KINETICS OF DEUTERIUM SESQUIOXIDE radiation to the sample. There was some evidence for a second-order component but results were not very reproducible. They serve only to place an upper limit of 100 M-' sec-l on kzz and to extend the range in which reaction 1 is sufficient to explain available data to pH 13.3.
3213 Acknowledgment. We are indebted to Mr. Jechiel Ogdan for the operation of the linear accelerator. L. M. D. and H. A. S. wish to thank the Department of Physical Chemistry of the Hebrew University for the kind hospitality and financial support extended during our stays.
Kinetics of Deuterium Sesquioxide in Heavy Water
by Benon H. J. Bielski Chemistry Department, Brookhaven National Laboratory, Upton, New York 1107.9 (Received March 18,lQ70)
The kinetic properties of deuterium sesquioxide have been studied in heavy water as a function of acidity and temperature. D208 decomposes to D20 and 02 by first-order kinetics with an activation energy of 17.5 kcal/ mol in 0.01 M DC104. The species has a maximum half-life of 139 sec at 0 "in 0.027 M DC104. The isotope effect on the rate of decay kEIOa/kD208 is 6. Long pulse experiments have been carried out in which the concentration of sesquioxide was increased to 0.5 mM.
Introduction The formation of hydrogen sesquioxide was originally deduced by Czapski and Bielski' from experimental results obtained when an acidified air-saturated aqueous solution was irradiated with an intense electron beam in a flow apparatus. Later Bielski and Schwarzz upon discovering the absorption spectrum of H203 rechecked the kinetic properties of this species under pulse radiolytic conditions, obtaining good agreement with the earlier results. The purpose of the present investigation is to determine the kinetic properties of Dz03 and to study experimental conditions which might eventually lead to the isolation of this species. As will become apparent from the results, there are advantages in studying the deuterium analog as its half-life is considerably longer than that of HzOa. Experimental Section A pulsed 1.95-MeV Van de Graaff generator served as an electron source. The pulse length was kept constant a t 0.1 sec throughout this study. All jrradiations were carried out in a Supersil quartz cell (2 X 2 X 0.8 cm) with one 2 X 2-cm window thinned to 0.4 mm to allow penetration of the electrons. Yields were determined by ferrous sulfate dosimetry. Analyzing light from a deuterium lamp passed through the cell three times with a total optical path length of 6.1 cm. The emerging light passed through two in tandem coupled Bausch and Lomb f/3.5 monochromators to a 7200 RCA photomultiplier. The
signal generated by the photomultiplier was subsequently fed into an oscilloscope where it was recorded photographically as a function of time. The scattered light a t 200 nm was less than 0.3% of the total light signa1 measured. Since relatively slow scanning rates were used, interference from Cerenkov radiation or from luminescence could be neglected. A flow apparatus described in detail by Czapski and Bielski' was used for scavenging experiments. An airsaturated 0.01 N DC104 solution passed through a continuous beam of 1.95-MeV electrons and was mixed with the scavenging solution of ferrous sulfate (1.6 N in DzSOJ after any desired time interval above 5 msec. The iron(II1) formed was subsequently assayed spectrophotometrically at 305 nm. An experimentally determined molar extinction coefficient of 2186 was used at 24". The DzO was purified by preirradiation with 'Wo y rays and subsequent distillation from chromic acid and alkaline permanganate. The acidity of the solutions studied was adjusted by the addition of either DC10, or D2S04.
Results and Discussion In view of the similarity of the radiolytic yields in heavy3-7 and light water, it is safe to assume that the (1) G. Czapski and B. H. J. Bielski, J . Phys. Chem., 67,2180 (1963). (2) B.H.J. Bielski and H. A. Schwarz, ibad., 72, 3836 (1968). (3) d. Jortner and G. Stein, Int. J. AppZ. Radiat. Isotopes, 7, 198
(1960). The Journal of Physical Chemistry, Vol. 74, No. 17, 1970
3214
BENON H. J. BIELSKI I
I
I
I
i I
I
I
i
I
I
I
I
I
I
I
4
I
0
I
1
1
2 3 S C C O N D S
I
I
4
5
Figure 1. Example of first-order decay of DzOain 0.01 N DClO, a t 23.5'. The concentration of DzOais proportional to [Fe(III)]t - [Fe(III)]-. Irradiation conditions were such that [Fe(III)], = 20.45 p M ; [Fe(III)]o = 86.00 p M ; (total dose/run) = 8.36 krads or 13323 M MFe(II1).
