928 NATHANIEL F. BARRAND AUGUSTINE0. ALLEN Vol. 63 those observed for fast electrons shows that the aqueous solutions.1° That such reactions do not decreases found in the heavy particle studies are contribute in a more important way is on first not due entirely to effects which occur at the reflection somewhat surprising. Diffusion of the extreme range of the track but rather to reactions radicals out of the track apparently competes which gradually increase in importance as the LET successfully with the bimolecular process. These increases. results are in accord with the previously observed The fact that the differellces in yields between fact that the over-all radiation chemistry of these fast electrons and heavy particle radiations are as simple hydrocarbons is relatively unaffected by the small as those indicated here demonstrates that LET of the radiatior2 Only through very detailed most of the radicals readily escape from the quantitative comparisons of the effects of light and ionization track. For radiations having a LET heavy particles such as the present is it possible t o of 2 e.v./A. one radical is produced 011 the average discover significant effects of LET On the radiaalong every 10 A. of path length. For a solution tion chemistry o f t h e system. 10-3 jlf in iodine the track must expand to a radius Acknowledgment.-We wish to thank Dr. A. 0 . of several hundred %ngstromsbefore it itself contains Allen for extensive discussions on the subject of sufficient iodine to quench all of the radicals formed. this Paper, l’~Ir.Irving MeYer for his aid in the Reactions between radicals within the track, which development of the very fine irradiation cells compete with the scavengiiig process, might well which have made these measurements Possible and be expected, because of the high initial colicentra- the memhers of the staff of the Brookhaveii 60-inch tion at which the radicals are formed and because cyclotron for their assistance with these measureof depletion of scavenger within the track. Such ments. (io) N. F. Barr and R. H. Schuler, THISJ O U R N A L , 63, 808 (1959). reaCtiolls are of lesser importance here than in
HYDROGEN ATOMS IN THE RADIOLYSIS OF WATER’ BY NATHANIEL F.BARRAND AUGUSTINE0. ALLEN Contribution f r o m Department of Chemistry, Brookhaven National Laboratory, Upton, N . Y . Received February 16, 2969
In the radiolysis of aqueous solutions containing Hz, HZOZand 02, the “H atom” formed by free-radical oxidation of H2 is shown to react with Oz much faster than with HzOz; but in solutions containing Oz and Hz0zonly, the “H atom” formed from water by radiolysis attacks Oz and HzOza t comparable rates. It i s concluded that the two kinds of “H atom” are in fact different, and may be basic and acid forms of H, L e . , the solvated electron, or the ion Hz+.
The mechanism of water decomposition by radi- When only oxygen and peroxide are present the ation, including the back-reaction, appears to be OH reacts with the peroxide and H with both oxyfairly well understood.2 Water is dissociated into gen and peroxide. free radicals, denoted H aiid OH. Some of these OH + HzOr = HOr + H2O (2) combine to form moIecuIar Hz and H20z; others H + HzOv = OH + HSO (3) escape this initial recombination and react with H + 0 2 = Hop (-1) dissolved materials present. In the absence of other materials the radicals act on the molecular HOI + Hop = HZ02 + Or (5) decomposition products. Peroxide is thereby converted in part into molecular oxygen, and hydrogen If only oxygen is present peroxide formed t ~ as result is combined with this oxygen and with the peroxide of reactions 4 aiid 5 is partially destroyed by re2, but since the free H is formed in larger to re-form water. To elucidate the process, a action quantity than free OH a net formation of peroxide number of studies have been made in which soluis observed with a yield equal to G H ~ o ~ GH tions of these decomposition products in water have G o H . ~ As soon as appreciable peroxide accumubeen irradiated with y-rays. As long as only two of the three decomposition lates, however, the rate diminishes and a steady is reached in which the peroxide is present a t products are present, the observed reaction kin- astate concentration about one-third that of the oxygen.s etics are satisfactorily explained by plausible reactions of the free radicals H and OH. Thus Not only the steady state levels a t different oxygen concentrations, but the entire course of the curve when solutions containing hydrogen gas and hy- shown in Hochanadel’s latest publication6 on this drogen peroxide are irradiated with y-rays the two subject agree with the above mechanism if the solutes react together to form water, with a high specific rate constants for reaction of H with 0 2 yield which indicates that a chain reac tion OCCUIS.~ and HzOz are of the same order of magnitude. This This chain must presumably be written as result has also been checked by measurements made OH + Hz H + HzO (1)
+
H
+ HzOz = OH + HzO
(3)
( 1 ) Research performed under the auspices of t h e U. S. Atomic Energy Commission. (2) A. 0. Allen, Proc. Intl. Con/. Peaceful Uses of Atomic Energy, 7 , 513 (1055)(United Nations). (3) C. J. Hochanadel, J . Phys. Chem., 66,587 (1952).
