3736
JOSEPH RABANIAND SIGURD 0. NIELSEN
Absorption Spectrum and Decay Kinetics of 0,- and HO, in Aqueous Solutions by Pulse Radiolysis1
by Joseph Rabani Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel
and Sigurd 0. Nielsen Department of Chemistry, Danish Atomic Energy Commission, Research Establishment, Rial, Denmark (Received February 18, 1969
+
The absorption spectrum and decay kinetics of 02- radical ions have been investigated in H20zand H2 O2 solutions by the method of pulse radiolysis. The peak absorption at 2400-2450 A has an extinction coefficient of 1970 ~4f-lcm-l. In the pH range -5 to 9.7, optical absorption assigned to 02- was observed. The decay of 02- absorption is consistent with ~ ( H o ~ + o = ~ - 7.9 ) X lor M-'sec-l, and I C ( O ~ - + O ~ - ) < lo6 IM-l sec-l. I n the pH range below 5, absorption due to HO2 was also observed. The extinction coefficient of HO2 at 2400 A was measured and equals 1150 A4-I crn-l. Using both our own data and those of Bielski and Schwarz recently reported results, we could account for both sets of data with ~ ( H o ~ + H o=~ )6.7 X lo5 M-l sec-l. Both kinetic and spectroscopic results indicatethat the pH of HOZradicals may be slightly higher than the previously reported value of 4.5, namely 4.8.
Introduction The absorption spectra and decay kinetics of H 0 2 and 02-have been investigated previously. Czapski and Dorfman2 reported optical absorptions due to HOzand 02-,in pulse radiolysis of acid and near neutral solutions, respectively. In alkaline solutions they reported a relatively long-living optical absorption with a spectrum indistinguishable from 02-but with lifetimes and decay kinetics that were different from those expected for 02-. This absorption was attributed to 02- or OZ2-. Adams, Boag, and Michaela supported the suggestion made before by Baxendale4 that 02decays away by the reaction with HOz, except at relatively low or relatively high pH's, where radicals of the same type interact. (H02with H 0 2or Oz- with OZ-at low and high pH's, respectively.) Gordon, Hart, and Thomass investigated the optical absorption of HOZand the rate of formation of HOz from hydrogen atoms and 02.Bielski and Schwarzs investigated the decay of HOz and 02-in the acid pH range. Despite the extensive work done on HOz and 0 2 - , many features of the system remain unsolved. There is no agreement between different authors concerning rate constants, extinction coefficients, and ratios of rate constants to extinction coefficients in either acid or neutral p H ranges. The situation is even more complex in the alkaline pH range, where new intermediates have been invoked to explain the systemq2 The main purpose of this work was t o investigate the spectrum and decay kinetics of 02-,particularly in near neutral and slightly alkaline solutions. We used solutions of H2 and Oz, to convert all the radical species obtained in the pulse-irradiated solutions into The Journa2 o j Physical Chemistry
HO2 and 0 2 - within a few microseconds. Our system has the advantage of being relatively simple, because reactions due to OH, 0- or to intermediates formed from OH and 0- (other than H 0 2 or 0,-) can be neglected. Possible complications due to such reactions are thus avoided. I n some experiments H202 solutions were used as a source of 02- radical ions. The spectrum of 02-in such solutions was identical O2 solutions, with the spectrum obtained in t,he H2 as will be shown later. This spectrum, together with revised extinction coefficients of 02- and our new kinetic data have made it possible for us to present a reaction mechanism which accounts for our observations in the whole p H range studied from 2.0 to 9.7, without the need to invoke other light-absorbing species other than HOZand 02-.
+
Experimental Section The linear accelerator at Ris@was used as an electron pulse source. The pulsing technique, syringe filling, pulsed Xe lamp, the optical and electronic detection system, and pressure cell were described previously.' A 7.1 cm long cell was used for all the H2 (1) The experimental part of this work was carried out at Ris6 under the auspices of the Danish Atomic Energy Commission and the Danish-Swedish pulse radiolysis project operated jointly with AB Atmenergi, Sweden. (2) G.Czapski and L. M. Dorfman, J . Phys. Chem., 68, 1169 (1964). (3) G.E. Adams, J. W. Boag, and B. D . Michael, Proc. Roy. SOC., A289,321 (1965). (4) J. H . Baxendale, Radiat. Res., 17, 312 (1962). (5) 8. Gordon, E.J. Hart, and J. K. Thomas, J . Phys. Chem., 68,1262 (1964). (6) B. H. J. Bielski and H. A. Schwarz, ibid., 72, 3836 (1968).
