2318
M. Faraggi
Equation 4 may be written R = RI+ R2, where R1 and R2 are the rates of the primary and secondary processes consuming 03.Under the conditions of our experiments R2/R1 reached a maximum value of 2.5 and this is only possible if short chains of ozone decomposition occur. Such chains occur in the O3 alkene systems and have been shown to be thermochemically feasible for O3 C2H4.l Although the identity of the chain carriers for the O3 CzF4 system cannot be established from our work, such a chain leads to the catalytic destruction of 03.This would normally form 0 2 as a major product but, as previously discussed, the resulting 0 2 is consumed in this system to form oxygen-containing polymers.7 In summary, the reaction of O3 with CzF4 is an extraordinarily complex system involving chains of both reactants and exhibiting both 02 production and 02 uptake. We have attempted to account quantitatively for the observed rate law and have given a qualitative description of the complex aspects of the kinetics.
+
+
+
Acknowledgment. We thank the Rutgers University Research Council for the support of this work. Supplementary Material Available: Tables A-F of kinetic
data at 0,30,70, and 110 "C and effects of added SFG at 30 OC along with a computer program for obtaining rate constants (6 pages). Ordering information is available on any current masthead page. References and Notes (1)F. S.Toby, S. Toby, and H. E. O'Neal, int. J. Chem. Kinet., 8 , 25 (1976). especially ref 1-11. (2)R. S.Sheinson, F. S. Toby, and S. Toby, J. Am. Chem. Soc., 97, 6593 (1975). (3) J. Heicklen, J. Phys. Chem., 70, 477 (1966). (4) F. Gozzo and G. Camaggi, Chim. ind. (Milan), 50, 197 (1968). (5) F. S.Toby and S. Toby, int. J. Chem. Kinet., 6, 417 (1974). (6) See paragraph at end of the text regarding supplementary material. (7) F. Gozzo and G. Camaggi, Tetrahedron, 22, 1765 (1966). (8) F. Gozzo and G. Camaggi, Tetrahedron, 22, 2161 (1966). (9)F. S.Toby and S. Toby, ht. J. Chem. Kinet., Symp. 1, 197 (1975). (10)S. W. Benson, "Thermochemical Kinetics", W h y , New York, N.Y., 1968. (11) S.R. Abbott, S. Ness, and D. M. Hercules, J. Am. Chem. Soc., 92, 1128 (1970). (12)E. A. Ogryzlo and A. E. Pearson, J. Phys. Chem., 72, 2913 (1968). (13)Gozzi and Camaggi4 found small quantities of ozonide in the products of the liquid phase reaction at 0 OC at high 03/C2F4ratios, but not in the gas phase reaction. (14) W. B. DeMore, int. J. Chem. Kinet., 1, 209 (1969). (15)K. H. Becker, U. Schurath, and H. Seitz, lnt. J. Chem. Kinet., 6, 725
(1974).
Steady State and Pulse Radiolysis Studies of Molybdenum Octacyanate in Aqueous Solutions M. Faraggl Nuclear Research Centre-Negev, P.O.B. 900 1, Beer Sheva, Israel
(Received January 8, 1976)
The oxidation of oxygen saturated solutions of Mo(CN)s4- at different pH values was studied by steady state and pulse radiolysis techniques. I t was shown that Mo(CN)& is the oxidation product formed. G values of the Mo(V) octacyanate complex varied with the pH of the solution. It was of the order of 13 at pH 0 (1M HC104) and approaching zero at pH >7. The high G values in acid solutions are explained by the oxidation of the Mo(1V) complex ion in the reactions with OH and H02 radicals and by H202. The H202 reaction with Mo(1V) ion was found to be a very slow reaction and 75% effective only. In neutral and alkaline solutions the low G values are interpreted by the reduction of the MOW)ions formed by the 0 2 - radicals. Confirmation of the above mechanism was established by using the pulse radiolysis technique. The formation of Mo(CN)s3-, the oxidation product of M O ( C N ) ~in~ -the reactions with OH and HO2 radicals, was followed at 385 nm. The rate constants (in M-1 s-l) are (5.8 f 0.5) X lo9 and (5.7 f 0.6) X lo4, respectively. These values were unaffected by the presence of H+ or unreactive alkaline cations. 0- reacts with Mo(CN)s4much more slowly (estimated to be -1 X lo7 M-l s-l). 0 2 - reduces the Mo(CN)& to Mo(CN)x4-, the rate constant found was 3.0 f 0.5 X lo5 M-l s-l.