mechanism for the formation of deuterium sesquioxide is the same as for hydrogen sesquioxide. .0(
D, esol,OD, D2, D202
D2O -w+
D
+ 02 +DO2
esol
+ 02
--+
D + + O2-+
(5)
+
+
2[@~202 '/~(GD- GOD)] (11) where GDzo2is the molecular yield of deuterium peroxide and G(D202) is the total yield of deuterium peroxide in the absence of a scavenger. If the time between irradiation and scavenging with ferrous sulfate was reduced to seconds or less, surviving sesquioxide reacted to yield additional Fe(II1) according to 4Fe(II) D20a 4D+ +4Fe(III) 3D20 (7)
+
I
(4)
0 2
DOz DO2 --t D202 0 2 (6) The radiolytic yield for reaction 5 in 0.01 N DC104 wax determined by scavenging of the sesquioxide with ferrous sulfate in the flow apparatus, This method of chemical monitoring of &Oa is based on the comparison of the yield of Fe(II1) a t various times after irradiation since the sesquioxide decays by first order to products which do not react with iron(I1) (see reaction 13). Hence when the scavenger solution of ferrous iron is added several minutes after irradiation, that is, after all the sesquioxide has decayed, the yield of ferric iron [Fe(III) ] is G[Fe(III)] = 2G(D202) =
+
I
(3)
D203
+
I
(2)
02DO2
--+
t
(1)
+ DO2 D2O + OD + DO2 +
OD
(1)
+
The Journal of Physical Chemislry, Vol. 74, No. 17, 1970
The difference in Fe(II1) formed at time t and a t long times, [[Fe(III)], - [Fe(III)],] is proportional to the concentration of deuterium sesquioxide a t time t. A plot of the logarithm of this difference as a function of time yielded a straight line, demonstrating a first-order decay for this species. A set of experimental results for 0.01 N DClOd is shown in Figure 1. The same set of data is used for the computation of the radiolytic yield of deuterium sesquioxide in terms of G, molecules/100 eV absorbed. G is given by G(D203)= 1/4[G[Fe(III)o- G[Fe(III)I,I = 1.86
* 0.12mpt
(111)
where G(FeI1I)o is the ferric yield extrapolated to zero time, Based on this G value the relative probabilities of reaction 4 and 5 are 36 and 62%, respectively. The kinetic properties of deuterium sesquioxide were studied spectrophotometrically at 200 nm by pulse radiolysis as a function of acidity and temperature. At (4) K. Coatsworth, E. Collinson, and F. S. Dainton, Trans. Faraday SOC.,56, 1008 (1960). (5) E. Hayon, J . Phys. Chem., 69, 2628 (1966). (6) E. M. Fielden and E. J. Hart, Radiat. Res., 33,426 (1968). (7) E. J. Hart and E. M. Fielden, J . Phys. Chem., 7 2 , 577 (1968).
3215
KINETICS OF DEUTERIUM SESQUIOXIDE
1 I
480
1
I
I
I
400
-1
36011 320
280La
240
k\
200
0 O.o02 4I I
I
1
I
I
I
2
3
4
5
-log (H+or D*)
'"; 0 200
1
I 210
I 220
4
230 240 250 260 WAVE LENGTH, nm
270
280
1
Figure 3, Absorption spectrum of Dz08 ( A ) (determined a t the end of the electron pulse) and DZOZ( 0 )in heavy water containing 0.01 M DClOl a t 23.5'.
this wavelength interference due to the absorbance by the DOz radical is minimized since the extinction coefficient of both species is of comparable magnitude. Resolution of the recorded curves of absorbance vs. time into the two components constituted no difficulty since the first half-life of the DOz radical was always much shorter than the half-life of the DzOadecay. An example of such a decay curve for a 0.05 N DC104solution is shown in Figure 2. The initial rapid change in optical absorbance is due t o the bimolecular disappearance of the DOz radical by reaction 6. The part of the curve which foIlows first-order kinetics represents the decay of deuterium sesquioxide, reaction 13. The absorption spectrum of Dz03 shown in Figure 3 corresponds to the time at the end of the electron pulse. 'The extinction coefficient of deuterium sesquioxide is based on G(D20a) = 1.86 and was computed from the following equation E =
(absorbance of D203) (optical path length in cm)(dose) [G(D203)1 (IV>
The absorption spectrum of Dz02 (Figure 3) was determined in an air-saturated 0.01 N DClOd solution which had been irradiated by 6oCoy rays. The concentration of the peroxide in this solution was determined
Figure 4. Variation of the rate constant of decay of Dz03 with acidity a t 23.5': 0 , DzSOI; 0, DC10,. Curve A follows the theoretical eq V for DzOs. Curve B is the corresponding curve for & 0 3 (ref 2).
by the ceric sulfate method at 320 nm. A blue shift of the order of 16 for DzOzover H202 has been observed, The DzOs decomposition is dependent on acid concentration and is in agreement with the following mechanism.
