RADIOLYSIS OF NEUTRAL WATERBY CYCLOTRON PRODUCED DEUTERONS
April 20, 1959
OH --- P~Z(OH)Z[HNG(OH)~I~~‘ V PKI = 7.7 (7:7)
H
1801
7 Pb2O(OH)[HNG(OH)s]*+’
[HNG(OH)5]PbPb
[HNG(OH)5] +4
-
H
V
20H P K ) + p K , = 20.4 (two steps)
IV
Preparation and Analysis of the 3 :1 Lead N-Methylglucamine Solid.-A mixture of 0.9761 g. (0.005 mole) of Nmethylglucamine and 37.50 ml. of 0.4000 M (0.015 mole) lead nitrate solution was adjusted to pH 12.0 with sodium
[CONTRIBUTION FROM
THE
@=:
9.3 (9.4)
Pb202[HNG( OH)5]z+2
Pb20z[NG(OH)~lz4
tem.lb Above 50 mole yolead the optical rotation again becomes more positive, and a precipitate forms for mixtures in excess of 67 mole % lead. These data can only be explained by the presence of a relatively positive-rotating, soluble 2 : 1 complex (66.7 mole yo Pb). It is interesting to note that above 70 mole Yo lead the optical activity becomes strongly negative even though an appreciable amount of the mixture has precipitated. A sharp minimum occurs a t 75 mole yolead. These results can only be explained by concluding that the precipitate formed is a partially soluble, strongly levorotatory 3: 1 solid. Apparently this compound has a large enough solubility to strongly influence the optical activity of the solution.
I11
VI
hydroxide. The mixture was made to volume in a 50-ml. volumetric flask, immersed in a steam-bath for 4 hr. and allowed to stand overnight a t room temqrature. The white precipitate was filtered under an atmosphere of nitrogen, washed rapidly four times with boiled distilled water and twice with ethanol, and dried in uacuo (0.01 p) for 4.5 hr Anal. Calcd. for Pb3C7H13NO~:C, 10.15; H, 1.58; N, 1.69; Pb, 75.1. Found: C, 10.21; H, 1.72; N, 2.42; Pb, 73.4 (polarographically). The high results for nitrogen might be explained by inadequate removal of nitrate nitrogen by this washing technique. N o suitable solvent could be found for recrystallization. The analysis is sufficiently consistent with the calculated values to confirm that a 3: l solid was formed.
.
Acknowledgment.-The author wishes to acknowledge the assistance of Mr. Kiyoshi Burt Sagawa who carried out several of the preliminary measurements on this system. URBANA, ILLINOIS
CHEMISTRY DEPARTMENT, BROOKHAVEN NATIONAL LABORATORY]
Radiolysis of Neutral Water by Cyclotron Produced Deuterons and Helium Ions’ BY HAROLD A. SCHWARZ, JAMESM. CAFFREY, JR., AND GEORGE SCHOLES RECEIVED SEPTEMBER 12, 1958 The yields of H2, HzOZ,H and OH in neutral aqueous solutions have been determined for 18 MeV. deuterons, 32 MeV. helium ions and 11 MeV. helium ions. Hz yields were determined in KBr and KNOZsolutions, H202 yields in air-saturated and 0 2 . The dependence of these KBr solutions and H atom yields in solutions of HPand 0 2 and in solutions of C Z H ~ O H yields on solute concentration was studied. The results are interpreted in terms of the radical-diffusion mechanism and are found to be in good agreement with the predictions of Ganguly and Magee, based on a simple model of this mechanism except in the densest tracks, where the yield of water decomposition appears to increase. The mechanism whereby radiation decomposes water into free radicafi is discussed in the light of the existing data.
