed in dilute solutions are generally applicable. Recent pulse radiolytic measurements on the oxidation of ferrocyanide by OH, however, show a significant dependence of the yield on the ferrocyanide concentration2 and indicate that competition between the combination reactions of OH within the track and reaction with added solutes is similar to that for hydrated electrons, as previously described? It is now quite clear that the yield for reaction of OH with a given solute will be dependent on both the solute and N20 concentrations. For solutions saturated with N 2 0 at atmospheric pressures the pseudo-first-order rate constant for reaction 1is 1.9 X lo8 s-l so that one expects, from the treatment of electron scavenging by Balkas, Fendler, and S ~ h u l e rthat , ~ the yield for OH produced in reaction 1should be -3.2. This value has led us to suggest previously that the total yield for oxidation of the solute by OH radicals should be taken as 6.04 and such a value has, in fact, been observed for the reaction of OH radicals with M b e n ~ e n e .Schwarz ~ has commented6that conversion of hydrated electrons into OH radicals within the track does not necessarily make them available for reaction because they may be lost in subsequent track processes. It is clear that the exact value assumed in any particular instance for the yield of OH reactions is currently based to a considerable extent on speculation. It is particularly important to establish accurate values of the absolute yields for the reactions of OH radicals in pulse experiments since this type of study provides the most direct way to use scavenging methods to examine track processes. A convenient, and currently the most commonly used, dosimeter for pulse radiolytic experiments involves oxidation of M SCN- in N20-saturated solutions (eq 2 and 3). For this dosimeter the product of yield and ex-
+ SCN- 2OH- + SCN. SCN + SCN- + (SCN)z-
.OH
(2)
(3)
tinction coefficient of (SCN)z-at its (472 nm) absorption maximum is frequently taken to be in the range 38500-41500 (molecules/100 eV) X M-l cm-l (ref 7) based on an assumed yield of -5.4-5.8 and an extinction coefficient of 7100 M-l cm-l, as given by the early determination of Adams, Boag, and Michael8 in studies where the hydrated electrons were removed by reaction with 0 2 . Comparisons in this laboratory with the oxidation offerrocyanide,2 however, show that the extinction coefficient of (SCN), is higher and comparable to the value of 7600 .~ M-' cm-l given by Baxendale, Bevan, and S t ~ t t Either the yield for OH scavenging must be lower or the yieldextinction coefficient product greater than has been commonly assumed. We have undertaken to calibrate the thiocyanate dosimeter directly against the Fricke system in pulse radiolytic experiments. When this dosimeter is used as an intermediate referencelo the yield for oxidation of ferro0022-3654/80/2084-2088$0 1 .OO/O
OH + Fe(CN)t- -% OH- + Fe(CN):-
(4)
ficiently short (i.e., 1p s ) that the observations on reaction 4 should be uncomplicated by reduction of the ferricyanide by the hydrogen atoms1*or peroxide12 produced in the radiolysis. As briefly described here, the radiation chemical yield for reaction 4, which appears to be quantitative, is found, at pH -10.5, to be 6.1 for nitrous oxide saturated solutions lo-' M in ferrocyanide. This value should apply to similar cases where the pseudo-first-order rate constant is lo8 s-l but under other conditions must be modified to take into account variations in scavenging from the track. This present value is even slightly higher than our earlier estimate and it is now clear that recombination within the track of OH produced in reaction 1must be compensated for by a correspondingly high yield for the scavenging of primary OH. The comparisons referred to above were carried out at the Notre Dame Radiation Laboratory linear accelerator facility by using signal-averaging methods as previously described.13 Improvements in the beam-handling system now make it possible to make individual measurements of the integrated dose with a standard deviation of -0.5%. The optical detection equipment has been equipped with a backoff amplifier incorporating digital memory. This approach avoids electronic drift problems and allows observations on absorbances as weak as 0.02 to be carried out to times as long as 10 s. Differential pulse radiolysis methods have been used to compare the thiocyanate dosimeter directly against the oxidation of air-saturated 0.03 M ferrous ion in 0.8 N HzSO4, where the period for oxidation by the peroxide produced in the radiolysis is 0.3 s. In order to minimize systematic errors, these comparisons were made in a l-cm cell and under experimental conditions which were otherwise as similar as possible. In these experiments 20-11s pulses of 9-MeV electrons at doses 500 rd/pulse (initial radical concentrations of -3 pM)were used so that reactions second order in primary radicals should not contribute significantly. The ratio of absorbances per unit dose for the thiocyanate and Fricke systems, measured respectively at 472 and 304 nm, was found to be 1.368 f 0.018 at 25 "C. This ratio takes into account the 2.5% higher stopping power of the Fricke system. Pulse radiolytic comparison of the oxidation of M ferrocyanide (NzOsaturated) with the thiocyanate dosimeter, measured at 420 and 472 nm, respectively, gives an absorbance ratio per unit dose of 0.1355 f 0,0009. Combining these two comparisons, taking the yield for the Fricke dosimeter as 15.4514and the extinction coefficients of ferric and ferricyanide ions as 2197 and 1027 M-' cm-', one obtains a G value of 6.13 f 0.09 for scavenging OH by loY2M ferrocyanide, The accuracy of this measurement is limited primarily by low-frequencynoise (