3737
ABSORPTIONSPECTRUM AND DECAY KINETICS OF 02AND HOz
+
O2experiments. A 2.54 cm long cell with an optical path of 5.08 cm was used for the experiments with H202solutions. H2 was purified as described before.' The solution was introduced into the cell under an 0 2 atmosphere, using a continuous flow of O2 through the cell. Special precautions were taken while adding the high-pressure H2 (up to 50 atm) to avoid compression of more 0 2 from the filling system. This was done by pumping the filling system before the H2was added. The actual pressure of O2in the pressure cell after equilibration with 50 atm of H2was 1 atm. H202,Analar grade, was used without further purification. I n some experiments, H202 was prepared by pulsing a neutral solution saturated with N2O in the absence of an irradiated gas phase to avoid contamination from possible radiation products in the gas phase. For this purpose, syringes containing water were saturated with N20. After removing the gas phase, the solutions were cooled to about 0" in order to increase gas solubility and prevent irrradiation gas products from escaping out of the solution. Electron pulses were given, until the total irradiation dose was about 1 Mrad. During the irradiation, the electron beam was scanned so that all the volume of the syringes was homogeneously irradiated. After the end of the irradiation, we usually found one small bubble of 1-2 mm diam in each syringe due to gaseous products formed by the irradiation (Hz, 0 2 , and Nz). These bubbles were probably not present during the irradiation, but were slowly formed and increased with postirradiation time. Except for gaseous products which can easily be removed, our "self-made" H20zsolutions should be of extremely high purity. The pulse radiolytic results obtained in these solutions were identical with those obtained in solutions of Analar grade H202. The concentration of HZ02 obtained using a total dose of about 1 Mrad was about 1 mA!f. Oxygen, 99.7% pure, was normally used after passing through an NaOH solution and through triply distilled water. I n some cases, O2 was prepared by the reaction of Ce4+ with H202. All other chemicals used were of analytical grade and used without further purification. Stock solutions of 20 M NaOH were used for the preparation of alkaline solutions. The temperature was about 24" in all the experiments. All the solutions containing H202were analy~edto determine the exact concentration of H202 at the time of the pulse. Occasional measurements of optical transmission were made at 4300 especially at pH 13, in order t o find the fraction of oxidizing radicals which reacted with 0 2 (formed by the decomposition of H202) rather than with H202. The method for analysis of HzO2 and of correcting for 08-formation were described previously. Since the method used for pressure cell filling is time consuming, we used to irradiate each pressurized solu-
X
solutions, Figure 1. Spectrum of Oz- in NzO-saturated H~OZ 40 psec after the electron pulse: 0, 0 , pH 12.98 [HzOZ]= 3 X 10-4 M , made by irradiation of NzO solutions; pulse duration, 1 psec; total ([OH] [O-1) formed by the electron pulse was 2.20 x 10-6 M; dotted line, experimental optical density corrected for a small amount (less than 10%) of 0 and OH which reacted with 02 and formed OS-. The OSabsorption was disregarded. 0 , calculated absorption decreased due to HzOz disappearance; G( -HaOz) = 4.5 was used; 0, corrected spectrum of 02-. Use the scale to the right that represents the extinction coefficient of 0 2 - , taking G(O,-) = 5.4. x , corrected spectrum of 0 2 - (use scale to the left) a t pH 12.98, using [HzOZ]= 8.3 X 10-4 M (Analar grade), [O-1) = 4.04 X M. saturated NzO, at 1 atm, ([OH] D values multiplied by 4.65. Pulse duration was 0.7 psec. The electron beam was scattered by a thin aluminum plate. The normalization factor of 4.65 is somewhat lower than the dose factor of 5. This agrees with G(OZ-)= 5.8 in these solutions compared with G(Oz-) = 5.4 estimated for the results with 3 X 10-4 M HzOz. See further explanation in the text.