Water is decomposed into four products, Hz, H and OH, by ionizing radiation.2 The ratio of the molecular products, HZand HzOz, to the radical products is greater for radiations with a high rate of energy loss, -dE/dx, than for radiations of low - dE/dx. The Hz and HzOz are generally postulated to be formed by combination reactions of the two radicals H and OH in the regions of high radical concentration along the track of the radiation. H202,
H+H+Hz OH ---f H20z OH +H20
OH H
+ +
(1) (2) (3)
The last reaction has not been observed, but is inferred from the other two. Radicals that do not combine react with solutes present, giving rise to the “radical yields.” In the case of 7-rays and fast electrons, these radicals are produced in isolated (1) Research performed under the auspices of the U. S. Atomic Energy Commission. ( 2 ) A. 0 . Allen, Radialion Rerearch, 1, 85 (1954).
spurs of a radius of about 10 A. containing 5 to 10 radicals3 (ie., about 100 e.v. of absorbed energy). For particles of higher -dE/dx, the spurs may overlap. For a 1 MeV. electron, -dE/dx is 0.02 e.v./k4; for an 18 MeV. deuteron, 0.5 e.v./A.; for a 32 MeV. helium ion, 2.3 e.v./A. and for a 3.5 MeV. a-particle, about 10 e.v./A. Thus a t high -dE/dx, the spur structure is lost and the track consists of a concentrated column of radicals. This mechanism accounts qualitatively for the variation of the product yields with -dE/dx and for the decrease in molecular yields upon the addition of so!utes reactive to H and OH.5-6 The yields of the molecular and radical products in 0.4 M HzS04 for various cyclotron radiations have been determined by Schuler and Allen,4 (3) A. H. Samuel and J. L. Magee, J . Chem. P h y s . , 21, 1080 (1953). (4) R. H. Schuler and A. 0. Allen, THISJ O U R N A L , 79, 1505 (1957). (5) T. J. Sworski, ibid., 76, 4687 (1954). (6) H. A. Schwarz. i b i d . , 77, 4960 (1955).
1802
HAROLD A. SCHWARZ, JAMES M. CAFFREY, JR.,AND GEORGESCHOLES
VOl. 81
Hart, et al.,? and Barr and Schuler.8 We have verged from theirs by 10%. we were always within 375, of determined these yields in neutral solutions for their values. The results obtained during that period were in a relative manner only, as in determining the rela18 MeV. deuterons, 32 and 11 MeV. helium ions used tive yields of air saturated and OTsaturated solutions. in order to compare them with the yields in acid The beam energy was determined by reducing the energy solution and have also measured the solute con- with aluminum absorbers and the Pyrex cell window to apcentration dependence of the molecular yields proximately 4 MeV. and measuring the rate of FeSO, oxidation in the cell.4 The energy was found from the curves of a t these energies. We have interpreted our results Schuler and Allen relating yield per coulomb t o energy. in terms of a model proposed by Ganguly and The initial energy was then determined from range energy tables.lg Reduced energy helium ion beams were obtained Magee.g The Hz yield was determined in deaerated KBrlO by removing a known amount of this absorber. Cell window thicknesses were determined both by the and KNOz solutions1' and the HzOz yield in air- microscope technique described by Schuler and Allen and saturated KBr solution.6*12 The radical yields by determining the residual range in each cell, as above. were determined in solutions containing Hz and As was found by Schuler and Allen, these two methods gave OZl3and in solutions containing C2H50H and 02.14 excellent agreement but indicate that the stopping power of Pyrex is 10% greater than the stopping power of alumiThese systems were chosen because they all have num. The cell windows used in this work ranged generally been studied thoroughly with y- and X-rays. from 30 to 60 mg./cm.*. These thicknesses could be measHart has proposed that another radical product, ured t o i l mg./cm.*. The stirrers in the cells were rotated at a rate of about HOz, is formed in the radiolysis of water.16 Donaldr . ~ . m . The ~ beam current entering the cell was kept son and Miller have found additional evidence for 800 between 1 X lo* and 5 X lo4 ampere, except when this product." It probably is due to secondary checking intensity dependence. The Fe(II1) and HZOIanalyses were performed spectroreaction in the track and hence increases sharply photometrically using 2 cm. cells in a Beckman DU ultraOH HzOz +HO? H10 violet spectrophotometer. Fe(II1) was measured at 305 with -a/&.This product amounts to only mp. The cells were not thermostated, but the lamp housing on the spectrophotometer was water-cooled and the tem16% of the hydrogen yield for PoZl0 a-rays.16 perature of the sample was measured after analysis. It was We have neglected it as it probably occurs to only always within 1' of room temperature, The optical densia small extent in the -dE/dx region we have ties were corrected t o 26.5' assuming a temperature coeffiof +0.6% per degree.% The extinction coefficient studied. Our values of GOH, GH, and G-H,o are cient 26.5' was taken as 2217.4 not affected by including this reaction. The a t Hydrogen peroxide was determined by the method of values of GH and GH~o,are low and high respectively Gh0rm1ey.l~ Five cc. of sample was made to 10 cc. with the iodide reagent and the optical density measured a t 350 by the extent of this reaction.
