Reactionsof OH, IdQ2, and 02-Radicals
3703
Pulse Radiolysis of the Aqueous Ferro-Ferricyanide System. I . The Reactions HQ2, and Q 2 - Radicals ow Pekavi and Joseph Rabani” Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91 000, Israel
(Received March 24, 1972)
Pulse radiolysis of ferro and ferricyanide aqueous solutions is investigated at various H+ and salt concentrations. It has been found that association and ion pair formation may affect the reactivity toward OH, H 0 2 , and Q2- radicals. The following reaction rate constants were determined (in units of M - l sec-I): k((o)31+ F ~ ( C N ) G ~=- )(1.25 ) d= 0.1) X 1O1O; k ( ( 0 H HFe(CN)e3-)) = (9.0 f 0.9) X lo9; k((OH C H2Fe(CN)62-)) = (1.7 f 0.5) X 10’; k((HO2 + Fe(CN)e4-)) = (3.0 f 1.5) X lo4; k((H02 HFe(CN)63-)) = (1.4 f 0.1) X lo5; k((HO2 + H z F ~ ( C N ) ~ ~=- (1.0 ) ) f 0.3) X lo4; k((HO2 KFe(CN)63-)) = (3.0 f 1.5) X 10%k ( ( 0 2 - -!= Fe(CN)e3-)) = (2.7 f 0.9) X lo2; k((O2- + KFe(CN)e2-)) = (6.2 f 0.6) X 105. The last two values are calculated for zero ionic strength. The formation of ion pairs has only a little effect ( 10% decrease) on the reactivity of ferrocyanide toward OH radicals.
+
+
Introduction In recent years, extensive work has been published on the steady (+y and X-rays) and pulse radiolysis of aqueous ferro- and ferricyanide ~ o l u t i o n s . l 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. The radiolysis of water can be expressed by
, OH, &o+,OH-, H2, H202
(1)
It is generally assumed that in the ferro-ferricyanide system, the OH radicals oxidize ferrocyanide and the eaq- and H radicals reduce ferricyanide according to reactions 2-4, while Hz does not react with ferrocyanide nor ferricyanide. Fe(CP464- 4- OH-.Fe(CN)63-
F ~ ( G N ) G4-~eaqFe(CN)& I- H
-
-
+ OH-
Fe(CN)s4-
!?e(cN)e4- + H+
-+-
HO2
-
Fe(CN)&
+ HO2-
Experimental Section The experimental procedure for the pulse irradiation has been described elsewhere.22A linear accelerator was used as an electron pulse source of 5 MeV and 200 mA. The pulse duration was varied between 0.1 and 1.5 ysec. The inductive current obtained by the electron beam in a coil was used to
(2) (3) (4)
(5)
It has been proposed that the IFe(CN)5H20]3- complex is formed together with E”e(CN)64-in the reduction of ferricyanide by eaq-,5,9H,5 and 0 2 - .5 The aquapentacyanoferrate(I1) has a peak of absorption at -445 nm in neutral solutions.17-19 It absorbs light at 420 nm where ferricyanide is usually measured by its peak absorption, and a t wavelengths longer than 500 In addition to the aquapentacyanoferrate(II), small yields of aguapentacyanoferrate(II1) have been proposed.58 In acid solutions ferrocyanide ions become protonated 20,21 This fact has been disregarded in previous
-
radiation chemistry work. Consequently, the possibility of the effect of pH on the reactivity of ferrocyanide has not been considered. The purpose of this work is to elucidate the mechanism of the OH and peroxy radicals reactions in the ferro-ferricyanide aqueous system, in both neutral and acid media.