Introduction In recent years, two studies have been published on the steady state (y rays) and pulse radiolysis of aqueous molybdenum(1V) octacyanate solutions.l.2 Sharmal reported that M o ( C N ) ~ ions ~ - in 0.4 M H2S04 solutions (oxygenated) react with OH radicals via a single electron transfer reaction to give Mo(CN)s3-
+
M o ( C N ) ~ ~ -OH
+
Mo(CN)s3-
+ OH-
The Journal of Physical Chemistry, Voi. 80, No. 21, 1976
(1)
This study' also reports that G(Mo(CN)s3-) = 2.7 and concludes therefore that neither HO2 nor Hz02 oxidize the Mo(CN)& complex. However, a close examination of his results (ref 1, Figure 1) indicates that G(Mo(CN)&) = 1.3 rather than 2.1. Waltz et a1.2 measured the rate constant of reaction 1via the competition pulse radiolysis method3 and found a value of (5.8 f 0.6) X lo9 M-' s-l. Cyanide complexes have been extensively studied in the
2317
Study of Molybdenum Octacyanate in Aqueous Solutions
I
n
\ I 2
6
4
PH
Figure 1. Mo(V) octacyanate ions yield (G value) as function of
pH.
ferro-ferri system.4-20 This system has been used for the determination of radical and molecular yields in the radiolysis of water and as a dosimeter in pulse radiolysis studies of aqueous solutions. It i s generally known that OH radicals oxidize ferrocyanide and eaq- and H atoms reduce ferricyanide. Absolute rate constants for these reactions were measured by the pulse radiolysis technique. The peroxy radical in its acid form (HOz) oxidizes the ferro complex while its alkaline form ( 0 2 - ) reduces the ferri complex. The aim of this work was to study the radiation chemistry of Mo(CN)s4- and to compare its behavior to Fe(CN)fi4-.
Experimental Section Materials. Triply distilled water was used to prepare all solutions. Mo(CN)s4- was prepared according to Furman and Millerz1 and was recrystallized before use. Mo(CN)s3- was prepared from Mo(CN)s4- by electrolytic oxidation at 1.0 V. The purity of the complexes was confirmed analytically and spectrophotometrically.zz Solutions of these. light sensitive complexes were prepared and studied in blackened vessels. All other materials used were of analytical grade. y Irradiations, Irradiations were performed with a 17 kCi cobalt 60 y source providing a dose rate of about 1.2 X lo6 rads/h as determined by ferrous sulfate dosimeter assuming G(Fe3+) = 15.6. Saturation of the solutions was made by continuous bubbling of purified gases (Matheson Co. Inc.) for 30 min. The pH of the solutions was adjusted by addition of either HC104 or NaOH. The yield of Mo(CN)s3- was determined spectrophotometrically using a Cary 17 spectrophotometer at X 386 nm. A t this wavelength the absorption coefficient (e) of M o ( C N ) ~ ~ is 150 M-l cm-1 and that of Mo(CN)s3- is 1300 M-l cm-1.22 Pulse Radiolysis. The experiments were carried out using the Hebrew University linear accelerator. It was operated at 5 MeV, 200 mA and pulse length of 0.1-1.0 ps giving a dose range of 150-2000 rads per pulse. Other experimental procedures, the apparatus optical detection system, cell filling technique, and the evaluation of the kinetic curves have been previously described.23
+
Results and Discussion Steady State Radiolysis. Oxygen saturated solutions of different Mo(CN)s4- ion concentrations and pH were irradiated. Linear plots of Mo(CN)s3- concentration, as measured