+
+
Experimental The usual purity precautions were observed.lO The radiation cells were of the type described by Saldick and Allen, with glass enclosed, magnet driven stirrers." The cell volumes were about 25 t o 30 cc. The solutions containing HZ and 0 2 were prepared by passing each gas separately through a flow meter, calibrated under operating conditions, then through a fritted glass barrier with water above it, into a small mixing chamber. The mixed gases were bubbled through the water in the radiation cell at a rate of about 30 cc. per minute for 5 t o 10 minutes prior t o the irradiation. The entrance and exit tubes were fitted with water-lubricated stopcocks, which were closed after saturation, so that the gases did not pass through the solution during irradiation. 01saturation was accomplished in a similar manner. Air-free samples for HZanalysis were prepared as described by Johnson and Allen.lo The methods developed by Schuler and Allen for using the Brookhaven 60-inch cyclotron and measuring the beam energy and current were used essentially without change.'* M FeSO,, M The rate of oxidation of air-saturated in NaCl and 0.4 M in HaO,, studied extensively by Schuler and Allen,' was determined each day, and our resdlts were normalized to their yields. Actually, our measurements were absolute, but in order t o avoid any variation in resistor calibration (used in calibrating the current integrator), we used their determination as standards. Except for one period when for several months our results inexplicably di(7) E. J. Hart, W. J. Ramler and S. R . Rocklin, Radiation Research, 4 , 378 (195F). ( 8 ) N. F. Barr and R. H . Schuler, to be submitted for publication. (9) A. K. Ganguly and J. L. Magee, J . Chcm. Phys., 46, 129 (1956). (10) E. R . Johnson and A. 0. Allen, THISJOURNAL, 7 2 , 4 1 4 7 (1952). (11) H . A. Schwarz and A. 0. Allen, ibid., 77, 1324 (1955). (12) A . 0. Allen and R . A. Holroyd, ibid., 77, 5852 (1955). (13) C . J. Hochanadel, J . Phys. Chem., 66, 587 (1952). (14) G. G. Jayson, G. Scholes and J. Weiss, J . Chem. SOL.,1358 (1957). (15) E. J. Hart, Radiation Research. 8, 33 (1955). (16) D. M . Donaldson and N. Miller, Trans. Faraday Soc., SI, 652 (1956). (17) J. Saldick and A. 0. Allen, J . Chem. Phys., 8 2 , 4 3 8 (1954). (18) R . H. Schuler and A. 0. Allen, Reo. Sci. Inslr., 36, 1128 (1955).