Absolute rate constants for the OH radica1,s the H radica1,ll and the eaq- radical ion4 reactions in this system were measured by the pulse radiolysis technique. Hydrogen peroxide oxidizes ferrocyanide t u ferricyanide in acidic solutions and reduces ferricyanide to ferrocyanide in basic solutions, but these processes are slow in comparison with reactions 2-4.l6 In aerated solutions of ferrocyanide, eaq- and H radicals react with oxygen to form peroxy radicals. The peroxy radicals oxidize Ferrocyanide only whlen sufficient acid concentrations are present according to Fe(CN)s*-
+
(11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
J. Rabani and G. Stein, Trans. FaradaySoc., 58, 2150 (1962). (a) F. S. Dainton and W. S: Watt, Nature (London), 195,-1294 (1962); (b) F. S. ,Dainton and W. S. Watt, Proc. Roy. Soc.. Ser. A , 275,447 (1963). (a) G. Hughes and C. Willis, J. Chem. SOC.. 4848 (1962); (b) G. Hughes and C. Willis, Discuss. Faraday Soc., 36, 223 (1963). (a) S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, J. Amer. Chem. SOC., 85, 1375 (1963); (b) S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani. and J. K. Thomas, Discuss. FaradaySoc., 36,193 (1963). (a) E. Masri and M. Haissinsky, J. Chim. Phys., 60, 397 (1963); (b) M. Haissinsky, A. M. KoulkBs, and E. Masri, ibid., 63, 1129 (1966). (a) E. Hayon, Trans. Faraday SOC.,61, 723 (1965); (b) E. Hayon, ibid., 61, 734 (1965). G. E. Adams, J. W. Boag, and 8. D. Michael, Trans. Faraday Soc., 61,492 (1965). (a) J. Rabani and M. S. Matheson, J. Amer. Chem. SOC.,86, 3175 (1964); (b) J. Rabani and M. S. Matheson, J. Phys. Chem., 70, 761 (1966). C. E. kurchill, F. S. Dainton. and D. Smithies, Trans. Faraday SOC., 63, 932 (1967). S. Ohno and G. Tsuchihashi. Radioisotooes. 16. 26 (19671. J. Rabani and D. Meyerstein, J. Phys. Chem., f2, 1699 (t968). G. Czapski and E. Peled, lsr. J. Chem., 6,421 (1968) J. Sobkowski, Nukleonika, 14, 253 (1969). G. E. Adams and R. L. Willson, Trans. Faraday Soc., 65, 2981 (1969). G . C. Barker, P. Fowles, and 8.Stringer, Trans. Faraday SOC.,66, 1509 (1970). J. Sobkowski, Roczn. Chem. 43, 1729 (1969). M. Ottolenghi and J. Rabani, J. Phys. Chem.. 72, 593 (1968). G. Emschwiller, Colloq. Int. Cent. Nat. Rech. Sei., 191,307 (1970). (a) G. Stein, Isr. J. Chem., 8, 691 (1970); (b) Ni. Shirom and G. Stein, J. Chem. Phys., 55, 3379 (1971). J. Jordan and G. J. Ewing, lnorg. Chem.. 1, 587 (1 962). G. I. H. Hanania, D. H. Irvine, W. A. Eaton, and P. George, J. Phys. Chem., 71,2022 (1967). D.Zehavi and J. Rabani, J. Phys. Chem., 76, 1738 (1971). The Journal ot Physics, Chemistry, Vol. 76, No. 25, 1972
Dov Pehavi and Joseph Rabani
3704
procedure described before.8b The temperature was 23 i 2". A Beckman Model H3 pH meter was used for pH measurements with standard Beckman buffer solutions for calibrations. A CD6400 computer was employed for the calculations of complex kinetics, using Schmidt's23program.
350
40 0
450
500
WAVELENGTH ( n m )
Figure 1. The spectra of ferricyanide and ferrocyanide in neutral aqueous solutions a, absorption spectrum of ferricyanide (5 X M ) , b, absorption spectrum of ferrocyanide M), c, a difference spectrum (a - b), 0, the spectrum in the pulse irradiation of M K4Fe(CN)6in oxygenated neutral solutions. The optical density at 420 nm is 0.04.
monitor the pulse intensity. A 150-W Osram xenon lamp, a halogen lamp (for the long time range) and D2 lamp (for the uv range) produced the analyzing light. A 4-cm cell with 12.2-cm light path was used unless otherwise stated. An IP28A photomultiplier, a Bausch and Lomb high-intensity monochromator, and a Tektronix 556 double-beam oscilloscope were used. Spectra were recorded using a split analyzing light beam. One light beam a t constant wavelength (normally at the peak of the absorption) served as a pulse intensity monitor. Ferrocyanide and ferricyanide solutions undergo thermal and photochemical decompositions. In addition, ferrocyanide is oxidized by oxygen. These processes are enhanced un both acid solutions and at relatively concentrated neutral solutions. To minimize these effects, the following precautions were taken. (a) The solutions were prepared in syringes just before irradiation and were irradiated within about 20 min, Ferrocyanide was dissolved only after saturating the solutions with the appropriate