+
+
tion with several electron pulses (up to 25 pulses of 1-psec duration or their equivalent). Each solution was checked for the effect of accumulated radiation products by comparison of the kinetics and initial absorptions of the first and last pulses. No radiation product effect could ever be detected, and the last (7) (a) P. Pagsberg, H. Christensen, J. Rabani, G. Nilsson, J. Fenger, and s. 0. Nielsen, J . Phys. Chem., 73, 1029 (1969); (b) H. Christensen, G. Nilsson, P. Pagsberg, and S. 0. Nielsen, Rev. Sci. Instrum., 40,786 (1969). (8) We wish to thank Mr. P. Pagsberg for suggesting this method for the preparation of H2Oa solutions. (9) J. Rabani, Advances in Chemistry Series, No. 81, American Chemical Society, Washington, D. C., 1908, p 131, Volume 75,Number 11 N O V E ~ 1069 IET
3738
JOSEPH RABANIAND SIGURD 0. NIELSEN
I
I
I
I
I
.15-
“OH”
I 1
- ISOO+ Z W
I->-
I
k!
5i
w z a
9
-
+ “H202”
+ H2O + H+
+0 2 -
(2) We use the quotation notation to indicate that the reaction may involve species equivalent to OH ( i e . , 0-) and H20z(i-e.,HOz-). At p H 13, H atoms are quickly converted to eaq- according to reaction 3 . 1 2
LL II W
+ OH +e,-(ka
= 2 X 107M-1sec-1),
(3) 4: Therefore, at pH 13, if no radical radical reactions 0 \ Iue ((tOH7, + “OH” and “OH” u 0,-) were involved, Q z 0 I= Ge GH GOH = 5.8, and G(-H,O2) = G(O2-) x .05-500 Ge GH GOH- G H ~ = o ~5.1. These conditions are \ h valid for the experiments described in Figure 1 (crosses). A Other experiments represented in Figure 1 were carried out with a relatively low H202 concentration and a higher (X5) pulse intensity. Under such conditions 2000 2200 2400 2600 2800 3000 we expect some radical-radical reactions to take place, especially reaction 4 (2k4/2 is of the order of lo9 M-1 Figure 2. Spectrum of 02-in NzO-saturated HZOZsolutions: sec-’ a t pH 131a). 0, corrected spectrum of Oz-. (Use scale to the right); l-psec pulse with a dose of 2.38 X lozoeV/l. per pulse. [H~OZ] ‘LOH” + “OH>7+I(H20211 (4) = 6.4 x 10-4 M. (HzOzmade by irradiation of NzO solutions); pH 11.50. 0 , calculated decrease of absorption For these experiments with low Hz02 concentration and due to HnOz disappearance. (B(-HzOz) = 4.52 was used); high pulse intensity, G(02-) = 5.4 and G(-H202) = pH 11.50. A, corrected spectrum of Ot-, (use scale to the 4.6 were estimated 40 psec after the pulse. The ex(Analar grade), 0.7 psec left), using 8.3 X 10-4 M H%O% cellent agreement (shown in Figure 1) between both pulses, with an intensity of 4.75 X 10l8eV/1. per pulse (with scatter plate); pH 11.48, calculated D values multiplied sets of results at pH 13 obtained, at two pulse intensiby 4.13. The normalization factor, 4.13, was used instead of ties differing by a factor of 5 , justifies the neglect of See the dose factor of 5 due to differences in ~ ( O Z -values. ) radical-radical reactions other than reaction 4 such as text for further explanation. X, corrected spectrum of 02.10
\
-I
H
0
-looo;
I
+ +
(use scale to the left), at pH 13 taken from Figure 1. The corrected D values were multiplied by 0.76. The factor of 0.76 is due in part to differences in G ( O Z - ) . There is also probably an insignificant difference of about 10% in the absorption of 02- which is higher a t p H 13 than at pH 11.5. Note: the scales on the right of Figures 1 and 2 give the extinction coefficient for the corrected 02-spectrum.
pulse always gave results identical with the first pulse within experimental error.