mp in a 2 cm. cell. The H102 concentration was found to be 38.8 pM per optical density unit. One batch of iodide reagent was prepared a t the beginning of the day. The reagent blank increased slowly, but linearly and reproducibly during the day. Several blanks were run, and the time of analysis was noted in order to find the proper blank. I t should be noted that this reagent is light sensitive, and we found best results storing it in dark bottles. The method of H2 analysis (combustion in 0 2 on a Pt filament) has been described previo~sly.~g
Results and Discussion The yields of the four products of the radiation, expressed as molecules per 100 e.v., are denoted by G H ~GH~O,, , GH and GOH. The observed yield o h a t e r disappearance is denoted by G- H ~ Oand ~ is equal to ~ G H , GH or ~ G H ~ o , GOH. The actual observed yield of H202, which is a function of the four products, is denoted by G(H202). This work was performed a t three energies, 18 Mev. deuterons, 32 MeV. helium ions and 11 Mev. helium ions. Since several different radiation cells were used a t various times, it was not possible to keep the beam energy constant. In practice, the beam energies were usually within 1 MeV. of these values. The helium ion beam energies in the studies of the Hz yield varied between 30 and 33 MeV. I n order to compare the yields with the results in the other systems, GH, was corrected by -0.007 per hfev., which appears reasonable from the work of Schuler and Allen4 This correction amounted to 3% a t most. The reduced energy helium ion Hz02 yield in air-saturated KBr solutions was determined a t 12.7 Mev. instead of 11 MeV. Again, in order to compare results, we as-
+
+
(19) W. A . Aron. B . G. Hoffman and F. C. Williams, Document AECU-063, 1951. (20) R . Bastian. K. WCberling and F. Palilla, Anal. Chem., 26, 284 (1953).
April 20, 1959
m I O L Y S I S OF
NEUTRAL WATER BY CYCLOTRON PRODUCED DEUTERONS
1803
sumed the Same yield to apply a t 11 MeV. In Table I it is seen that this yield is almost independ"O ent of energy, so the slight extrapolation should not introduce appreciable error. Hydrogen Yields in KBr and KNO, Solutions.0.8 The hydrogen yield observed in solutions of KBr and KNO, is the molecular hydrogen yield, GH,.~OJ~ NOt- reacts efiiaently with H atoms, and consequently GH,is a function of the NOZ-concentration.' The HZ yields observed in KNOZ solutions for 0.6 18 MeV. deuterons and 32 MeV. helium ions are given in Fig. 1. The data of Schwarz for Cow %I*. y-rays are included.6 GH, increases with -dE/dx and decreases with NO,- concentration, as is expected on the basis of the radical diffusion 0.4 mechanism. In deaerated 2 X M KBr solution, GH, was found to be 0.71 f 0.01 for 18 MeV. deuterons, 0.97 0.02 for 32 MeV. helium ions and 1.2 for 11 MeV. helium ions. (Two KNOZ solutions, 2 X 10-4 M also were irradiated with 0.2 11 MeV. helium ions. GH,was 1.2, the same as with the KBr solutions. Yields in this region are less precise, approximately f 575, and solute dependI ence studies were not performed.) Schuler and Allen found that GH, was 1.01 in 0 IO+ 10-1 air-saturated FeSO4 solutions, 0.4 M in Ha04 for 10-8 33 MeV. helium ions4 Barr and Schuler, studying (NO;), M. the air-saturated FeSOd to deaerated FeSOd ratio Fig. 1.