gas and the removal of air (by bubbling the solutions with Ar, N20, or 0 2 for 20 min). Gas chromatographic measurements showed that less than 2 x 10-7 M residual 0 2 was left after 20 min bubbling with Ar. (b) Appropriate light filters (Corning or Schott and Gen.) were used between the light source and the irradiation cell. (c) A shutter, operated mechanically, was used between the irradiation cell and the light source. The solutions were exposed for periods of less than 1 sec before irradiation. (d) The analyzing light signals were recorded simultaneously with the corresponding absorption traces, so that errors in the optical density due to photoproduction of small concentrations of products before pulsxng were eliminated. (e) The syringes, the filling system, and the irradiation cell were protected from light. Materials were of AR grade. K4Fe(CN)6.3H20 (Mallinckrodt), K3Fe(CN)6 (BDH), HClO4 (Merck), H2S04 (BDH), &SO4 (H & W), and KOH (Riedel-De Haen) was used without further purification. The water used was triple distilled. Ultrahigh-purity argon and oxygen (Matheson Co.) were used. N2O (Matheson Co.) was purified by the The Journal of Physical Chemistry, Vol. 76, No. 25, 1972
Results 1. T h e Reaction of OH with Ferrocyanide i n Neutral and Acid Solutions. Rabani and Mathesons reported k 2 = (1.07 f 0.10) x 1O1O M - I sec-I in the pulse radiolysis of neutral and acid (pH -3) solutions of ferrocyanide. Irradiations in the presence of 0.3 M Na2S04 in neutral solution gave similar results.8 It was assumed that ferricyanide was the only reaction product absorbing at 420 nm. In order to check whether the reaction of ferrocyanide with OH produces exclusively ferricyanide, neutral and acid solutions of ferrocyanide were irradiated and the optical absorption spectrum in the visible region was determined. The results are presented in Figure 1. that the spectrum of ferricyanide in the range 330 to 500 nrn does not depend on the presence of ferrocyanide (checked in 0.1 M ferrocyanide) or on the pH (examined at pH 3 and 1).The optical absorption of ferrocyanide changes with the pH due to protonation but it does not influence the difference spectrum (c) above 400 nm. Under the conditions of Figure 1 the OH radicals react with ferrocyanide while the eaq- and H radicals react with 0 2 . The absorption formed is stable for at least 100 psec. Identieal spectra have been obtained in the irradiations of (a) IO-? M ferrocyanide in NzO-saturated neutral solutions (about 1 psec after the M ferrocyanide in oxygenated electron pulse) and (b) HC104 solution at pH 1 and 5 x _M ferrocyanide in airsaturated 3 M HC104 solutions (I psec after the electron pulse). In all these experiments no absorption was found in for electron pulses the range of 500 to 700 nm. ( D < 5 X identical with those of Figure 1). This indicates that the only product formed under these conditions is ferricyanide. In Table I we present the reaction rate constant h2 measured in neutral and acid solutions. Values of k 2 were determined from plots of In (D2 - Dt) vs. t , where Dt and Dz are the optical densities at time t and a t the end of the reaction 2, respectively. D2 was measured before any decay of the ferricyanide had taken place, or corrected €or it. I n solutions containing 0 2 , electrons and H atoms produce 0 2 - and HO2, respectively. These products maintain a p6J dependent e q ~ i l i b r i u mUnder . ~ ~ our conditions the reactions of ferricyanide or ferrocyanide with WQz or 0 2 - alre well separated in time from reaction 2, so that 0 2 could be measured directly. In neutral solutions containin trons are quickly converted to OH radicals. -0.6) may reduce ferricyanide (reaction 4 in competition with recombination reaction 7 ) . N,O
H +H
+ eaq- % N~
+
-I- OH
+ OH-
HZ (k7 = 7.8 X 109 M-1 sec-l
(6) 25)
(7)
In acid solutions, reaction 4 becomes more important owing to the formation of additional H atoms by reaction 8. In eaq-
+ Elaq+
-+
M
(8)
K. H. Schmidt, ANL Report 7199, Argonne National Laboratory, Argonne, ill., 1966. (a) J. Rabani and S. 0. Nielsen, J. Phys. Chem., 73, 3736 (1969). (b) D. Behar, G. Czapski, J. Rabani, L. M. Dorfman, and H. A. Schwarz, ibid., 74, 3209 (1970). P. Pagsberg, H. Christensen, J. Rabani, 6.Niisson, J. Fenger, and S. 0. Nieisen,J. Phys. Chem., 73,1029 (1969).