Results (a) The Absorption Spectrum of 02-Radical Ions in H202 Solutions. I n Figures 1 and 2 we present the optical absorption obtained in H202solutions, saturated with 1 atm of NzO, at pH 11.50 and 12.98. The same results were obtained no matter whether H202selfprepared by irradiation or Analar grade H202 was used. Under our conditions, reaction 1 esq-
+ N 2 0 +N2 + 0-
+ N2O
N2
+ OH + OH-
suggested first by Dainton and Peterson,lo leads to the formation of OH or 0- in much less than 1 psec. The evidence for the quick formation of OH (or 0-) is summarized in a previous paper.” OH (or 0-) radicals react quickly with hydrogen peroxide and form 0 2 radical ions by reaction 2. Xhe Journal of Physical Chemistry
+
02-
--+
0 2
+ OH-(kj
+
=
1 X 1010 M-1 sec-’)
(5)
At p H 11.5 corrections for competing reactions are more important. Using the set of low-intensity experiments (Figure 2, triangles) , we estimated G(O,-) = 5.6 and G(-H20z) = 5.5. The relatively high G(-H20z) is due to the reaction of H atoms with HzO2 according to reaction 6. H
+
H202
+HzO
+ OH(ks = 1 X 108 M -1 sec-l)
14a
(6)
For the set of data with higher intensities (Figure 2), G ( 0 2 - ) = 4.62 and G(-Hz02) = 4.52 were computed, using the above given mechanism, and the following additional reactions and rate Constants : k(“OH”+“HaOa”) 9 = 2.2 x lo@,2 k ( H + H ) 1.6 x 2k(1’OH”+“OH”)1S = 1.1 x 10’0, k(H+OH)140 = 2 X 1Olo M-l sec-’.
(1)
or
e,-
OH
+ +
(10) F. S. Dainton and D. B. Peterson, Nature, 186,878 (1960). (11) J. Rabani, “Radiation Chemistry of Aqueous Systems,” Stein, Ed., The Weizmann Science Press of Israel, Jerusalem, 1968, p 229. (12) (a) J. Rabani, Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 242; (b) G. Stein, “Hydrogen-Bonded Solvent Systems,” A. K. Covington and P. Jones, Ed., Taylor and Francis LCd., London, 1968,p 87. (13) J. Rabani and M. S. Matheson, J . Phya. Chem., 70, 761 (1966). (14) (a) J. P. Sweet and J. K. Thomas, {bid., 68, 1363 (1964); (b) S. 0.Nielsen, P. Pagsberg, J. Rabani, H. Christensen, and G. Nilsson, Chem. Commun., 1523, (1968); (c) H. A. Schwarz, Radiat. Res. Suppl., 4, 89 (1964).
3739
ABSORPTIONSPECTRUM AND DECAY KINETICSOF 02-AND HOZ Since the radical combination was relatively small, the error in the computed G(O2-) value is not large, perhaps not more than 5%. The extinction coefficient of 02-,eoz-, was obtained with the aid of the ferrocyanide dosimeter (lo-* M ferrocyanide, NzO saturated, irradiated at the same pulse intensities and pH's as the HzOz solutions; initial D values were taken for the dosimetry and were assumed not to be affected by reduction of ferricyanide by H). We can assume that the primary yields in the HzOz solutions and dosimeter solutions were equal. The absolute values of the yields are not important for the evaluation of eo2-, because only the ratios of G values in HzOzand in the dosimeter solutions were used. 0 2 ( b ) The Absorption Spectrum of 0 2 - in H2 Xolutions. When appropriate concentrations of H2 and 0 2 together are present in an aqueous solution, H atoms and eaq- react with Oz to form HOz and 0 2 - , respectively, and OH radicals react with Hz to form H. The relative concentrations of HOz and 02-are determined by the pH of the solutions, the pK of H 0 2 reported as 4.5.'5-1*
+
+ H+ OH + Hz eaq-
0 2
0 2
+02-(k, = 2 X 10'O
M-' sec-')I6
(7)
-+ HOz(k8 = 2 X 1O1O M-' ~ e c - ' ) ~ (8)
HzO
+ H(ks
=
lo7M-'
4.5 X
sec-')'7
(9) 2
Under our conditions, using one atmosphere of 0 and 45 atm of Hz, reactions 7, 8, and 9 are practically the only reactions by which the radical species OH, H, and eaq- disappear. The result of reactions 7, 8, and 9 is the conversion of all the radicals formed by the HOZ) = GR radiation into 0 2 - and HOz. G(OzG, GOH. 'The optical density (D) scale in Figure 3 can be easily converted into an extinction coefficient scale. This is done by dividing D of Figure 3, curve a, with C X 1 -- 8.2 X (for X 7.1 = 5.8 X pH 9.7) and dividing D of Figure 3, curve c, with C X 1 = 6.8 X loF6 X 7.1 = 4.8 X Calculated extinction coefficients of 0 2 - at 2400, 2450 (the peak), and 2600 A are given in Table I.