-The variation of GH* with N G - concentration: found GHZ to be 1.05 .8 These yields are somewhat ( A ) Corn y-rays (Schwarz6); ( B ) 18 MeV. deuterons; (C) higher than we have observed for neutral solutions, 32 MeV. helium ions. The curves are calculated by Ganguly so we irradiated 2 x M KBr in 0.4 N Ha04 and Magee.s and found GHZ = 1.05 f 0.02, in agreement with the other authors. Apparently GHZis 8% higher must be added to this scheme, since HZOZ competes in acid solution for 33 MeV. helium ions. effectively with 0 2 for H atoms. The ratio of rate Hydrogen Peroxide Yields in Air-saturated KBr constants k4/k10 is 1.85.21 In air-saturated KBr Solutions.-Allen has proposed that GH,o, can be solution, then, the mechanism consists of reactions determined in air saturated water, suggesting the 4, 6-8 and 10. The rate expression for this mechmechanismZ anism is
+
,
Hz0 +HI, HzOt, H, OH H Os +HOs OH Hs02 +H02 Hz0 2H02 +H202 Oe
+ +
+
(4) (5) (6)
+
The O1protects the HzO2 from attack by H atoms Each H atom produces l/2 HZOZwhile each OH radical destroys l/z HzOz. Hence, the yield is given by G(HzOZ)= GH,o, '/z (GH - GOH)or (using the materid balance relation, ~ G H GH ~ = ~ G H @ , GOH)G(Hz0z) = ~ G H ~ O IGH~. Sworski has found that the system behaves similarly in the preserice of KBr,6 except that reaction 5 is replaced by
+
+
+
+ Br Br + H202+HBr + HOz OH
followed by
+ Br-+
OH-
(7)
(8)
He found that G H , ~decreased with increasing Br- concentration and also that the mechanism in the absence of Br- was inadequate, requiring the addition of the step OH
+ H2
-----)
H20
+H
(9)
which leads to additional HzOz production o b (4) and (6). Reaction 9 is eliminated in the presence of Br- due to reaction 7. As HZOZbuilds up in the solution, another reaction H
+ Hz02 +OH + Hz0
(10)
For small values of (HzO2) (expanding the logarithmic term in the integral) the solution of the rate equation is Gak10(Hz02)*= Go(Hz02)(dose) ( 11) (Hs02) 4- GO(H t O e ) k ~0,) (
where Go(H202) is the initial yield of HzOz and is equal to 2G~,o,- GH,. GH will be discussed in the next section, and Go(HzOZ)can be estimated with sufficient precision from the data for its use in the correction term. The correction term is never larger than 5% of the H202 concentration. Two of the three curves shown in Fig. 2 represent data collected in this system, one from air-saturated water and the other from air-saturated 2 X lo-* M KBr solution, both for 18 MeV. deuterons. The curves a t the other energies and Br- concentrations were similar to these. The solid lines represent Go(H,OZ) calculated from equation 11 and the dashed lines are the corresponding curves for H20z production. It can be seen that the deviations of G(Hz02) from Go(H~02)are small. (21) A. 0.Allen and H. A. Schwarr. Proc. Infcrn. Conf. Peaceful User Atomic Energy, (1958).
HAROLD A. SCHWARZ, JAMES M. CAFFREY, JR.,
1SO4
AND
GEORGESCHOLES
VOl. 81
Hence GH, in air-saturated water would be the same as in 5 x M KN02, and in 02-saturated water would be the same as in 2.5 X M KNOZ. These yields can be found in Fig. 1, i.e., &,(air) = 0.66 for 18 MeV. deuterium ions and 0.90 for 32 MeV. helium ions. From the data of the previous section on GH,, it is seen that &(air) is probably about 1.1 for 11 MeV. helium ions. G H , is ~ given in Fig. 3. The data of Allen and Holroyd for Co60 y-rays are included.12 GH~O, increases regularly with - dE/dx, as expected, and Br- depresses G H , ~ in , all cases, also as expected.
35
30
25 0 .
