J. Phys. Chem. 1982, 86,2066-2069
2006
Ultraviolet Spectrum and Decay of the Ozonide Ion Radical, 03-, in Strong Alkaline Solution K. Sehested," J. Holcman, E. BJergbakke,and Edwln J. Hart' Accelerator Department, Riser National Laboratory, DK 4000 RoskiM, Denmark (Received: October 7, 1981, I n Final Form: January 11, 1982)
Pulse radiolytic experiments in strong alkaline solution (pH 13-14) on the formation and the decay of the ozonide ion radical, 03-,are reported. The high-pressure cell technique was used applying 4 MPa of nitrous oxide and 0.1 MPa of oxygen. The spectrum of 03-in the UV was measured and the decay product was identified as the peroxy radical ion, 02-.A complete mechanism for the decay of 03-is based on the following reactions: 03-+ 0- 202- and 02+ 0- + H 2 0 O2 + 20H-. A computer simulation of the high-pressure system as well as the atmospheric-pressure oxygenated system supports this mechanism and yields a rate constant, k(oj+o-, of (7.0 f 1.0) X lo8 dm3mol-' s?. As a consequence of our mechanism, a k(O,-+O-Tof(6.0 f 1.0) X lo8 dm3mol-' s-l is also derived from the computations.
-
-
Introduction The ozonide ion radical, OL, has been studied very extensively by means of pulse radiolysis and flash photolysis as one of the key species in irradiated oxygenated alkaline water. The results were summarized about a decade 8go.l~~ The absorption spectrum of 03-is well-known above 300 nm. However, there is still some controversy about its absorption in the UV. Some claim an intense absorption below 300 nm, whereas othersw claim that 03has no absorption in the UV. The decay of 0; is not very well understood, but first-, second-, and mixed first- and second-order decay mechanisms have been postulated. As products of the 03- decay, species such as 03, ,':O 042-, and 02have been suggested, but a satisfactory mechanism has not yet been proposed or proved. The present investigation has been undertaken by using the pulse radiolytic pressure cell technique,l0applying high pressure of N20 in order to form the ozonide ion radical, 03-, almost exclusively. By this means we were able to separate the formation of 03-from its decay and thereby to identify the decay products. Experimental Section Pulse Radiolysis. The 10-MeV HRC linear accelerator at Riser with a pulse length of 1ps and a dose varying from 1 to 10 krd pulse-' was used throughout the work. The optical detection system has been described previous1y.l' It consists of a 150-W Varian high-pressure xenon lamp, a Perkin-Elmer double quartz prism monochromator with a 1P28 photomultiplier. The data were recorded on a Nicolet Explorer I11 digital storage oscilloscope and stored on disks. The data treatment was performed by using an on-line PDP8 computer whereas the simulation of the reactions was done on the Burroughs 6700 at Rise, with a program developed for homogeneous kinetics on the basis of the algorithm DIFSUB by E. A. Gear.12 The experiments were done in the high-temperature pressure cell described elsewhere.'O In this study only high pressures were used, and the temperature was always ambient temperature, -20 "C. The pressure cell consists of a 20-mm thick stainless-steel vessel fitted with optical Suprasil windows. Inside the pulse radiolysis cell, a 25-mm long Suprasil cell with optical windows is situated. A magnetic stirrer is hung in a gallows in the opening at the top of the cell so that it just touches the surface of the solution when rotating. This arrangement is necessary for 2115 H a r t Road, P o r t Angeles, WA 98362. 0022-365418212086-2066$0 1.2510