+
+ +
Table I: Extinction Coefficients of 2600 A System
PH
Near neutral* Near neutral" 7.12 (phosphate) 9 . 7 (NaOH) 11.5 (NaOH) 13 (NaOH)
02-
-Extinction coefficients ata2400 ii 2450 8, 26008,
+ Oz + Hz + H P + Oz HZ HP
1860
0 1
1990
0 2
1930 2000 1710 2120
HzOz HzOz
a I n units of M-1 cm-1. ~ K H O=Z4.80.
a t 2400, 2450, and
* Taking ~
1860 2020 1930 2000 1720 2150
1550 1760 1650 1720 1600 1760
K H= o 4.45. ~ ' Taking
.15
I
I
I
I
2G03
2800
I
e
0 2ccO
I
2200
l------l 2400
I
h(A)
3m
+
Figure 3. Spectrum of 0%obtained in H Z 0 2 solutions, 10 psec after the electron pulse; 1 atm of OZand 45 atm of Hz; light path, 7.1 om; total ([HOZ] [OZ-1) = 8.2 X M based on initial G(ferricyanide) = 5.8 in neutral, Nz0 saturated (at 1 atm) M ferrocyanide, and G([H02] [OZ-]) = 5.8 in the H Z 0 2 systems; curves b, c, and d coincide above 2600 A. (a) Spectrum of 0 2 - at pH 9.7 and pH 7.13 (pH is before the 1-psec pulse). The result at 2100 A was corrected for disappearance of OH- (D was raised by 0.007 unit); 0, pH 9.7; 0 , pH 7.13, phosphate buffer (see Table 11). (b) Spectrum a t near neutral conditions of the equilibrium mixture of 0 2 - and HOZ(was neutral before the ~ 4.45, the pH of this solution is electron pulse). If P K H O= 5.17, [02-] = 6.8 X lo-' M and [HOz] = 1.4 X M. If ~ K H =o 4.80, ~ [OZ-] = 6.0 X M and [HOz] = 2.2 X M . (c) Spectrum of 6.8 X M 0 2 - at p H 5.17 obtained from (b) by subtracting the absorption of HOZ. (d) Spectrum of 6.0 X lo-' M 0 2 - a t pH 5.22 obtained using , 4.8. (e) The absorption of the results of (b) taking ~ K E o= M HOZ.?~This absorption was subtracted from 1.4 X curve b t o give curve c.
+
+
+
(c) Kinetics between p H W and p H 9.7. A typical oscilloscope trace, showing the formation and decay of 02-is shown in Figure 4. In the upper trace (fast sweep) we observe the increase of optical density with a half-life of about 1 psec. This is longer than the halflife of about 0.5 psec expected for reaction 9 under our conditions. We suppose that this observation is due to the dissociation of HOZ to give 02-. 02- has a greater absorption than has H 0 2 at 2400 A (see below). The half-life of the spontaneous ionization of HOz may well be about 1psec. At pH 2, under similar conditions to Figure 4, the building up of optical density is very fast and is lacking the tail at the end with t l / , S 1 psec which we see in Figure 4. This is expected below the pK of HOz. As no appreciable dissociation of HOa to form 02- takes place a t pH 2, no tail is observed in the buildup of optical density. Experiments a t other (15) K.Sehested, 0.L. Rasmussen, and H. Fricke, J. Phys. Chem., 72,626 (1968). (16) S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, J.Amer. Chem. SOC.,85, 1375 (1963). (17) H. A.Schwarz, J . Phys. Chem., 66,255 (1962). (18) J. Rabani, W.A. Mulac, and M. S. Matheson, ibid., 69, 53 (1965). Volume 75,Number 11
November 1969
3740
JOSEPH RABANIAND SIQURD 0. NIELSEN
IO 0 % light
TIME
-
Figure 4. Formation and decay of 0 2 - at 2400 A. Both traces were recorded with the same 1-psec pulse; 42 atm of I& and 1 atm of 0 2 ; pH (before pulse) = 6.65 (3 X lo-* M NaHIP04 and 1 X M Na2HP04); light path, 7.1 em. 01
pH values than 2 and 6.5, using various buffers, HC104, and NaOH for the control of pH are in agreement with above considerations. The decay of optical density was always observed to be much slower than its formation and could easily be separated from the formation and analyzed. A plateau is observed in Figure 4, upper trace, after about 5 psec or more. I n all the kinetic work described here, the plateau optical densities of the traces taken with 10 psec/div sweep rates were equal to the initial optical densities of the decay traces recorded in the milliseconds to seconds time ranges (e.g., Figure 4, lower trace). I n addition, the optical absorption appeared to decay away to zero in one stage. We conclude that one reaction or several