P X
In 2 0
2I
$; IO
1.2
I
O.f
'
I
I
I
1
5
0
0
12
4
16
COULOMBS X IO',
Fig. 2.--Typical
hydrogen peroxide production curves. M KBr irradiated with 18 MeV. deuterons; 0 , air-saturated water irradiated with 18 MeV. M C2HsOH solutions dcuterons; 0, 02-saturated irradiated with 32 MeV. helium ions. 0 , air-saturated 2 X
The initial yields in this system are given in Table I. Go(H202) is seen to be a function of Brconcentration. In the y-irradiation of air-saturated KBr solutions, Go(H202) varies linearly with the cube root of Br- c o n c e n t r a t i ~ n . ~This , ~ ~ relation does not hold accurately for the cyclotron results. Go(H202) for cyclotron beams does extrapolate smoothly to the yield observed in the absence of KBr, in contrast to the results with yrays of Sworski and of Allen and Holroyd, who noted a sharp increase in Go(H202) below TABLE I TIIE INITIAL YIELDS OF H202I N AIR-SATURATED KBr SOLUTIONS
-
I
I
\ @
I
0.4 I
10-5
10 - 4
I
10-3
I
1
10- 2
(Br-), M . Fig. 3.-The variation of G H ~ owith ~ Br- concentration : (A) 0, Cow y-rays (Allen and Holroyd)12; (B) 0,18 MeV. deuterons; (C) 0 , 32 MeV. helium ions; ( D ) B, 11 MeV. helium ions. The curves are calculated by Ganguly and Magee.g
Go(H202)is given in Table I1 as a function of acid and oxygen concentration. In M H2S04, 1.21 1.24 0 1.31 both Go(H202) and GH,o, are greater than in neutral 1.10 1.18 2 x 10-5 1.21 solution. Barr and Schuler find GH,o, = 1.25 for 0.98 1.06 2 x 10-4 1.10 32 MeV. helium ions in 0.4 M indicating a 0.81 0.85 2 x 10-3 0.90 further increase in G H , ~ , . ~Allen and Holroyd 0.47 0.43 2 x 10-2 0.48 found the same effect with y-rays. Increasing the AI KBr due to the onset of reaction 9. Apparently 0 2 concentration also increases Go(H202). A small reaction 9 does not occur with cyclotron radiation. part of the increase in 0 2 solution is due to a de, in neutral solution, The only obvious explanation is a dose rate.effect. crease of about 3y0 in G H ~but The energy input per unit volume a t the currents GH~o,is definitely higher a t the higher O2 concentrawe employ is orders of magnitude greater than tion. O2 concentration in this region does not M H2SO4, however. An exobtained in cobalt sources. The most logical source affect G H ~ oin~ planation of these effects is apparent in the studies of this effect is the reaction of Sworski5 and of HochanadelZ3on this system a t OH + HOP +HzO + OP (12) which would occur instead of ( 5 ) and (9) a t high high H2Oz concentration. The effect of reaction dose rate. The over-all rate expr-ession would still 10 is less noticeable in acid solution. Acid probe the same except that the complication of re- tects H202 from attack by H atoms, probably via Reaction 10 is very efficient,22 action 9 is avoided. Additional evidence for this formation of €Iz+. occurring on most encounters,6so that it is probably dose rate effect is reported in the next section. important as a track reaction, ;.e., a H atom and an In order to evaluate G H ~ we o ~ need to know G?. We have not measured in 0 2 solution, but in H202from the same spur or track react before difprevious work, it was noted that a given oxygen fusing intertrack distances. This reaction would concentration has the same effect on GH, for Co60 lower GH,o, and would be repressed by acid and 0 2 remove the H atoms. Since HZOZis not y-rays as twice that concentration of N O Z - . ~ . ~which ~ ( B r -.), M
18 M e v . D +
32
k!:?$i 1 1 Me". He+;
(22) J. A . Ghormley and C. J. Hochanadel, Radiation Research, 3, 227 (1955).
(23) C J. Hnchanadel, Proc. I n l e r n . Conf. Peaceful Uses A f o m i c Energy, 7, 5 2 1 (1956).
RADIOLYSISOF NEUTRALWATERBY CYCLOTRON PRODUCED DEUTERONS
April 20, 1959
present when the spur is formed, the majority of the reaction would occur while the reactants are in a looser distribution than the initial one. Hence solutes would have a greater effect per unit concentration on this reaction than on reactions 1-3.