equilibrating the solution with the appropriate gas used at high pressure. The cell can be filled and emptied by a syringe through a capillary fitted with a standard taper joint. The optical cell fits into the steel vessel with a distance of 0.5 mm to the optical windows of the vessel. The cell is surrounded by water, saturated with the same gas. The equilibration with, e.g., 40 atm (4MPa) of N20 takes 15-20 min of stirring. The light path is 25 mm perpendicular to the electron beam from the accelerator. The pulse radiolytic cell is filled by an oxygenated solution against a moderate flow of oxygen. The cap of the steel vessel is closed and the pressure is equilibrated to 1 atm (0.1 MPa). The cell is then pressurized to 40 atm (4 MPa) of N20and the magnetic stirrer is operated for 15-20 min. After the first electron pulse a small amount of hydrogen peroxide is formed, but the absorption spectrum of 03-is not altered by several electron pulses of a few krd pulse-'. The experiments for obtaining the spectrum of 03-were repeated several times with fresh solutions, and the results averaged. For kinetic measurements the first pulse was always used, but even for the kinetics a few pulses did not make any significant difference. Solutions. All solutions were prepared from triply distilled water. The water was saturated with oxygen at neutral pH and the sodium hydroxide was added just before closing the syringe to minimize the content of carbonate. One millimole of carbonate was also shown to have no significant influence on the spectrum or the kinetics at these pHs. N20 and O2were used without further purification. The dosimetry was performed with the hexacyanoferrate(I1) dosimeter13using G = 5.4 and td2,, = 1000 dm3 mol-' cm-'. (1) B. H. J. Bielski and J. M. Gebicki, 'Advances in Radiation Chemistry", Vol. 2, M. Burton and J. L. Magee, Eds., Wiley-Interscience, 1970, p 248. (2) G. Czapski, Annu. Reu. Phys. Chem., 22, 171 (1971). (3) G. E. Adams, J. W. Boag, and B. D. Michael, Nature (London), 205, 4974 (1965). (4) W. D. Felix, B. L. Gall, and L. M. Dorfman, J. Phys. Chem., 71, 384 (1967). (5) V. R. Landi and L. J. Heidt, J . Phys. Chem., 73, 2361 (1969). (6)G. Czapski and L. M. Dorfman, J . Phys. Chem., 68, 1169 (1964). (7) D. Behar and G. Czapski, Zsr. J . Chem., 6, 43 (1968). (8) J. Rabani, Ado. Chem. Ser., No. 81, 131 (1968). (9) D. Behar and G. Czapski, Isr. J. Chem., 8, 669 (1970). (10) H. Christensen and K. Sehested, Radiat. Phys. Chem., 16, 183 (1979). (11) H. C. Christensen, G. Nilsson, P. Pagsberg, and S. 0. Nielsen, Reu. Sci. Instrum., 40, 786 (1969). (12) C. W . Gear, Commun. ACM, 14, 185-90 (1971), Algorithm 407, DIFSUB, for solutions of ordinary differential equations. (13) J. Rabani and M. S. Matheson, J. Phys. Chem., 70, 761 (1966).
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982 2067
UV Spectrum and Decay of 03-
t
O.D. 0,20
I
A
-
r
250nm
0,15
430 n m
L 5 ms 0.10
B 0.05
250nm
0%
0.0.0.0.
0,
350
300
250
450
400
500
55'
h(nm) Figure 1. Optical absorbance of 03-in a solution containing 1.2 X lo3 mol dm3 (0.1 MPa) oxygen, -0.9 mol dm3 (4 MPa) n b w s oxide, and 0.1 md dm3 sodium hydroxide (dose, 5.2 krd; 1-ps pulse; optical path length, 2.5 cm): (0)measured optical densities; (0)corrected for Go*- = 0.55 and GHOi = 0.6.
Results and Discussion Spectrum of Os-.The spectrum of 03-was measured in 0.1 M NaOH with 4 MPa of N 2 0 and 0.1 MPa of O2 (Figure 1). The dose was 5 krd in a 1-ps pulse. The spectrum shows the well-known absorption maximum at 430 nm.14 The absorption below 300 nm is rather small and exhibits a minimum at 280 nm and then increases continuously down to 220 nm, where the absorption is about one-fourth of the maximum absorption at 430 nm. The spectrum was taken about 10 1.19 after the pulse. At pH 13 with 4 MPa of N20 substantially all of the hydrated electrons are converted into 0- radicals. The high concentration of N20 (-0.9 mol dm-3) has an appreciable effect on the radical yield as spur reactions such as eaq- + e, H2 and ea; + OH OH- are lessened by eaq- scavenging by N20. Consequently no attempt was made to measure the extinction coefficient under these conditions. Under the specified conditions the H atoms (G = 0.55) will react principally with oxygen forming 02even though they can react with N20 and OH-: H+02 HOz(Oz-) (1) k = 2 X 1O'O dm3 mol-' s-l (ref 14)
-
-
-
+
-
H N2O OH(0-) + N2 k = 2.5 X lo6 dm3 mol-' s-l (ref 14) H
+ OH-
-
eaq-
+ H2O
k = 1.5 X lo7 dm3 mol-' s-l (ref 14) At pH'13, and a t concentrations of 1.25 X
(2)
53L
I
430 nm
Figure 2. Oscilloscope traces at 250 and 430 nm obtained in pulseirradiated 0.1 mol dm3 sodium hydroxide solutions (dose,10 krd; optical path length, 2.5 cm): (A) Containing 1.2 X lo3 mol dm-3 oxygen and mol dm-3 -0.9 mol dm-3 nitrous oxide; (B) containing 1.2 X oxygen.