(4) The net yields of the radical and niolecular products from water radiolysis, in molecules produced per 100 e.v. absorbed, are denoted liere by 0 with an appropriate subscript. Actual observed yields of various products are denoted by d(Product)/d(Dose), taken again in units of molecules per 100 e.v. ( 5 ) C. J. Hochanadel, Proc. I n t Z . Coiif. Peaceful Uses of Atomic Enevgy, 7 , 521 (1955)(United Nations).
June, 1959
HYDROGEN ATOMSIN
THE
RADIOLYSIS OF WATER
929
in this Laboratory by Schwarz,6 who obtains initially containing 45 p M 02 and 440 fiM Hz do not for the rate constant ratio ICq/k3 the value 1.85. follow this expected curve a t all. Data shown in I n solutions containing only hydrogen and oxy- Fig. 6 of his paper3 indicate that the peroxide keeps gen, the two molecules combine to form hydrogen on climbing a t almost the initial rate until the peroxide. The rate is shown by Hochaiiadel to be expected dose for complete consumption of oxypractically independent of the concentrations of gen is exceeded. Then the concentration of perhydrogen and oxygen, and one must conclude that oxide drops very suddenly, presumably by the all free OH is reacting with H2 to form H ; all chain reaction between hydrogen and hydrogen H then reacts with O2 to form HOz which in turn peroxide, which is well known from the study of eventually reacts with itself to form peroxide. solutions containing only these two components. The number of molecules of peroxide formed here, It seemed that the competition between 02 and 3.2 per 100 e.v., is therefore taken as a measure of H202for H atoms was not proceeding at all in the half the total yield of free radicals plus the direct presence of hydrogen as expected from experiyield of molecular peroxide; G o H / ~f G H / ~i-ments on solutions not containing hydrogen. Other G H ~ o=~3.2. From these and similar experiments, data on these systems also seem to indicate difthe radical and molecular yields are obtained: ficulties. We therefore undertook a more detailed G H ~= 0.45, G H ~ o=~0.70, GH = 2.75, GOH = 2.25. study of the course of peroxide formation in When all three substances, hydrogen, hydrogen aqueous solutions of hydrogen and oxygen experoxide and oxygen, are present together the posed to y-rays. system no longer behaves as expected on the basis Experimental of the above reactions. An example of such disWater was purified by multiple distillation and radiolysis crepancy is found in Hochanadel's work2 on solutions initially containing much hydrogen and rela- a8 previously described.7 Water was saturated with hydroby bubbling and the two solutions mixed by drawing a tively little oxygen, when the reaction continues gen portion of each into a glass hypodermic syringe. A glass into regions where the peroxide concentration ring was placed in the syringe to ensure thorough mixing becomes comparable to that of the oxygen. In when the syringe was shaken. Irradiations were made at in the syringe using the -pray source described by this system, since hydrogen is in large excess over 23" Schwarz and Allen.* At intervals the syyinge was removed peroxide, practically all OH reacts with H, rather from the source, a sample of liquid ejected for analysis, and than HzOz, and the rate of consumption of oxygen the remainder replaced for further irradiation. Special experiments showed that the amount of oxygen leaking is expected to be constant and equal to G o ~ / 2 the syringe from the air during these manipulations Reaction 3 will reduce the peroxide yield into was negligible compared to the amounts of oxygen present in but should not affect the yield of oxygen consump- the experiments. tion, since the OH formed in (3) will react with Results Hz by (1) to regenerate H which can then react with 02. In a solution containing hydrogen and A number of experiments showed that for solua relatively m a l l quantity of oxygen the dose a t tioiis of different oxygen concentrations with which the oxygeii is all consumed can then be hydrogen at about 360 pill the initial yield of calculated. At approximately half this dose the peroxide appeared to be independent of the oxygen oxygen and peroxide will be present in equal con- concentration and equal to 3.2, in exact agreement centrations, and according to the experiments men- with Hochanadel's value. At a concentratioii of tioned above on solutions containing only oxygen H2 of 160 fiM the initial yield appeared to be about and peroxide the O2 and Hz02should be competing 9% smaller. This effect, if real, is reminiscent of for H and the rate of peroxide formation should be the drop in peroxide yield found by Jayson, greatly reduced. Reaction 3 destroys one mole Scholes and Weiss,9 in aqueous solutions of oxygen of peroxide, while (4) followed by ( 5 ) produces and alcohol, when the alcohol concentration is reone-half mole. Then when the rate of (3) exceeds duced to very low levels. We believe these effects one-half the rate of (4), the effect of the radical may be ascribed to a direct reaction between reactions is to give a net destruction of peroxide. OH and HOz which sets in when the amount of all If Ic4/Ics = 1.85, this condition will occur when the other materials capable of reaction with OH is HzOz concentration becomes greater than 1.85/2 small. Figure 1 shows the results obtained from a or 0.925 of the O2 concentration. As further oxy- number of solutions initially containing approxigen is consumed the rate of peroxide destruction by mately 700 pild H, and much smaller concentrations radicals will exceed its rate of formation from the of 0 2 . In every case the form of the curve rewater, the peroxide concentration should then sembles that reported by Hochaiiadel; i.~., the begin to drop, and by the time the oxygen is all peroxide concentration continues to rise until the consumed no peroxide should be left. The equa- total dose reaches a value a t which oxygen is tion for peroxide concentration on the above expected to be completely consumed; then at a mechanism, consisting of reactions 1, 3, 4 and 5 slightly greater dose a precipitous fall occurs. and including the contribution of the molecular The results are highly reminiscent of those reported yield by assuming reaction 2 to be negligible, is by Hartlo for solutions containing formic acid and hydrogen peroxide in the presence of oxygen.