1805
small amounts of HzOz, reactions 5 and 10 can be neglected. The rate expression from this mechanism is simply G(H202) = GH~o, '/z (GH GoH), or, using material balance, G(H202) = G H ~ GH. G ( H 2 0 ~is) given in Fig. 4 for this system irradiated
+ +
+
TABLE I1 I
THE DEPENDENCE OF THE INITIAL YIELDSOF HZOZAND THE MOLECULAR H20zYIELDSON OXYGEN AND ACID CON-
m
o
CENTRATION
--Go(HsG)--GH*+ 2 x 10-5 2 x 10-4 2 x 10-6 M KBr M KBr M KBr
32 MeV. He++, air-sat. 0 2 sat. 0.01 M H,SOd. airsat. 32 MeV. He++, 0.01 M HzSOI, 0 2 s t . 18 MeV. D + , air-sat. 0 2 sat.
1.10 1.23
0.98 1.17
1.00 1.05
2
x
IO-'
M KBr 0.94 1.02
1.09
1.27 1.31
1.09 1.10 1.32
0.88 0.96 I."
10-9
0 ' 0
When 0 2 concentration is increased, there is another track reaction that would tend to increase GH,o~,;.e., H HOz + HzOz. The increase in G H ~from o ~ this reaction should parallel the decrease in GH¶,since the HOz formed would maintain a similar distribution to its H atom precursor and would have a similar probability of reacting with another H atom. Ghormley and Hochanadel find that Go(H202) is 1.10 for solutions bubbled with 0 2 while being irradiated with Corn y-rays.24 Since the HZ is being removed by bubbling, this should represent the value of 2G~,o,- GH in the absence of KBr. It is 10% higher than is found by extrapolating to infinite dilution of KBr the results obtained in airsaturated solution by Allen and Holroyd.12 In view of the otherwise excellent agreement between the two laboratories, this difference is probably real and is of the same origin as our oxygen concentration effects. In an attempt to study GH,o, a t higher (Br-) in M acid solution with 32 MeV. helium ions, we found that the apparent G(H202) increased rapidly when (Br-) was greater than M and leveled off a t about 2.5 to 3 around 2 X 10+ M (Br-), but was irreproducible. This product was not all H202but contained some bromine, as shown by following its rate of reaction with the iodide reagent in the absence of the molybdate catalyst. No Br2 a t pH 2 was noted with CoB07-rays a t (Br-) below 0.1 M . This difference from the cydotron work must be due to the increased dose rate with the helium ion beam. Brz catalyzes the decomposition of HzOz after the i r r a d i a t i ~ n ,ac~~ counting for the lack of reproducibility. Hydrogen Peroxide Yield in Solutions of Hydrogen and Oxygen.-Hochanadel measured the H atom yield for Co60 7-rays by studying solutions containing HZand 0 2 . 1 3 He gave the mechanism as reactions 4, 9 and 6. This system has been studied further by Barr and Allen.26 They find that for
+
(24) J. A. Ghormley and C. J. Hochanadel, THISJ O U R N A I , 1 6 , 3351
(1954). (25) W.C. Bray a n d R. S. Livingston, i b i d . , 10, 1654 (1928). (26) N. F. Barr and A. 0. Allen, presented at -4.C.S. meeting, San Francisco, April, 1958.
10-8
BEAM CURRENT. AMP
Fig. 4.-The variation of G(H202) with beam current for 18 MeV. deuterons: lower curve, solutions containing 500 p M HZand 420 p M 02;upper curve, O2 saturated solutions of IO-' M GH60H.