Reactions of OH, H02,an0 O2 Radicals TABLE I : Rate Constants far the OH
3705
Ferrocyanide Reaction
__
lO4[K4Fe(CN),]. M
-.___l__l__lD#
Additivesu
Near neutral Near neutral Near neutral Near neutral
N2O 1 atm N1O 1 atm
0.3
0.5 1.o 2.0 1 .o
1 atm o r N 2 0 1 atm N 2 0 1 atm o21 atm 1 x 10-5 M H+ NzO 1 atm 4 X M H+ 0 2 1 atm 1x M H+ N 2 0 1 atm 2 X M H+ M H+ O2 0.8 atm 9.3 X N 2 0 0.8 atm 9.3 X M H+ O2 1 atrn 1X M H+ Air saturated 1 X M H+ O2 0.8 atm 2.8 X M H+ M H-b W 2 0 0.8 atm 4- 9 . 3 X O2 0.8 atm 9.3 X M H+ O 20.8 atm 2.8 X M H+ O2 0.8 atm 9.3 X M H+ I V 2 0 0.8 atm 9.3 X 1 OW2 M H+ 0.46 M H f 0.46 M Hi 0 2 0.8 atrn 0.46 M H + O2 0.8 atm 0.46 M H+ 0 2 0.8 atrn 0.465 M H + Air saturated 1 M H+ Air saturated 3 M H+ 0 2
+ + + + + + + + + + + +
1.01
1 .o 1.01 2.6 2.56 1.01
1.o 2.6 2.56 2.6
5.0 5.0
5.5 4.5 4.27 3.82 3.1 1 3.16 3.08 3.04 2.57 2.04
2.07 1.65 1.17 1.18 0.45 0.45 0.45 0.45 0.45 0.1 0.4
+
2.56 2.6 2.6 2.6 2.6
+ + +
5.0 5.0 5.0
+
+
*
I 0 ’O k i , d M sec
’
1 .‘I3 1.26 1.27 1.29
0.037 0.039 0.048
0.045 0.046 0.057 0.045 0.056 0.037 0.075 0.026 0.043 0.037 0.051 0.036 0.038 0.038 0.037 0.033 0.016 0.01 7 0.036 0.038 0.049 0.052
1.24 1.16 1.17
0.99 0.79 0.89 0.78 0.83 0.66 0.71 0.59 0.39 0.31 0.36 0.23
0.25 0.23 0.22 0.23 0.18 0.13
HClOh was used to obtain acid pH values. Measured before irradiation. The pH of the 3 M solution is an extrapolated value from a pH vs. [H’ 1 plot. 0.009[4-pH (measured)] for pH -Willson. ~t al,*X (26) K. Sehested, 0. L. Rasmussen. and H. Fricke, J Phys. Chern.. 72, 626 (1968) (27) W. A . Eaton, P. George, and G. I. H. Hanania. J. Phys. Chem., 71. 2016 (1967). (28) R . L. Willson, C. L. Greenstock. G. E. Adams. R. Wageman, and L. M. Dorfman. In! J Radial. Phys. Chem.. 3, 211 (1971).
The Journal of Physical Chemistry, Vol. 76. No. 25. 1972
Dov Zekavi and Joseph Rabani
370
converted into peroxy radicals, HO2 and 0 2 - . H82 and 0 2 are in equilibrium, determined by the pK of € 4 0 2 , which is 4.8.24 Neither HzOa nor 0 2 - is reactive toward ferrocyanide under our conditions.?J6 We have confirmed this by pulse irradiations of M ferrocyanide in oxygenated neutral solutions and 0.1 M ferrocyanide in oxygenated slightly basic (pH 9.6) solutions. After reaction 2 has been completed, no further change in the optical absorption has been noticed (at 420 nm) up to 10 msec in the neutral solutions. I : o i I sec) was obIn the basic solutions, a partial decay ( t 1 / 2 04L served, which can be attributed to the reaction of ferricyanide with 0 2 - (this will be discussed later). In conclusion, under our conditions, the oxidation of ferrocyanide uia / reaction 5 is well separated in time from other reactions I L 0 I 2 3 4 5 6 of ferri- and ferrocyanide. PH We have examined the spectrum of the product of reacFigure 2. The dependence of the apparent k ( ~ ~ + f ~ ~tion ~ 5~. The ~ ~ratio ~ D5/Dz ~ i d (where ~ ) D2 is the optical density a t on t h e pH (HCIQ4). T h e experimental results are those of Table of reaction 2 before any oxidation by MO2 radicals the end I : 0 , oxygen or air saturated solutions; A , N20 saturated soluhad begun and 0 5 is the optical density a t the end of reactions; 0 ,deaerated (Ar saturated) solutions; a, calculated curve M ferrocyanide solutions tion 5 ) , determined in 2.5 X for the apparent k(oH+ferrocyan,de); b, the fraction of ferrocyanide present as Fe(CN)e4--;c, the fraction of ferrocyanide in acid range, did not change with the wavelength in the present as HFe(CN)63- ; d , the fraction of ferrocyanide present range 400-500 nm. No absorbance had been noticed in the as H2Fe(CN)$-. range 500-700 nm. This indicates that ferricyanide is the only product of reaction 5 . Association of ferrocyanide with positive ions to form ion The kinetic results are given in Table HI. Values of the pairs has a small ef€ect on k2. Some results are presented in overall reaction rate constant of peroxy radica!s were deterTable 11.In the Li-t solutions, about 50% of the ferrocyanide mined from plots of In (Us - &) us. t, which showed linear is present as ion pairs. In all other solutions there is practidependence in all the experiments (Figure 3). In calculating cally a full association with one i0n.