1537
PULSE RADIOLYSIS OF AQUEOUS THIOCYANATE SOLUTIONS
Pulse Radiolysis of Aqueous Thiocyanate Solutions. Nature of the Intermediate Transient Species by D. Behar, P. L. T. Bevan, and G. Scholes* Laboratory of Radiation Chemistry, School of Chemistry, University of Newcastle upon Tyne, NE1 7 R U , Ewland (Received October 7 , 1971) Publication costs borne completely bu The Journal of Physical Chemistry
The pulse radiolysis of NzO-saturated aqueous solutions of KCNS has been studied under both neutral and alkaline conditions, and the optical absorption spectra of the precursors of the (CNS)z- radical anion have been obtained. In neutral solution the precursor has A, 330 nm with €380 900 M - I ern-', while in alkali it 390 nm and e390 4600 M-I cm-1. Kinetic considerations lead to their assignment as CNS and has a A, CNSOH-, respectively. The oxidation of CNS- ions by OH radicals in these solutions is interpreted in terms of the reactions: OH + CNS- +.CNSOH-; CNSOH- $ CNS + OH-, K = 3.2 X 10+ ill; and CNS (CNS)z-, K = 2 x 106 M-l. The rate constant for the dissociation of CNSOH- has been found CNSto be >5 x 107 sec-1. In alkaline solutions, oxidation of CNS- by 0 - also takes place, viz., 0 - + CNSCNSOZ-, k = 3.7 x 109 M - 1 sec-l; CNS02- HzO CNSOH- OH-. Under the conditions used (up to 0.8 X NaOH) the doubly charged adduct is converted completely to CNSOH-.
+
+
Introduction The mechanism of the oxidation of CNS- ions by OH radicals in aqueous systems was first investigated by Adams, et ~ 1 . 1 ~ 2Pulse radiolysis studies showed the formation of a transient species which absorbed strongly 480 nm), and this absorption was in the visible,,X,( first assigned to the CNS radical, formed according t o
OH
+ CNS- +CNS + OH-
(1)
It was pointed out,2 however, that the experimental data would also be consistent with the transient species being (CNS)2-, produced by the secondary equilibrium reaction CNS
+ CNS- e(CNS)z-
(2) Subsequent detailed kinetic studiesaf4of the formation of the transient at low [CNS-] gave results which could be quantitatively explained if the absorption was entirely due to (CNS)2-; it was reported that ICl = 2.8 X 1O1O M-l sec-l and Kz = 2 X lo6M-l. More recently,s it has been shown that irradiation of frozen alkaline glasses (at 77°K) containing KCNS leads to the formation of an intermediate (A, 380 nm) which, on annealing, decays to produce (CNS)2-; thus, in the glassy matrix, it was proposed that reaction of CNS- with the radiation-produced holes (e.g., 0-) can produce a transient complex radical ion. I n view of this observation, aspects of the pulse radiolysis of neutral and alkaline thiocyanate solutions have been reinvestigated, particularly with a view to the detection of OH(0-) adducts. Experimental Section The pulse radiolysis apparatus has been described.6 I n the present work two linear accelerators were used,
+
-+
of 5- and 10-MeV energy, respectively. Generally, pulses of 0.1 psec in length were used, giving doses from 0.1 to 1.0 krad. Spectra were obtained using the kinetic spectroscopy method. The KCNS and NaOH used were of analytical reagent grade, and solutions were made up in triply distilled water. Results and Discussion Transient absorption spectra from M KCNS solutions (saturated with NzO to remove the radiationproduced solvated electrons, eaqN2O + N2 OH OH-) were recorded at various times after the electron pulse. Figures 1 and 2 show, respectively, the results obtained from solutions a t natural pH (pH ~ 5 . 6 )and from solutions containing 0.1 M NaOH. The presence of a precursor of (CNS)Z- is clearly indicated by the kinetic features, namely, that the formation of (CNS)2- at 480 nm is accompanied by a decay at lower wavelengths (see oscillograms). Moreover, the optical absorption spectrum of the precursor (the difference spectra in Figures 1 and 2) is different under the different pH conditions, viz., X,,,(neutral) 330 nm, X,,,(alkaline) 390 nm. I n view of this evidence, it is concluded that tlie oxidation of CNS- ions in these irradiated systems cannot be described simply in terms of reactions 1 and 2.