at X 386 nm, vs. dose were obtained. From the slope of these curves G(Mo(CN)&) values were determined. Figure 1shows the results as function of pH a t two different concentrations (5 X 10-4 and 10-2 M). It should be pointed out that due to the slow reaction between HzOz and Mo(CN)s4- (vide infra) measurements of the Mo(CN)s3- ion concentration were made at different periods after irradiation. This was at least 15 h, during which the irradiated solutions were kept a t dark, for the 10-2 M Mo(CN)s4- solutions. In the 5 X M Mo(CN)s4- solutions the determination of the Mo(V) cyano complex concentration was made immediately after irradiation (-5 min). Figure 1 clearly demonstrates that at high acid and Mo(CN)s4- concentrations the G(Mo(CN)s3-) reaches a value of approximately 13 f 0.5 (plateau value) reflecting the fact that under these conditions the oxidation reactions of Mo(CN)s4- cannot be explained only by the reaction of the complex ion with OH radicals (reaction 1).Hence, the reactions of the HOz radicals, produced via the H + 02 reaction, and the H202 with Mo(CN)a4- are also suggested to participate in the oxidation process Mo(CN)s4Mo(CN)s4-
+ HOz
+ HzOz
H+
+ HzOz
(2)
+ OH + HzO
(3)
Mo(CN)s3-
H+
Mo(CN)s3-
Thus:
+
+
G(Mo(CN)s3-) = GOH 3 G ~ o 2~G ~ , o * = 2.95 3 X 3.65 2 X 0.8 = 15.524
+
+
The difference between the calculated value and the experimental one is probably the result of the fact that reaction 3 compared to reactions 1and 2 is slow and is only 75-80% effective. This behavior is similar to that observed in the H202 Fe(CN)fi4- reaction.25 Between pH 2.5 and 4 another plateau value is found and G (Mo(CN)s3-) = 6.2 f 0.3. This result shows that in this pH range only reactions 1 and 2 are taking pldce. Thus, G(Mo(CN)s3-) = GOH+ G H O = ~ 6.0 in good agreement with the experimental results. Similar results were found in the 5 X lov4M concentration indicating again that at this low solute concentration and after a short period after the irradiation, reaction 3 does not take place even at high acid concentration. When solutions of higher pH values were irradiated G(Mo(CN)s3-) decreased approaching to zero at p H >7. At this pH value the perhydroxyl radical is transformed to its basic form 0 2 -
+
HOz
~ ; 0t 2 -
+ H+
(4)
with a pK of 4.88 f 0.Lz6 The G values at the neutral pH solutions found could be explained if it is assumed that the Mo(CN)s3- produced via reaction 1 is reduced by 0 2 - produced via eaq- O2 ---* Ozaccording to
+
MO(CN)s3- + 0 2 -
Mo(CN)s4-
+
(5) and that H202 does not react with Mo(CN)s4-. Thus, as GoH < G,,,GH then +
0 2
+
G(Mo(CN)s3-) = GOH - Go2- < 0 Reaction 4 is similar to that proposed for the reaction of ferricyanide in the ferro-ferri irradiated system. In separate experiments reaction 3 was followed in oxygen saturated aqueous solutions containing M Mo(CN)~~-, The Journal of Physical Chemistry, Vol. BO, No. 21, 1976
M. FI-
2918
5 x 10-5 to 5 x 10-4 M H202,and 1M HC104. It was found that the oxidation reaction is a slow process and approximately only 75% of the H202 reacted via reaction 3. Mo(V) equilibrium concentration was attained after a t least 15 h. Both the MOW)yield and the time to attain equilibrium were found to depend on Mo(CN)& ion concentration. The results are again similar to those obtained in the Fe(CN)s4--H202 acid solution system. The Mo(CN)& H202 reaction is acid dependent. T o attain the equilibrium concentration value