Consequently the spectrum of 03-has been corrected for Go,- = 0.55. Another minor correction, which was made, is due to the molecular hydrogen peroxide formed (G = 0.6). Decay of 03-.The decay of the 03-in the pH range 13-14 with 0.1 MPa of O2 and 4 MPa of N 2 0 is followed by a simultaneous buildup in the UV (Figure 2A). After the decay of 03-the resulting spectrum in the UV is that of 0, with an absorption maximum at 240 nm (Figure 3). The lifetime of this absorption is several seconds and is the same as that observed with only oxygen in the solution or with oxygen and 10 MPa of hydrogen instead of N20. Furthermore, the lifetime increases with NaOH concentration. On this basis we conclude that the species remaining after the 03-decay is that of the peroxy radical ion, Of. The yield of Of is a little less than half of the original Os-yield in these high-pressure experiments, but about the same as is obtained in a saturated oxygenated solution. The decay of 03-is neither first nor second order, but complex. We use the following reactions to explain the decay of 03-and the formation of 0 ~ : k4 = 3.3 X lo3 dm3 s-l (ref 15) 03- 0- O2 (4) k-, = 3.0 X lo9 dm3 mol-l (ref 16)
+
(3) mol dm-3
O2and -0.9 mol dm-3 NzO, the kc for formation of 02is about tenfold higher than for the other two reactions. (14)M.Anbar, Farhataziz, and Alberta B . Ross, Natl. Stand. Ref. Data Ser. (US., Natl. Bur. Stand.), No. 61 (1975).
03-+ 0-
- + - +
+ 0HOB- + 002-
[0,2-]
H20
0 2
02-
202-
(5)
20H-
(6)
OH-
k7 = 4.0 X lo8 dm3 mol-' s-l (ref 17)
(7)
The Journal of Physical Chemistry, Vol. 86, No. 7 7, 7982
2068
Sehested et al.
0.0.1
A
t
430 n m
9
1 Y 250nm
O"I
o l
' 220
230
I
I
I
I
I
I
240
250
260
270
280
290
G 0
O
X(nm)
Flgure 3. Absorption spectrum taken 30 ms after pulse, when most of the 03-has decayed in 0.1 mol dm-3 sodium hydroxide solution containing 1.2 X mol dm-3 oxygen and -0.9 mol dm-3 nitrous oxide (dose, 4 krd; optical path length, 2.5 cm).
Reaction 4 is well established and is thought to be the only way in which 03- (as 0-) can react with nonradical sol ~ t e s . ~Reaction J~ 5 has been suggestedls but has also been dismissed because of the lack of a simultaneous buildup of a UV absorption in solutions containing only oxygen.2 We, too, found almost no change in the UV absorption in oxygenated solution (Figure 2B), but this is interpreted merely as a net effect of reactions 5-7, which is fully justified by the computer simulations (see later). In solutions with 0.1 MPa of O2 and 4 MPa of N20, the observed increase in absorption in the UV, at an early stage of the 0, decay (-10% decay), corresponds to the formation of about one molecule of Of for every molecule of 0;. This, we believe, confirms the stoichiometry of reaction 5. Reaction 6 has been postulated,lg but its rate constant has not yet been measured, and it has never been considered in a context of the 03-decay mechanism.1+2Reaction 7 is of only minor importance in the mechanism as long as the concentration of H202is low. Two other reactions have been suggested to play a role in the 03-
-
03-+ 03- product
Os-
-
+ 02-
product
(8) (9)
These reactions were studied by increasing the oxygen concentration under the same dose and pH conditions as previously. It turns out that the lifetime of 03is almost proportional to the oxygen pressure up to 10 MPa of O2 (-0.120 mol dm-3 02). The lifetime of ozonide ions under these conditions was several seconds, and the decay did not conform with the second-order rate law. The half-life of 03-imposes a limit on the rate constants of reactions (15) B. L.Gall and L. M. Dorfman, J.Am. Chem. SOC.,91,2199(1969). (16) Farhataziz and Alberta B. Ross, Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.), No. 59 (1977). (17) H.Christensen, K.Sehested, and H. Corfitzen, J. Phys. Chem., 86, 1588 (1982). (18)G. Czapski, "RadiationChemistry of Aqueous Systems", G. Stein, Ed., Weizmann, Jerusalem, 1968. (19) B. H. J. Bielski and J. M. Gebicki, ref 1, p 243.