+
+
+
(1) G. E. Adams, J. W. Boag, and B. D. Michael, PTOC. Chem. Soc., 114 (1964). (2) G. E. Adams, J. W. Boag, J. Currant, and E. D. Michael, “Pulse Radiolysis,” Academic Press, New York, N. Y . , 1965, p 117. (3) J. H. Baxendale and D. A. Stott, Chem. Commun., 699 (1967). ( 4 ) J. H. Baxendale, P. L. T. Bevan, and D. A. Stott, Trans. Faraday Soc., 64, 2389 (1968). (6) D. V. Bent, N. B. Nazhat, and G . Scholes, 4th International Congress of Radiation Research, Evian, Abstract 74, 1970. (6) J. P. Keene, “Pulse Radiolysis,” Academic Press, New York, N. Y., 1965, p 1.
The Journd of Physical Chemistry, Vol. 76, No. 11, 1972
D. BEHAR,P. L.T. BEVAN,AND G. SCHOLES
1538
OH
+
+ OH-
0- HzO (5) for which pK, = 11.9' and by the relative magnitude of the rate constants ka and led. Given that the adducts may then enter into the pH-dependent equilibria CNSOHCNS0'-
+ HZO
+ OHCNSOH- + OH-
=
CNS
(6)
(7) it then follows that (CNS)2- can possibly arise from three equilibrium processes, vix., reactions 2, 8, and 9.
+ CNSCNS02- + CNS-
CNSOH-
h , nm.
Figure 1. Transient absorption spectra in pulsed aqueous
KCNS solutions (lod4M ) saturated with NaO a t natural pH: (0,a) observed spectrum after 1 psec, (0, a) contribution from (CNS)2-, ( A ) difference spectrum. Oscilloscope traces: (a) X 480 nm, time scale 2 psec per division, 3.5% absorption per division; (b) h 330 nm, time scale 2 psec per division, 0.9% absorption per division.
+ OH(CNS)2- + 20H(CNS)Z-
Ha0
(8) (9)
On this basis, the observed precursors could be either the pair CNS-CNSOH- or the pair CNSOH-CNS02-. Representing D as the optical density due to (CNS)2at equilibrium, where (CNS)2- and the precursors coexist, and DO as the absorption if full conversion of OH(0-) radicals to (CNS),- were to be attained, then the variation of the ratio Do/D with [OH-] can be predicted for various reaction schemes. Thus, assuming that the precursors are CNS and CNSOH- and that they enter the equilibria 2 and 6, it can be shown that
D_ OD -
+
1 [OH-] Kz[CNS-] -I- KJT2[CNS-]
(1)
If the reaction of CNSOH- with CNS- is also included in this scheme, an equation similar to I would then apply, since K$iz = Ks. On the other hand, for a reaction mechanism involving the pair CNSOH-CNS02-, and the equilibria 7, 8, and 9, equation I1
D Do
1
[OH-]
}
(11)
(4)
would apply. In order to distinguish between these reaction schemes, the effect of [OH-] on the equilibrium concentration of (CNS)2- was studied. The solutions (N20 saturated) contained a fixed amount of thiocyanate (7.85 X lov6 M ) , the [OH-] being varied from -0.2 to 0.8 M . Electron pulses of relatively small dose were used so that the equilibrium values of (CNS)2- could be attained before appreciable decay of this species had taken place. It was found that the data could be expressed by eq I and not by eq 11. Figure 3 shows the results plotted according to eq I, setting the intercept from the known values of [CNS-1 and Kza4 We therefore conclude that the species absorbing at 330 and 390 nm correspond to CNS and CNSOH-, respectively. Although the absorption spectrum of the CNS radical in water is unknown, it has been determined in the gas
The relative extents of these two reactions will be governed by the dissociation process
(7) J. Rabani and M. 8 . Matheson, J . Amer. Chem. Soc., 86, 3175 (1964).