+
where (02)will be given by (OZ)O- '/~(GoH.+ GH) (Dose). Hochanadel's data 011 a solution of
(6) A. 0. Allen and H. A. Schwarz, Second I n t l . Con/. PeaceJul Vses Afomic Energy, (19581, in press (1958).
(7) A. 0. Allen and R. A. Holroyd, J . Am. Chem. SOC.,77, 5852 (1955). (8) H. A. Sohwara and A. 0. Allen, Nucleonws, 12, No. 2, 58 (1954). (9) G. G. JRyson, G. Soholes and J. Weiss, J. Chem. Soc., 1358 (1057).
(IO) E. J. Hart, J . A m . Chem. Soc., 7 3 , G8 (1951).
NATHANIEL F. BARRAND AUGUSTINE0. ALLEN
930
9-76
/
25 -
Vol. 63
tration started to decrease. The result was a second rise and subsequent fall in peroxide, similar to that produced initially. I n Table I the values TABLE I CHARACTERISTICS OF THE PEROXIDE CONCENTRATION-DOSE CURVES (Oh rnoleo.)l. X 10-18
Dmm, e.v./l. X 10-10
(Oz)o/Dm.,, mole4 100e.v.
(HzOa)mar,
molec./l. X 10-18
(Hz0z)m.J
(Oh 1 6.79 3.1 2.19 5.15 0.76 2 15.07 6.9 2.18 12.1 .80 2' 13.66 6.3 2.17 17.9 a 3 15.56 7.4 2.11 13.0 .84 4 26.19 12.6 2.08 21.8 .83 5 32.42 14.8 2.09 24.2 .75 6 36.97 17.3 2.14 27.2 .75 a Calculated (HZ02)mex = 16.6. For this run equation (B) was integrated with a different boundary condition, since (H~OZ) was not zero at the start. Expt.
I
5
IO
ENERGY ABSORBED IN SOLUTION
15
1 1OZoev/ LITER].
Fig. 1.-Peroxide formation in Hz-saturated solutions containing various amounts of oxygen (see Table I).
of the dose required to produce maximum peroxide concentration are shown together with the ratio to this dose of the initial oxygen concentration. If the rate of oxygen uptake is constant a t G H / ~f G o H / ~ ,the dose to consume all oxygen should be 2.5 times the initial oxygen concentration. The required dose to maximum peroxide actually exceeds this value by about 15%, iustead of being considerably smaller as might be expected from the competition for H between HzOz and 02. Figure 2 replots the points from experiment 2 of Fig. 1. The two lower curves are calculated from equation A, assuming the rate constant ratio Ic4/lc3 to have the value 1.0or 2.0, respectively. Discussion The present data show that solutions containing hydrogen, oxygen and hydrogen peroxide behave quite similarly to those containing formic acid, oxygen and hydrogen peroxide. The indication seems clear that as long as any oxygen is present, hydrogen peroxide will not react with hydrogen to form water under y-radiation. As soon as oxygen is all used up a rapid chain reaction sets in between the hydrogen and the peroxide. This reaction is stopped by very small quantities of oxygen. In the case of formic acid this behavior was explained'O by supposing that the chain reaction between peroxide and formic acid was carried by the reactions OH
+ HCOOH
HCOO
HCOO + HzO + HzOz = H2O + COz + OH 3
This chain is stopped by small quantities of oxygen because the chain carrier HCOO reacts very much more rapidly with oxygen than with peroxide (io2'ev/ L I T E R ) , from expt. 2 replotted to show comparison
ENERGY
ABSORBED IN SOLUTION
Fig. 2.-Points with curves obtained by integration of Equations A and B.