O,'
0
L
t
C 430 nm
LYl5zEL D 250nm
0 0
10
20
30
40
50
mS
Figure 4. Calculated optical density traces with experimental points for 03-decay (430 nm) and 02buildup (250 nm) in 0.6 mol dm-3 sodium hydroxide: (A and B) containing 1.2 X mol dm-3 oxygen and -0.9 mol dm-3 nitrous oxide; (C and D)containing 1.2 X loT3 m d dm3 oxygen. Dose, 9.8 krd; optical path length, 1 cm. = 2000 dm3 mol-' cm-', eo2-(a5qnm) = 1900 dm3 mol-' cm , and em)27250nm) = 210 dm3 mol-' cm- were used in the calculations.)
8 and 9, which is k, or k, C lo4 dm3 mol-' s-l. Hence, reactions 8 and 9 do not play any role under our pulse radiolysis conditions. Computer Simulations. The chemical equations make up a system of differential equations which cannot be solved analytically. Handling the kinetics of the 03-decay in the 'classical way has led to the conclusion that the general mechanism for the decay should conform to a mixed first- and second-order mechanism.2 This mechanism in turn requires an impurity, which removes 0- by a first-order process. For the computations a complete reaction scheme for water radiolysis was used. This consists of 44 chemical equations including reactions 1-7. All of these reactions are well established14J5with the exception of reactions 5 and 6, which were treated as parameters. k5 has been determined previously,' but without specifying the product of the reaction. As can be seen from Figure 4,the calculations are in remarkably good agreement with the experimental results. A high-quality fit is obtained in the pH range 13-13.8 for solutions containing 0.1 MPa of O2 and 4 MPa of N20 and for solutions con-
J. Php. Chem. 1982, 86, 2069-2072
TABLE I: G Values for Primary Radicals, Molecular Products, and Ions Used in the Computationsa pH 13.8 pH 13.0 4 MPa