I , nm. Figure 2. Transient absorption spectra in pulsed aqueous KCNS solutions (10-4 M ) saturated with NzO containing 10-1 M NaOH: (0)observed spectrum after 3.5 psec, (0) contribution from (CNS)a-, ( A ) difference spectrum. Oscilloscope traces: (a) X 480 nm, time scale 2 psec per division, 3.1% absorption per division; (b) h 390 nm; time scale 2 psec per division, 2.1% absorption per division.
As an alternative mechanism, one may suppose, in the first instance, that the primary reaction with CNSions leads to the formation of adducts, according to
+ CNS- +CNSOH0- + CNS- +CNS02-
OH
The Journal of Physical Chemiatry, Vol. 76, No. 11, 1978
(3)
PULSE RADIOLYSIS OF AQUEOUS THIOCYANATE SOLUTIONS phase8 and absorption bands are present in the range 330-440 nm. The molar extinction coefficient of CKS, eCNsa3O 900 M-’ cm-l, was obtained by kinetic treatment of the results (see below), using the reported value4 of e(CNs)z-475 7600 M-l cm-’. Support for the assignment of the 390-nm absorption to CNSOH- arises from a comparison with the recently describedg-ll physical properties of the transient thiocyanate-halide complexes, CNSX- (Table I). The equilibrium constant of reaction 6 was determined from Figure 3; KZK6 = 6.45 X lo3,hence K6 = 3.2 X iM. Using these values and the equilibrium optical densities at differing pH values, the molar extinction coefficient for the CXSOH- radical ion was assessed.
1539
Kinetics in Neutral and Acid Solutions The kinetics of formation of (CNS)?- from OH radicals are derived for the proposed mechanism
+ CXS- -% CNSOHCNS + OHK6 3.2 X lo-’
OH ks
CKSOHCNS
M
=
k -8
1s2
+ CNS-
(CNS)z-
Kz
=
2 X IO5 M-’
k -2
The concentrations of the transient species are proportional to their optical densities. For each transient, let D ,and D be the optical densities at time t and at equilibrium, and Do the optical density attained if all the OH radicals were converted to the relevant transient. At low [OH-] (neutral and acidic CKS- solutions), we neglect the reverse reaction of equilibrium 6 (rate a k-6 [OH-]). Under these conditions, the following equations express the buildup of the transients after the electron pulse. for CKS D - ,-- D -
- k--2)
k3’(k6
(k6 - k)’)(k6
DO
(k6
- kz’
-
e-kst
- k-2)
k.ti(k3’ - k-2) - k3’)(k3’ - kz’ - k,)
+
,-kaq
kZ‘k,’k6
COHI, M (NaOH).
Figure 3. Variation of the equilibrium concentration of (CNS)a- with [OH-].
The ratio D3!)0/D475 at pH 13 and 14 was found to be 0.43 and 1.50, respectively. Given the relationships 0 4 1 5 = (e(CN8),-475) [(CNS)z-] and 0 3 9 0 = ( ~ ( c N s ) ~3 90) [(CKS),-] 4-(€CNSOH-390)[CNSOH-1, we calculate that ECNSOH-390= 4600 M-’ cm-l. It can be seen in Table I that these various properties of CNSOHfall very well into the general pattern for the thiocyanate complexes. Questions arise about the relative kinetic significance of the equilibria 2 and 8 in the formation of (CNS)z-, it being evident that eq I cannot distinguish between them. It follows from (8) that [(CXS)z-]/ [CNSOH-] = KzK6 [CNS-]/ [OH-]. For the solutions of natural pH (5.5), this leads to the relationship [(CNS),-]/ [CNSOH-] = 2 X l0lz[CNS-], indicating that exM ) would tremely low CNS- concentrations be sufficient to allow full conversion of CNSOH- t o (CNS)z- by reaction 8; this is clearly not observed. Furthermore, since the forward reaction of equilibrium 8 is suppressed by alkali, we are of the view that this particular process does not contribute t o (CNS)zradical-ion formation under the conditions described here.