reactions occurring simultaneously are responsible for the decay of the optical density. No conversion of the initially formed equilibrium mixture of 0 2 - and HOz into other intermediates could be observed by means of the time dependency of the optical density. These comments and conclusions are valid for the entire pH ranges investigated (pH 2-9.7). The same features were found for other wavelengths in the range 2100-3000 A, whenever a full spectrum has been taken. At 2000 we observed sometimes different kinetics in the 10 psec/div pictures. This could be explained by the optical absorption of OH-, but a detailed investigation of this observation is beyond the scope of this paper. The features described above and demonstrated by Figure 4 are valid for all the experiments in which decay kinetics were observed, irrespective of whether the decay was first or second order, and independent of the pulse intensity. The decay kinetics were usually The Journal of Physical Chemistry
0
I 10
I
I
20
30
I 40
50
t (msec)
Figure 5 . Second-order test of the decay of 02-. Conditions as in Figure 4. Intensity varied by changing the pulse duration from -0.5 to ~3 psec.
intermediate between second and first order. Three different pulse intensities were always used. The relative contribution of the first order component decreased with increasing dose per pulse. Typical second-order plots, i.e., 1/D vs. t plots, are shown in Figure 5. The pulse intensities used in Figure 4 are also typical for all other pH’s used. It can be seen that deviationa from a straight line occur more rapidly at lower pulse intensities. I n addition, the initial slopes of the 1/D vs. t curves decrease with increasing intensity. However, by varying the pulse intensity almost tenfold, the initial slope varied by only a factor of about 2. This dependency on pulse intensity is typical for the case when both first- and second-order reactions take place simultaneously. The relative contribution of the first-order reaction depended not only on the pulse intensity but also on pH. Thus, sometimes a tenfold change in intensity gave up t o about fourfold change in initial d(l/D)/dt. I n general, the higher the pH, the greater was the relative contribution of the first-order component. With a single solution or with identical solutions irradiated on the same day, the reproducibility was excellent. However, sometimes a solution known to give mainly second-order kinetics showed first-order kinetics. Whenever this happened, the first-order decay was much faster than the mixed second- and
3741
ABSORPTION SPECTRUM AND DECAY KINETICS OF 02AND HOz 110
2
!
( Initial)
GO
20
n
Figure 6. Demonstration of the separation into second- and first-order components (see text): X , lower and left scales; 0 , upper and left scales; 0, lower and right scales.
first-order decays. We have found that by cleaning the valves in the pressurized H2 system from traces of brass powder, slowing down the rate of H2flow through the molecular sieve, and extracting the membrane filter with water, the decay of optical density slowed down and was found to possess the relatively small first-order component mentioned above. The cleaning of O2did not have any effect on the results. The same results were obtained with 0 2 taken directly from the gas cylinder as with O2 that had first passed through a bubbler containing NaOH and then through water, or with O2 made in a glass container from solutions of Ce4+and Hz02. On the basis of these results, indicating that impurities were responsible for the first-order reactions, we have disregarded all the experiments in which the decay, of D was only first order with no second-order component. It can be shown that when second-order and firstorder reactions take place simultaneously, e.g. A+A+p'
(10)
A-P
(11)
and when only A absorbs light, eq 12 describes the dependency of the slopes of 1/D vs. t curves on D. d(l/D)/dt =
x
~~