of N,O + 0.1 MPa of 0 ,
e aq
H OH HO; H2
o*-
3.7 0.55 3.7 0.45 0.205 0.02 4.22 0.5
4 MPa
of N,O t 0.1 MPa 0.1 MPa 0.1 MPa of 0 , of 0 , of 0 , 2.8 0.55 3.3 0.45 0.455
0.02
3.32 H' 0.5 OHFor explanation see Appendix.
3.7 0.55 3.4
2.8 0.55 3.0
0.6
0.6
0.205 0.02 4.22 0.5
0.455 0.02 3.32 0.5
+ OH-
pK = 11.9
Appendix G Values of the Primary Radicals at p H 13 and 13.8 Used in the Computations. From the absorption of the ozonide ion radical at 430 nm, we can for a given dose calculate the G value of 0; formed if we use an extinction coefficient of 2000 dm3 m01-l.~ The GO3;= 7.1 and 7.6 at pH 13.0 and 13.8, respectively. The increased yields, compared with usual conditions at normal pressure, originate from spur scavenging of the electron by the high concentration of N20 and by a slight increase in the OH yields at the expense of molecular hydrogen peroxide. The yield of molecular hydrogen from ea; + ea; H2 is assumed to decrease to GH*= 0.2 because of spur scavenging by N20 at solute concentrations of 0.9 mol dm-3. Also the yield of molecular hydrogen peroxide is believed to decrease slightly as a consequence of the fast reaction OH OH- 0- + H 2 0 ( k = 1O'O dm3 mol-' s-') which, together with a fivefold slower recombination of 0- 0(k = lo9 dm3 mol-' s-') compared to OH + OH (k = 5 X lo9 dm3mol-' s-l) at high concentrations of OH-, influences the molecular product diffusing out of the spurs. The molecular hydrogen peroxide yield is assumed to be 0.6 and 0.45 at pH 13.0 and 13.8, respectively, in agreement with the increased yield of 03-in oxygenated solutions at these pHs. The yields of 03-are then accounted for by Gesc = 2.8 + 0.5 = 3.3 and GOH = 2.8 0.2 = 3.0 at pH 13.0 and G O H = 2.8 0.5 = 3.3 at pH 13.8, plus the hydrated electrons and OH radicals from eag-scavenging of the spur by N20 in the reaction ea; + OH OH-. The yield from this last reaction necessary to add up to the full experimental yield of 03-is 0.4 of each radical. This gives a theoretical yield of 03-at pH 13 Go; = G," GJH) GJoH) GOHO + GOH(OH) + GOHle)
-
taining only 0.1 MPa of oxygen a t different doses. In order to avoid reactions of OH radicals from the equilibrium
0-+ H 2 0 F' OH
2069
(10)
which may have a small influence even at pH 13, the parameter study leading to evaluation of the rate constants k, and k6 has been carried out at pH 13.8. (The reactions of the OH radical in alkaline oxygenated water will be a subject of a subsequent paper.) Reaction 6 is only of minor importance in the early stages of the 03-decay in solutions containing 0.1 MPa of O2 and 4 MPa of N20, whereas, in solutions containing only oxygen, reactions 5 and 6 are equally important. It is therefore possible to estimate the rate constants k5 and k, simultaneously. The best fit is obtained for k5 = 7 X lo8 dm3 mol-'s-' and k6 = 6 X lo8 dm3 mol-' s-'. A 10% change in the ratio k 5 / k 6or in one of the rate constants results in a deterioration of the fit quality. We therefore suggest k(O-+03-) = (7.0 f 1.0) X lo8 dm3 mol-' and 40-+0*1- (6.0 f 1.0) X lo8 dm3 mol-' s-'. Our value for k(O-+03-) agrees with the rate constant of (8 f 2) X 108dm3 mol-' s-' given by Behar and C z a p ~ k i .The ~ G values of the primary radicals used in the computations are given in Table I, and an explanation of these values is given in the Appendix.
Acknowledgment. We thank H. Corfitzen and T. Johansen for technical assistance during the work.
+
-
+
+
+
-
+
+
+
+
+
+
+
= 2.8 0.5 + 0.4 2.8 0.2 0.4 = 7.1 and GO3- = 7.4 at 13.8. G H = 0.55 at pH 13.0 will add less than 1%to the total 03-yield because more than 90% of the H atoms will react with oxygen forming 0;. At pH 13.8, however, one-third of G H may contribute to 03-formation because of the reaction H + OH- e?;. Therefore 0.2 is added to the 03-yield at pH 13.8 giving a theoretical yield of 7.6 as measured from the experiments. The 0.35 left is assumed to form 02-.
-
Formation of Ozone in the Reaction between the Ozonide Radical Ion, 03-,and the Carbonate Radical Ion, COB-, in Aqueous Alkallne Solutions J. Holcman," K. Sehested, E. BJergbakke,and Edwln J. Hartt Accelerator Department, Ris0 National Laboratory. DK 4000 Roskiide, Denmark (Received: November 18, 1981)
Ozone forms in aqueous alkaline solutions by a reaction between the 03-and COS- radical ions. This reaction has been demonstrated under conditions favorable for the generation of suitable concentrations of these ions by a high-pressure pulse radiolysis technique. The reaction is 03-+ C03O3 + CO-:. Its rate constant k(03-+C03-) of (6 f 2) X lo7 dm3mol-' s-' has been determined by computer simulation of the reactions involved.
-
Introduction In a previous paper we have described the decay of the ozonide radical ion in alkaline solutions.' As carbonate 2115 Hart Road,
Port Angeles, WA 98362. 0022-3654/82/2086-2069$0 1.25/0
is a common impurity in alkaline solutions, it is of interest to study its influence on the 0 3 - decay. The 0 3 - radical (1) K. Sehested, J. Holcman, E. Bjergbakke, and E. J. Hart, J. Phys. Chem., preceding paper in this issue.
0 1982 American Chemical Society