(kz’
+ k.-z)(k3’ - kz’ - k-z)(ka -
k2’
- k-2)
e - (kz’+k-z)t
X (111)
for (CNS)z-
kZ‘k6 (k6
(kz’
-
k3’)(k3’
+ k-2)(k3’
-
e -ks’t
- kz’ - k-2)
+
k z ‘k3‘k, X lC2’ - k-z)(lc, - ka’ - k-2) e - (kz‘+k-z)t
(IV)
where kz’ = kz[CNS-] and 1c3’ = k3[CNS-]. Baxendale, et ~ l . showed , ~ that at times >0.2 psec the kinetics of (CNS),- formation in pulsed neutral and acidic solutions could be formally accounted for by a primary oxidation step (represented by the OH + CNS OH- (1))but authors as CNSwhich is replaced here by (3)) followed by (2). It is thus concluded that k6 >> k3[CNS-] and (kz[CNS-]
+
+
(8) R. N. Dixon and D. A. Ramsay, Can. J . Phys., 46, 2619 (1968). (9) M. Schormhofer and A. Henglein, Ber. Bunsenges. Phys. Chem., 73, 289 (1969). (10) M. Schonsshofer, Int. J . Radiat. Phys. Chem., 1, 505 (1969). (11) M.Schoneshofer and A. Hendein, Be?. Bunsenges. Phys. Chem., 74, 393 (1970).
The Journal of Physical Chemistry, Vol. 76,N o . 11, 1978
1540
D. BEHAR, P. L. T.BEVAN,AND G.SCHOLES DJDo =
Table I : Comparison of Some Physical Properties of Thiocyanate Radical Ion Complexes (CNSX-)
CNSICNSBrCNSOH CNSC1-
420 400 390 390
1.3 X 6 X 3.2 x 1.5 X
9.2 7.3 4.6
4.7
lo-* lo-* 10-2
10-l
+
X2 - X(k6
k-2). Therefore, particularly at longer reaction times, the first exponential term of both eq 111and IV can be neglected. Introduction of these conditions reduces eq 111 and IV to V and VI, respectively. for CNS
for (CNS),-
k2 ' @a'
- k,' -
e -ka't k-2)
+
kZ'k3'
(k2'
+ k_z)(k3' - k2'
e-
(ki'+k-zJt
- k-2)
(VI)
Equation VI is identical with that previously reported.* as 2.8 X Taking the values for k3(=k1), kz, and 1O1O M-' sec-', 6.8 X lo9 M-' sec-', and 3.4 X lo4 sec-', respectively, concentration us. time curves for CNS and (CNS)z- formation were constructed from eq I11 and IV, using various values for k,. Comparison with our data from neutral solutions, as well as those of Baxendale, et u Z . , ~ indicates that k6 > 5 X lo7 sec-l. At the reaction times studied (>0.2 psec), the kinetics are thus accounted for by ka, kz, and IC-,. The molar extinction coefficient of CNS a t 330 nm was calculated from the difference traces a t 330 nm. from the known Equation I11 was used to find Doa3@ = bNSS3' values of k2', k-2, k3', kg, D t 3 3 @ , and t. From [OH]@and = ( ~ ( c N s ) ~ - [OH]o, ~~~) it follows that e ~ = (D0330€(CNg)z-475)/D~475. ~ s ~ ~ ~From the measured values of vix., the optical density at 475 nm for complete conversion t o (CNS),- and ~ c N s ) ~=- 7600 ~ ~ ~ M-1 cm-l (ref 4), we obtain € c N s 3 " = 900 M-' cm-l.