of Mo(CN)& ions a t a reasonable time scale solutions containing 1 M HC104were necessary. This might indicate that prototropic equilibria involving the M O W )complex exist in this acid range and reaction 3 might be observable only for protonated complexes. The slowness of the reaction could be a result of the high dissociation constant of the comple~,~' leaving a low concentration of the protonated form. Pulse Rodiolysis. The reaction of Mo(CN)s4- with OH radical was followed directly by the observation of the production of Mo(CN)gl- at 385 nm. The rate constant calculated was found to be (5.8 f 0.5) X 109 M-' s-I in neutral solutions (pH 6.5). In order to check that reaction 1 produces exclusively Mo(CN)&, neutral and acid solutions were irradiated and the optical absorption in the 300-500-11111 wavelength range was determined. It was found that this spectrum is similar to that of MO(CN)~R--MO(CN)~'-difference spectrum.22 It is stable for at least 100 ps, independent of Mo(CN)s'- concentration (up to 5 X 10-2 M) and PH.'-~A similar spectrum was observed in N20 saturated solutions. However, the optical densities values obtained in this solution doubled as compared to the oxygen saturated one. This is the result of eBq- conversion to OH radical in the N20 saturated solutions via
+
eaq-
+ N20+ H20
-
-
OH
+ N2 + OH-
and that Ceaq- COH.The results in oxygen and N20 saturated solutions seem to indicate that Mo(CN)& is the species produced. Table I shows the reaction rate constant k l measured in the presence of different additives and as a function of pH (acid and neutral). This table shows that except for the effect of 0.1 M of Kf no other change in the rate constant could be observed. The behavior is different from that obtained for Fe(CN)e4- hy Zahavi and RabaniZ0where association and ion pair formation between ferrocyanide and H+ affected the reactivity of the OH radical. Our results may be due to the fact that Mo(CN)s'- does not associate with the added hydrogen ions to a greater extent or, that the associated ion pair(s) reacts with the OH radical with similar rates. However, Kolthoff and Tomsicek2' studying cations effect on the standard electron potential of Mo(CN)s4- suggested that H4[Mo(CN)8]unlike H4[Fe(CN)61 is a strong tetrabasic acid. Thus, it seems that in our conditions most of the Mo(CN)s'- ions were not associated with H+. Association of Mo(CN)& with positive ions to form ion pairs has no effect or a small one on k l in agreement with ref 20. Rabani and Matheson" have shown that in alkaline solutions radicals dissociate to 0-radical ions according to OH + OH-
0-+ H20
TABLE I:Effect of Cations (Avdroeen and Alkaline Ions) on the Second-Order Rate Con&antof the Reaction of OH Mo(CN)&
+
5 x lO-5to 1.6 x lo-' .HCI04, 10-sto 1M LiCI04.10-2 to 1 M NaCIOn, lo-* to 1 M
h s ~ i o r m ~ , wvoi.~60, i NO. ~ 11.~ 1976 ,
6.6 f 0.7
CSCIO~.10-2 to 10-1 M
KCIOd:'10-3 to KCI04, 10-1 M
2. k-.
lunctim of pH.
M
lile Observed rate Constant 01 OH
5.5 5.4
* 0.7
i0.4
1.9 f 0.2
+ Mo(CN)84-,as a
Fipun 3. Oscilloscope trace at A 385 nm of lhe pseudo-lirstOrder reaction 01 H 0 2 radicals with Mo(CN)a'-. Oxygen saturated solution of 2X M MdCN)8'- at pH 2.5(HCIO.) imadiated with a 1.0.p~ pulse. Vertical displacement mnesponds lo change in Um optical bansmission ( 1 large division = 3.8%) horizontal Io time (1 large division = 5 ms).