+ D + +DaO+ + Oz +D + + DOsDOa- + D + DzO3 DOa- +OD- + 02 OD- + D203 +DzO + DOaDzOs
(8) (9) (10)
(11) (12)
As is apparent, this mechanism postulates that like Hz03,2the deuterium analog decomposes to molecular oxygen and heavy water DzOa
---t
DzO
+ Oz
(13)
The corresponding rate equation for the disappearance of DzO3, assuming steady-state conditions for DO3-, is
This expression predicts first-order kinetics over the entire acid range investigated. A graph of l / k o b s d 21s. (D') yields a straight line for the deuteron concentration range between 2 X and 5 X M with the equation l/kobsd
= 0.92
+ 1.025 x iO3(D+)
(VI)
The Journal of Phyeical Chemistry, Vol. 74, N o . 17, 1070
BENONH. J. BIELSKI
3216
Equation VI yields the value for klo/kll = 1025 and for k9 = 1.09 h 0.08 sec-l. The value for k8 = 1.00 f 0.10 M-' sec-l was determined from experiments a t high acid concentrations. The variation of ~ D with ~ o acid ~ concentration is shown in Figure 4,where the solid curve (A) is drawn according t o eq V. In computing the curve it was found that an assumed value of 2.2 X 1O1O M-l sec-I for lclz gave the best fit with the experimental points. The corresponding curve for H203(B),2also shown in Figure 4, is identical in shaped (within experimental error) with curve A except for the almost constant displacement due to the isotope effect. The ratio of kHnoa/kDzoais about 6. An approximate value computed for the pK of Dz03at 24" in D20is 10. A study of the effect of temperature on the rate of decay of DzOs in 0.01 M DC104led to the evaluation of the activation energy for the process
DtOa --j D +
+ OD- +
0 2
0, 0.1
AEz17.5 t 1.0
.O6
.021 0 .11-I-_I
,0031
..--LI-.-._LI ,0032 ,0033 .0034 .0035 .0036 IIT
,
OK-'
Figure 5 . Effect of temperature on the decay of D2O3in M DClO,. The activation energy for reaction 14 is 17.5 kcal/mol.
(14)
which was 17.5 kcal/mol. The experimental data are shown in Figure 5. From these results the computed maximum half-life for D203 is 138 see in 0.027 M DC104at 0". Extrapolation to lower temperatures yields half-life values of about 240 days at -65" and about 22 years at -80". Since sesquioxide does not react with alcohol, stable solutions of this species can be prepared by flowing a 0.027 M DC104 solution a t 0" through an intense electron beam into an alcohol solution at -80". An attempt to concentrate such a solution by vacuum evaporation at - 80" did not give the expected results. The relatively low yield was mostly due to change in pH as the system evaporated, since the rate of decay increases with increasing acidity. A technique will have to be developed bywhich the pH will be maintained at a reasonably constant value during the experiment. Alternately, the concentration of sesquioxide was successfully increased to 0.5 mM, when acidic heavy water solutions were irradiated with 2-MeV electron pulses lasting from 2 to 15 see. A comparison of the observed steady-state concentrations in air- and oxygensaturated solutions suggests that the limiting factor is the oxygen concentration. The formation of ozone and of a new transient was observed when acidic solutions saturated with oxygen were irradiated with an electron beam for 15 see. The total dose per pulse was 50-100 krads. Although the radiation-induced formation of ozone in alkaline solution has been observed by many researchers,s-18 its
The Journal of Phydcal Chemistry, Vol. 74,No. 17,107'0
---
formation in acidic solutions has not been reported before. The new transient decays by first order in the millisecond range a t pH 2 and has an absorbance in the low uv region. It is formed below pH 4 and is independent of the kind of acid present, e.g., H2S04,HC1O4, HaP04. Hence it is assumed that it is composed of oxygen and hydrogen only. Although its composition is as yet unknown, it is safe to assume that it is formed by radical attack upon sesquioxide. Acknowledgment. The author wishes to thank Dr. A. 0. Allen and Dr. H. A. Schwarz for their interest and help throughout this work and Mr. D. Comstock for his excellent assistance. This research was carried out a t Brookhaven National Laboratory under contract with the U. S. Atomic Energy Commission. (8) G. Czapski and L. M. Dorfman, J . Phys. Chem., 68, 1169 (1964).
(9) L.J. Heidt and V. R. Landi, J . Chem. Phya., 41, 176 (1964). (10) G.E.Adams, J. W. Boag, and B. D. Michael, Proc. Roy. Soc., Ser. A , 287, 321 (1965). (11) L.J. Heidt, J . Chem. Educ., 43, 623 (1966). (12) G. E.Adams, J. W. Boag, and B. D.Michael, Proc. Roy. Soc., Ser. A , 289, 321 (1966). (13) G.Czapski, J . Phys. Chem., 71, 1683 (1967), (14)W. D. Felix, B. L. Gall, and L.M. Dorfman, ibid., 71,384 (1987). (15) E.Hayon and J. J. McGarvey, ibid., 71, 1472 (1967). (16) Z.Dogliotti and E. Hayon, ibid., 72, 1800 (1968). (17) J. Rabani, Advances in Chemistry Series, No. 81, American Chemical Society, Washington, D. C., 1968,p 131. (18) G.Czapski, Technical Progress Report No.NYO-3763-3,U. 8. Atomic Energy Commission, 1968.