Kinetics in Alkaline Solutions Under alkaline conditions, the reverse reaction of equilibrium 6 can no longer be neglected and the kinetics become rather more involved. The general solution for the formation of the transient absorption now has the form The Journal of Physical Chemktry, Vol. 76, No. 11, 1976
+ k-6' + k2' + k-2) + + k6k-2 f k-e'k-2 1&3k2'
D/Do (VII)
=0
(VIII)
thus k-6 > 1.5 X lo9M-' sec-', and using the values of ICz and quoted above, it can be shown that, in solutions of pH > 12, A Z ( = k 6 k-6') >> X3; under these conditions, eq VI1 simplifies to
-+
+
DO
f
where al, a2, and a3 are functions of k3', k6, k-%', k,', and k-2, X I = kat and A 2 and X3 are the roots of quadratic equation VIII. Given that ICs > 5 X lo7 sec-', and
a Data for CNS- halide complexes taken from ref 9-11. Equilibrium constant for the reaction CNSX- e CNS X-.
D - D ,_ ~-
+ a2e-xzt+ a3e-Xst
D J D 0 = ale-"'
+ a3e-Xat
f D/Do
(IX)
Equation I X is formally similar to eq V and VI, replacing XI by k3' and X3 by ( 1 ~ 2 ' IC-,). For the formation and decay of CNSOH- (observed as the difference traces at 390 nm), the general equation now takes the form
+
Thus, the rate of formation of CNSOH- is governed by XI, and the decay by ha. Plots of log (Oh - D ) vs. t are linear at longer reaction time (see Figure 4), indicating that the term containing X1 has become negligible and that the first-order constant is X 3 . Values of ha obtained in this way for solutions containing varying amounts of CNS- and OH- are shown in Table 11. Also shown are values of XI calculated from the abovestated rate constants. (Note that A3 is independent of the values of k 6 and k-6' providing these are > 5 X 10' sec-'), and it can be seen that these agree quite well with the experimental values. Thus the decay of CNSOH- is adequately described by equilibria 6 and 2. Table I1 : Rates of Decay of CNSOH- in Pulsed Alkaline Thiocyanate Solutions [CNS-I
x
ha(0btld)
M
[OH-], M
1.0 1.13 0.4 2.0
0.1 0.1 0.02 0.3
104,
x
10-6,
Xa(oa1ed)
x 10-5,
seo-*
seo-1
2.1 2.3 1.9 1.8
2.00 2.21 2.02 1.66
The observed deviation from linearity at shorter reaction times (cf. Figure 4) represents the contribution from the term in XI. From eq X it follows that a logarithmic plot of the deviation, expressed as optical density, should be linear with slope X1 and an example of this procedure is included in Figure 4. Values of X1 for solutions containing different amounts of CNS- and OH- are collected in Table 111,and it can be seen that they conflict with the idea that the rate-determining
1541
PULSE RADIOLYSIS OF AQUEOUS THIOCYANATE SOLUTIONS
and that K7 > 1; therefore, for [OH-] < 1 M we can neglect k-,[OH-]. The formation of CNSOH- will then be described by the equation
DJD0 = 1
2.0-
a
-a& 1.8-
A2
1
6 1.6CII
3
1.4 .
psec.
Figure 4. Kinetics of CNSOH- in pulsed alkaline thiocyanate solutions, ([CNS-] = 10-4 M , [OH-] = 10-1 M ) : (a) logarithmic plot showing first-order decay a t longer reaction times, (b) logarithmic plot of deviation of (a) from first-order kinetics a t shorter reaction times (see text).