tion relating the effective rate constant ( h o w )for the OH (0-) reaction with the pH value of the solution. log
they reported a PKOHvalue of 11.9 f 0.2. Using revised rate constant values, Zehavi and Rabani reported a value of 11.85.'m Rabani and Matheson" have also shown that 0-reacts slowly, compared to OH, with Fe(CNk'-. They developed an equa-
5.8 f 0.5 6.4 f 0.7
("'how -
1) = pH - pK
Figure 2 shows a plot of k.w in the pH range of 11.2-13.3 according to this equation. From this figure the pK of OH is 11.8 i 0.2 in good agreement with the published value.1L.Q
Study 01 Molywenum Octacyanate In A q m u s Solutions
2319
oxidation reaction of HOz with Mo(CN)s4- is separated in time from other reactions of the molybdenum(1V) and molybdenum(V) octacyanate. In order to determine the oxidation rate constants of HO, with Mo(CN)& oxygenated solutions containing 5 to 10 M of the solute and 0.3 M formate (2 5 pH 5 4) were used.In these solutions the OH radical reacts mainly with the formate ions (>95%)
I I
I
x
OH
+ HCOO- -Con- + H20
with ke = 3.8 X 10sM-' to 02 via CO2Fipm 1.Oscilloscope trace a1 A 385 nm 01 the pseudo-firslader reHO, radicals with Mo(Chis4- Oxygen sal.rale.3 solution of 93X M MqCh)8'- ana 0 3 M ?!COO- ai pH 2 0 lkCl0.l inadiated wilha 1 &us PL se Verlica a s p acemenl C O W S D O ~ ~ S10 changes in the optical l r a n s m m on i I arge awision = 1 4 ' 0 I hoi zonlai 10 11mo ( 1 large division = 1 ms).
action 01
I
I I I
I Flgure 5. Osc~lloscope trace ai h 385 nm 01 the pseudo-Iwsl-order reOxygen saI.rale.3 soIu11on 01 3 1 X lo-' M MolCN),' , 4 0 X 10.' M MotCNla'- T 5 X 10.' M NaClO, at pH 10 0 IhaOHl # m aated with 1 0 us pulse Verlcal d s placemen1 cwresponds lo changes In transmission ( 1 large d vwon = 2 1 % ), horiionta IOlime 11 arge divlslon = 10 ms)
action 01 02- radica s w i n Mo(CN1,'-
(6)
The Cot- tranders its electron
+0 2
-
02-
+ co2
(7)
with k7 = 2.4 X lo9 M-16-1.S Thus, all radicals (eeq-, H, and OH) are converted to the peroxy radical and its reaction with Mo(CN)& could be followed. Figure 4 shows the experimental result. In these experiments only reaction 2 is studied. The value of kz was derived from the equation
and was found to be 6 f 1 X 10' M-' 9-1. Reaction of the 0 2 - Radical Ion with Mo(CN)&. This reaction was followed in oxygenated solutions containing Mo(IV) and Mo(VY&cyanate a t pH 8.3-10.4. Under these conditions Mo(CN)s4- reacted with the OH radicals amrding to reaction 1 to produce Mo(CN)s3- already present in the solution. Thus, after a short time, the system contains only Mo(IV) and MOW)octacyanate and 02-radical ions. At the pH studied the radical is relatively stable. Figure 5 shows the experimentid result from which it can be Seen the 02-reduced Mo(CN)& (reaction 5) with a rate constant of 3.0 i 0.3 X 105 M-' s-I. The reactivity of Mo(IV) and MOW)&cyanate toward OH, HOZ.and 0 2 - radical and HZOZis similar to that observed for ferro- and ferricyanide. It &ems however that the molybdenum complexes reactivities are less affecled by ion pair formation. Acknowledgments. The author wishes to thank J. Ogdm for the operation of the linear accelarator and Mrs. A. S a r d for her aid in the experimental work. References and Notes
From Figure 4 the rate constant for the reaction of 0-with Mo(CN)s4- was estimated to be -1 X lo' M-' s-I. Reaction o f H 0 2 with M o ( C N ) ~ ~0-2- saturated . solutions containing 1 X 10-3 to 1X 10-2 M Mo(CN)s4- in the pH range of 2.54.0 were investigated hy pulse radiolysis. Figure 3 shows the oscilloscope trace observed. The first step is the rapid formation of Mo(CN)& due to the rapid reaction of OH radicals with Mo(CN)s4-. The e,- and H atoms are converted into peroxy radicals HO2 and 0 2 - . The relative concentrations of the peroxy radicals are determined by the pK of H02 which is 4A2' As stated, the reaction of H202 with molybdenum(1V) &cyanate is a very slow reaction. The 02-radicals ion is also unreactive toward Mo(CN)s4-. This was confirmed by following pulse radiolysis experiments in oxygen saturated alkaline solution (pH 9)of M Mo(CN)s4-. After the formation of Mo(CN)$- via reaction 1was completed, its decay was observed in the 100-ms time range. This decay can he attributed to the reaction of Mo(CN)& with 0 2 - . Thus, the
ann..