~9The ion pairs are by h g , corrections have been made for reactions 12a and 12b as about 10% less reactive toward OH radicals as compared well as corrections for impurity effects on I 3 0 2 and 0 2 - , with free ferrocyanide ions. based on an empirical rate constant (from blank experiRabani and Mathesong reported pKoH = 11.9 j=0.2 (KoH ments) for a presumed reaction 13. is the equilibrium constant of OH $ 0 - H+).This value HO2 HOz H202 f 0 2 ( h -.. 0.76 X 106 M - 1 sec-1) is based on h2 = 1.07 X M-1 sec-1. Using the revised value k 2 = 1.25 X 1010 M-1 sec-1 and taking (124 h((OH NaFe(CN),3-)) = 1.15 X 101OM-lsec-l,pKo~= HOi ~02- H O ~ -+02 ( h = 8.5 x 107 M - 1 sec-l) 11.85 results. (12b) 2, T h e Reaction of HO2 Radicals with Ferrocyanide in Acid Solutions. Adams, Boag, and Michael? reported h5 = ‘“02’’ X P (13) 1.64 X lo5 M - I sec-l in 1Q-2N HzSO4 oxygenated soluThe contributions of reactions 12a, 12b, and 13 increase tions and assumed that the reaction product was ferricyawith pH. The relative amount of these reactions, in comnide. We have irradiated oxygenated solutions of ferrocyaparison with reaction 5 is indicated by the ratio ( 0 5 - D z ) / nide in the pH range 0.46 to 4.37 (HC104 or HzSO4). In 0 2 . When reactions 12a, 12b, and 13 can be neglected, this these solutions the OH radicals react very rapidly with ferratio equals to G(peroxy radicals)/G(OH) = (G, -k G H ) / rocyanide (reaction 21, while the eaq- and H radicals are GOH = 1.23 a t low [ferrocyanide]. Table 111 shows that the limiting ratio is approached below pH 2.5 (HC104). The TABLE II: The Effect of ion Pair Formation on ratio of the experimental ( 0 5 - Dz)/Dz divided by 1.23 is k(oH+ferrocyanl&) in Neutral Solutionsa approximately equal to the ratio “k5”/hexpti, where “ k 5 ” is the apparent pseudo-first-order rate conslant of the reaction of the total peroxy radicals (H02 + 0 2 - 1 with ferrocyaAdditive M ferrocyanide the value nide. In solutions containing G(peroxy radicals)/G(OH) = 1.13 was used in the correcNone 0.042 1.25 tions for reactions 12a, 12b, and 13. The ratio (G, f G H ) / 0.05 M Li2S04 0.042 1.28 GOHis expected to decrease as the ferrocyanide concentra0.5 M Li2S04 0.042 1.21 tion increases due to the increase in GOH3b 0.5 M NaC104 0.043 1.15 The values of k5 were derived with the aid of eq IV, in 0.05 M KzSOd 0.041 1.16 H+. which K is the equilibrium constant ofH02 s 0 2 - -I0.5 M K2SOa 0.043 1.04 0.05 M C S ~ S O ~ 0.042 1.15 r
-
+
+
-
+
+
+
0.5 M CszSBr,
0.041
1.18
0.025 M MgSCIa
0.042
1.10
0.025 M Ca(C104)~ 0.040 1.12 0.025 M BaGi,0.041 1.14 a T h e solutions were saturated with N2O and contained 1 X M potassium ferrocyanide. All t h e results are normalized to the same dose. Corrected for t h e recombination of OH radicals (reaction 9 ) . The Journal of Physical Chemistry, Vol. 75, No. 25, 7972
-
The results of Table 111 show that the presence of ferricyanide in the solutions did not affect the rate of reaction 5 . (29) “Stability Constants,” The Chemical Society, London, 1957, 1964, 1971.
3707
Reactions of OH, HOp, arid 0 2 - Radicals Experiments in the presence of 10--2M formate and M ferricyanide a t pH 2 (not included in Table 111) showed no reduction of ferricyanide due to HO2 within experimental error. From these experiments an upper limit ) M - l sec-1 can be calculated. h((HO2 + F ~ ( C N ) G ~