Table 111: Rates of Formation of CNSOHin Pulsed Alkaline Thiocyanate Solutions [CNS-I
0.4 2.0 a
Mean
[CNS-1
XI/
1.o 1.13
kq
+
IC-6'
+ + + k3'
k4')
k3'k-5'
+ + 'k,'k,'
=0
(XII)
Given the reported12values, k5 = 1.2 X 1O'O M-' sec-' and LE' = 9.2 X lo7 sec-', it follows that, in solutions above pH -12, X4(=k5' L5') >> X5. If one now makes the assumption that k7 > k6 > 5 X lo7 sec-I (this assumption being not unreasonable, since one would expect the rate of hydrolysis of CNS02- to be greater than that of CNSOH-), then k7 >> X5 at the concentrations of CXS- and OH- used. Equation XI then simplifies to
+
f,
M
= X(k5'
k5'kq'
v
104,
(XI)
where all as, and a3 are functions of k5', k - ~ ' , k3', kq', and k7, and h4 and A5 are the roots of the quadratic equation
-CII0
x
+ ale-k7t+ c ~ ~ e - "+~
[OH-], M
XI X 10-6,
0.1 0.1 0.02 0.3
5.4 5.9 4.2 9.1
980-1
X 10-9, M - 1 see-1
k4 X 10-9, M-1 ~ e c - 1 ~
5.4 5.2 10.5 4.5
3.6 3.5 3.8 4.0
= 3.7 =t 0.3 X 109 M-1 sec-1.
constant is ka'(=k3[CNS-], where k3 = 2.8 X 1O1O M-lsec-l). Under a1k:aline conditions, we must thus consider oxidation of CNS- by 0- (reaction 4). The rate of formation of CNSOH- would then depend on the following reactions
+ OH- E 0- + H2O OH + CNS- "3, CNSOHk6
OH
K5 = 7.25 X loa
k -6
k3
= 2.8 X 1O1O M-' sec-l
+ CNS- -% CNS02+ H20 CNSOH- + OH-
Dt/D
= 1
+ u3e-X5t
(XIII)
and the rate of formation of CKSOH- is determined solely by Xg. We can calculate X5 from eq XI1 using the known rate constants k5, k5,and k3, and various values of kq. The values of kq which correspond to X g at the various CNS- and OH- concentrations used are given in Table 111; a mean value of k4 = 3.7 X lo9 M-' 8ec-l is obtained. The formation of CKSOHin pulsed alkaline thiocyanate solutions can therefore be accounted for by reactions 5,3,4, and 7.
General Comments The production of transient OH adducts of the type XOH- should be a rather more general phenomenon, particularly from halides and pseudohalides. Indeed, the possibility of the formation of these in irradiated aqueous halide solutions has already been sporadically discussed in the literature.l38l4 We have recently15 obtained optical evidence for a precursor of It- in the pulse radiolysis of aqueous alkaline iodide solutions; this absorbs at -340 nm and, by analogy with the above observations, is probably attributable to IOH-. Similar adducts are produced in irradiated aqueous alkaline glasses at low temperature, as noted above for systems containing CKS- ions5 and also I- ions.I6 Symons and coworkers have recently identified XOHspecies (by esr spectroscopy) in certain solid systems irradiated at 77"K1 e.g., ClOH- from SrC12.6Hz0
0-
kl
CNS0'-
k --?
An important consequence of the observed variation of D(cNs)%with [OH-] is that equilibrium 9 is excluded
(12) G. V. Buxton, Trans. Faraday SOC., 66, 1656 (1970). (13) M. Anbar and J. K. Thomas, J. Phys. Chem., 68, 3829 (1964). (14) M. 8. Matheson, W. A. Mulac, J. L. Weeks, and J. Rabani, ibid., 70, 2092 (1966). (15) P. L. T. Bevan, M. Ebert, and G. Scholes, unpublished results. (16) D. V. Bent, S. K. Ismail, and G. Soholes, unpublished results. The Journal of Physical Chhemwtry, Vol. 76, No. 11, 1978
1542
G. LEMAIRE, C. FERRADINI, AND J. PUCHEAULT
crystals1' and BrOH- and IOH- from frozen alkali halide solutions. l8
atories, Christie Hospital and Holt Radium Institute, for providing facilities.