There also the peroxide was not consumed while any oxygen was present, but instead the formic acid and oxygen reacted together to form peroxide and COz. When all oxygen was consumed a chain reaction between peroxide and formic acid to yield C02and water suddenly set in. In experiment 2 of Fig. 1 more oxygenated water was added to the syringe after the peroxide concen-
HCOO
+ Oz = HOz + COS
to form the relatively unreactive HOz radical which cannot carry on the chain. In the case of hydrogen the corresponding chain reaction, as we have already seen, consists of reactions l and 3. Here also the data suggest that the chain carrier H must react much more rapidly with 0 2 than with HzOzin order to explain the inhibition of this reaction by small quantities of 02. Yet we have seen that in the irradiation of water containing only H202and 0 2 these two substances compete on approximately equal terms for reaction with the
HYDROGEN ATOMSIN THE RADIOLYSIS OF WATER
June, 1959
93 1
reducing free radical H formed from the water by radiation. 2.03 - tan-’ I n order to explain this discrepancy we have = 1.4438 tan-’ 221 2.313 2.313 reluctantly been forced to conclude that the “H atom” produced by the oxidation of H2in reaction 1 When ( 0 2 ) = 0, only the first term under the behaves differently from the “H atom” formed from logarithm remains; when y = w , the first tan-’ water by radiolysis. In writing reaction schemes becomes ~ / 2 . Then we consequently generally denote the product of reaction 1 by H’, and in systems where oxygen is present we can obtain the observed kinetics by assuming that H’ reacts exclusively with oxygen Thus the maximum (H202) is proportional to the in preference to peroxide. The reaction scheme for initial O2 concentration, and evaluation of the the formation of peroxide in the presence of hy- constant from the above equation gives (H202)max/ (02)0 = 0.833. Table I includes experimental drogen and oxygen then becomes values of this ratio obtained from the curves drawn HzO +H, OH, Hz, HzOz in Fig. 1. Agreement is remarkably good. Figure OH + Hz = H’ + Hz0 (1) 2 shows the curve calculated by integration of (B) OH + HzOz = HOz + HzO for (O& = 15.07 X 1OI8 molec./l. along with the (2) corresponding experimental points (from expt. 2 ) . H + H,Oz = OH + HzO (3) The main discrepancy of all the runs from the H + On = HOz (4) theory lies in the size of the dose required to conH’ + On = HOz (4‘) sume all oxygen, which is stated above is 15% HOz HOz = Hz02 0 2 (5) greater than expected. Some but not all the By equating rates of formation and consumption discrepancy may be ascribed to a slight contriof each of the intermediates H, H’, OH and HO2 in bution of reaction 2, which has been neglected in the usual manner, we find for the rate of H202 the above treatment. Obviously a t least one of the two entities, H and formation H’, must be something other than a simple hydrogen atom. In water, radicals are quite likely to exist in acidic or basic forms corresponding to loss or gain of a proton. Possibilities here are the 1 solvated electron, which may be regarded as the Ekl(HZ)_ 1 + - -G k.4(H2) basic form of H, and the much-discussed acidic 1 kz( HzOz) kdHz0~) form H2+. In neutral water the radical lifetime If (H202) is small compared to (H2), so that (2) may be too short to allow equilibrium to be estabcan be neglected, the equation reduces to lished between acidic and basic forms, so that these species react independently. The entity formed in water radiolysis could be a solvated electron, which might eventually react with water to form H, but only after a time long compared to (B) its ordinary lifetime in solutions containing apwhere again ( 0 2 ) may be assumed equal to (02)~preciable oxygen or peroxide concentrations. If ‘/2 (GOH f GH)(Dose). The course of the curves in Fig. 1 should be this were the case it must be supposed that solvated calculable by integration of equation B, with the elect>ronsformed close together can react readily assumption that maximum peroxide occurs when with one another to form molecular hydrogen. Another possibility is that the radical formed in all the oxygen is gone. Integration is accomplished water radiolysis is really a hydrogen atom, while by taking as variables (02) and y = (Hz02)/(02). the reaction of OH with He in aqueous environment Putting in the numbers for the various G’s and 1.85 for k4/k3,we find, for the boundary condition y = leads not to disruption of the H-H bond but merely to an electron transfer, giving Hz+ as the Owheii (02)= ( 0 2 ) 0 product. A suggestion has been offered“ that H2+ y2 + 2 . 0 3 ~ 2.368 may be a fairly weak acid. It would then be ex- In ( 0 z ) o = In pected to live long enough on the average in neutral 22, 2.03 - tan-’--)2 03 solutions to be able to react as such with active 2.313 solutes like oxygen. To evaluate the expected maximum peroxide Acknowledgment.-Throughout this work we corresponding to (02)= 0 and y = 00, we may have greatly profited by discussions with Dr. conveniently multiply the equation by 2 , combine Harold A. Schwarz. rep1ace Y under the logarithni the logarithmic (11) W. G. Rothschild and A. 0. Allen, Radiation Research, 8, 101 (1958). by (H202)/(02), a d find
(
+
+
rlL +
z
+
+
+-
1
+