(1) B. K. shsm\a. Csn. J. 46,2757(1988). 51, 2525 (2) W. L. W a k . S.S. Akhtar. and R. L. Eager. Can, J. m.. (1973). (3) L. M. Dxlnmn and Q, E. *68ms. N d , Ret &@ Ser.. MI!. Bv. stand. No. 46, (1973). (4) J. Rabani and 0. Stein. Trans. Faraday Sa..58,2150 (1962). (51 (a1 F. S. Oainlon and W. S.Wall. Nahq (Lon@m).195. 1294 (1962): (b) Rm. R. Soc..Ser. A. 275.447 (1963). (6) (a1G. Hughes and C. Willis. J. Chem. Sa..2848l1962); (b) Dlscurs. FeradaySoc.. 36.223 (1963). (71 (a) S. Gordon. E. J. Hart. M. S. MameMn. J. Rabani. and J. K. lhxnas.J. Am. Chem. Sa..65, 1375 (1963); (b) Dircuts. F&y Sa..36. 193 (1963). (8) (a)E.MayiandM.Haissinsky.J.Chim~..110,397(1963);(b)M.nb issinsky. A.M. Kwlkes. and E. k i . Ibld.. 63. 1129 (1966). (9) (a1 E. Hayon. Trans. Faraday Sa..81. 723 (1965); (b) ibld.. 61. 734
m.
(1965).
(10) G. E. A m . J. W. Boeg. and B. D. Michael. Tram. F d y S a . . 61.492 (1965). (11) (a)J.RaabanlandM.S.hWhewn. J.Am.chrmSa.. 86.3175(1964):@) J. Php.Chsm.. 70,761 (1966). (121 C. E. Bvchiil. F. S.@aimon. andD. Smihier. T m . F d y S a . . 63.932 (1967). 113) S. Ohno and 0.Tpuchihashi. Redioholopes. 16,26 (1967). (14) J. RabaniandD.Meyers1ein. J. mys. Chsm.. 72. 1599 (1966). I E. Peled. Isr. J. Chem.. 6.421 (19681. (15) G. C Z W S ~and T I W J C W W ~ ~ ~ ~ ~ S ~ C M Wvoi. Y , BO.^. 21. is76
2320
K.J. Kim and W. H. Hamill
(16)J. Sobkowski, Nukleonika, 14, 253 (1969). (17)G. E. Adams and R. L. Willson, Trans. faraday SOC.,65, 2981 (1969). (18)G. C.Barker, P. Fowles, and B. Stringer, Trans. faraday Soc., 86, 1509 (1970). (19)D. Zehavi and J. Rabani, J. Phys. Chem., 75, 1738 (1971). (20)D. Zehavi and J. Rabani, J. Phys. Chem., 76, 3703 (1972). (21)N. H.Furman and C. 0. Miller In "Inorganic Synthesis", Vol. 3,L. F. Audrieth Ed., McGraw-Hill, New York. N.Y., 1950,p 160. (22)J. R. Perumareddi, A. D. Liehr, and A. W. Adamson, J. Am. Chem. Soc.,
85,249 (1963). (23)M. Faraggi and A. Feder, Inorg. Chem., 12, 236 (1973). (24)A. 0.Allen, "The Radiation Chemistry of Water and Aqueous Solutions", Van Nostrand, Princeton, N.J., 1961,p 41. (25)J. Sobkowski, Roczn. Chem., 43, 1729 (1969). (26)D. Behar, G. Czapski, J. Rabani, L. M. Dorfman, and H. A. Schwarz, J. Phys. Chem., 74, 3209 (1970). (27)I. M. Kolthoff and J. W. Tomsicek, J. Phys. Chem., 40, 247 (1936). (28)G. E.Adams and R. L. Willson, Trans. faraday SOC.,65, 2981 (1969).