Acknowzedgment' The authors thank the staff Of the Department of Physical Chemistry, Hebrew University of Jerusalem, Israel, and of the Paterson Labor-
(17) R. C. Catton and M. C. R. Symons, J . Chem. Soc. A , 446 (1969). . . (18) I. Marov and M. C. R. Symons, ibid., 201 (1971).
Radiolysis of Water by Tritium P Rays: Scavenging of Hydrogen Peroxide Precursors by G. Lemaire, C. Ferradini, and J. Pucheault* Centre nationale de la Recherche Scientifique, 94 Ivry, France
(Received October 18, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
The influence of OH radical scavengers on the yield of HzOz formation in 0.4 M HzS04 solutions irradiated by tritium 6 rays was measured. Comparison with the results obtained with y rays shows differences which can be explained by the increase in the number of "short tracks" (in the terminology of Mozumder and Magee).
Introduction I n an earlier study,I we measured the initial yield of hydrogen peroxide formation in aerated 0.4 M HzS04 solutions irradiated by tritium P rays. The value obtained, Go(H202)= 1.5 f 0.05 molecules/100 eV, was in agreement with the molecular and radical yields determined by Collinson, Dainton, and Kroh,2that is: GH = 2.9, G H O= ~ 0, GOH = 2.0, G H ~ = o ~1.0, and G H = ~ 0.55. We felt it would be interesting to study the inhibition of molecular products by appropriate scavengers in order to see if this inhibition could be compared to that of y rays. From this point of view, the use of bromide and chloride ions which inhibit the formation of H202 in the presence of air is particularly well suited. The mechanism of their action has been clearly established notably by Sworski3 as due to the occurrence of halide r e a c t i o n ~i.e. ,~ OH+S--+S+OH-
(1)
S + HzOz ---t S- + H+ + HO2
(2)
in competition with those in the bulk of the solution OH
+
H202 ---t
H HO2
+
0 2
+ HO2
HOz
+ HzO
+HO2
+H2Oz
+
(5)
which do not change the formation yield of hydrogen (2) is equivalent to (3) peroxide since (1)
+
The Journal of Physkal Chemistry, Vol. 76, No. 11, 197.2
=
+
G H ~ o ~ '/z(GH
- GOH)
Using the equation of material balance ( ~ G H ~4-o ~ GOH= ~ G H GH), ~ this yield can be expressed in t e r m of the molecular yields
+
G(Hz0z)
=
~ G H~ G O H~ ~
(1)
On the other hand, when S- ions are present in high ~ ~ ~ concentrations, it is recognized that the G H molecular through the competition, yield is diminished by AGI.I~o~ in zones of high radical densities, of reaction 1 with the local formation of hydrogen peroxide OH
+ OH +HzO2
(6)
The fate of S, after diffusion, being identical with that described previously (reaction 2), it is as though supplementary OH radicals were recuperated by reaction 3, and the variation of the final yield can be reached by differentiating expression I, that is AG(H2oz)
=
~AGH~oZ
(11)
provided that the G H 2 yield is not modified by this heterogeneous process which can thus be expressed by
(3)
(4) 0 2
G(Hz0z)
(1) G. Lemaire and C. Ferradini, Radwchem. Radioanal. Lett., 5 , 175 (1970). (2) E. Collinson, I?. S. Dainton, and J. Kroh, Proc. Roy. Soc., 265, 422 (1962). (3) T.J. Sworski, J . Amer. Chem. Soc., 76, 4687 (1954). (4) E.J. Hart, Radiat. Res., 1, 53 (1954).