Direct and Indirect Effects in Pulse Irradiated Concentrated Aqueous Solutions of Chloride and Sulfate Ions Kang-Jin Kim and Wllllam H. Hamill' Department of Chemistry and Radiation Laboratory,' University of Notre Dame, Notre Dame, lndiana 46556 (Received April 15, 1976) Publications costs assisted by the U.S. Energy Research and Development Administration
Yields of Clz- have been measured in concentrated aqueous solutions of NaCl which contained ethanol to suppress ClOH- and HC03- to neutralize the acid spur. Yields of SO4- and Clz- have been measured in solutions of (NH&S04, some containing 50 mM NaCl to convert SO4- to Clz-. Yields of SO4- are considerably larger for solutions of (ND4)2S04in D2O. Optical spectra, extinction coefficients, and rate constants have been measured as needed for confirmation. Yields of oxidized species are attributed principally to electron transfer from the reagent R to ionized solvent S, with some contribution from direct effect. The effective electron fraction of R is 4 = f&]/(f~[R] fs[S])in terms of oscillator strengths f,with f R / f s adjustable. For direct and indirect primary yields G d o and Gio the yield is described by G = Gdo4 + Gio(l - 4)(ul[R]/(ul[R] vz[S])] where u1[R] is the frequency of electron transfer and vz[S]is the frequency of dry hole localization, e.g., by formation of H30+. For 1 - C#J 1,the direct and indirect yields are algebraically indistinguishable. Large yields of SO4- in DzO cannot be explained by 4. They are attributed to the vibration-limited frequency v 2 which increases the lifetime of DzO+.
+
+
-
Introduction The possibility of electron transfer from a solute to the primary positive hole in aqueous systems is of fundamental interest in radiation chemistry. Anions should be particularly suitable. For pulse radiolysis the oxidized species should provide an appropriate optical transient and OH, as precursor, capable of being excluded. Electron transfer from an anion to HzO+ in aqueous solution was considered by Anbar and ThomasO2 They observed first that chloride ion is rapidly oxidized by OH under pulse radiolysis in acidic aqueous solutions, the rate of formation of C12- being first order in the concentrations of OH, C1-, and H+.2The rate constant is ~ 1 0 M-2 ~ 0s-l. In neutral or slightly alkaline solutions Clz- was observed only at >0.1 M C1- and the reaction was considered to occur in spurs as a consequence of reactions HzO+ + HzO
-
and OH
+ OH
(la)
C1+ 2H20
Ob)
H30+
+ C1- + H30+
-
They also considered the following as a possible mechanism HzO+
+ C1-
-
HzO + C1
The Journal of Physical Chemistry, Vol. 80, No. 21, 1976
(2)
but thought it to be unlikely. Another mechanism which was also considered is
+ C1HOCl- + H+ OH
c1+c1-
-
HOCl-
(34
C1+ HzO
(3b)
Clz-
(34
-
The preceding results for large [Cl-] and neutral pH were subsequently attributed by Hamill to reaction 2.3 The possibility that Clz- a t neutral pH is produced principally by eq l b or 3b in the acid spur was examined by Peled et aL4 who showed that in 2 M NaCl the yield of Clz- was unchanged by 0.4 M Na2HP04 or 0.5 M NazS04 as buffers. They concluded that either (i) Hz0+ and e- do not annihilate, or (ii) partial annihilation is much too fast for interference by scavengers. Concurrently, Fisher and Hamill5 had found that the yield of Clz- was unaffected by 1 M NazS04 and concluded that reaction 2 occurred within s. Later work by Ogura and Hamil16showed that at pH -7 and large [Cl-1, yields of Clz- are -20% larger in D20 than in HzO while the rate constant for reactions 3a-c is 17% greater in H2O. Again, reaction 2 was invoked. Pucheault et al.7 studied neutral solutions of LiCl up to 14 M. The 100-eV yield, G (Clz-), was a maximum a t 3.4 in 9 M