with 18 MeV. deuterons, and it is seen that it does not obey these kinetics. There is a marked dependence on the beam current, even down to the lowest currents easily accessible. The same effect, more accentuated, was observed with 32 MeV. helium ions. The results support the conclusion of the last section on aerated water, that reaction 12 is important in these systems. The competition between (12) and (9) would produce the dose rate effect. Presumably, this effect would disappear a t higher Hz concentration, but we cannot work above atmospheric pressure with the required thin windows. All that can be said is that for 18 MeV. deuterons, G(Hz02) extrapolates to about 2.2 at low currents. Hydrogen Peroxide Yields in Solutions of Ethyl Alcohol and Oxygen.-Jayson, Scholes and Weiss have made an extensive study of solutions of CzH50H and 0 2 , concluding that this system can be used to measure G H . ' ~ Their results can be expressed by the mechanism Hz0 +Hz, HzOz, H, OH H 0 2 +HOz CzHbOH +CzHiOH H?O OH CzHiOH Oz + CHaCHO HOz
+
+
+ +
+
2H02 d H20z
(4) (13)
(14) (6)
Reaction 14 is not necessarily mechanistic, but agrees with the observed product distribution. It is possible that some organic peroxides are formed instead of I ~ z O Zbut , Jayson, et al., could not find any. They did establish that the most probable ones would react with the iodide reagent as if they were Hz02. The rate expression for this mechanism is simply G(H20.J = GH, GH, the same as in solutions of H:, and 0 2 . If the reaction H C2H60H -t C2H40H H2 is added to the scheme, the rate expression for G(HnO2) remains unchanged as C2H40H will also produce '/2 H202.
+
+
+
1806
HAROLD A. SCHWARZ, JAMES M. CAPPREY, JR.,
AND
GEORGE SCHOLES
VOl. 81
Garrison and co-workersn have studied 02saturated 0.05 M formic acid solutions a t pH 3 and 12 in which G(H202) is also believed to equal GH; GH. For 10 MeV. protons, which should be similar to 20 MeV. deuterons, they find G(HZ02) = 3.02, and for 30 MeV. helium ions they h d G(H202) = 2.14. These yields can be compared with ours for CZHSOH,OZ solutions, 2.27 and 1.91 for 18 MeV. deuterons and 32 MeV. helium ions, respectively. The difference may arise from pH effects or from the fact that the HzO2 yield in formic acid, 0 2 solutions increases with increasing HCOOH concentrations. The Radical Yields.-In the above C2HaOH,02 system, G(H202) = GH, GH. Both radical yields ca.n be found from this since G H and ~ GH~O, are known and from material balance, ~ G H , 1.90 GH = 2G=@¶ GOH. All of these yields are functions of the solute concentration, however, and 0 GLH,~,) 0 many different solutes have been used in these 0 0 studies. In order to avoid difficulties due to dif1.80 fering solute effects, we will attempt to construct the radical and molecular yields for an imaginary solute that reacts with equal efficiency with H and 10-3 10-2 lo-' OH radicals. This will facilitate comparison with (C, H,OH). M. the one-radical model of Ganguly and Magee. As noted earlier, 0.26 X M and 1.3 X Fig. 5.-The dependence of G(HlOn) on GHsOH concentration in air-saturated solutions irradiated with 32 MeV. M 0 2 decrease GH,to the same extent as 0.5 X lo+ M (NOz-), respectively. GH, helium ions: 0,beam current of 2 X lo4 amp.; 0. beam M and 2.5 X for the latter concentrations were extrapolated and current of 8 X l o 4 amp. interpolated from Fig. 1. These yields are given constant f 0.7% independent of C2H60H con- in Table IV for values of the parameter q, which is centration between and lo-' M . This was the concentration parameter employed by Ganguly the experience of Jayson, Scholes and Weiss in their and Magee in the diffusion theory and is essentially X-ray studies. M C&€60Hsolutions do not equal to the molar 0 2 concentration. For the exhibit the intensity dependence observed in 500 NOZ- solutions, q = 0.5(NOz-). Data for Co60 ~ , ~ 3.4 MeV. a-particles are included pM HZ solutions. Apparently OH radicals react y - r a y ~ l and with CzHsOH more readily than with H2. Since for comparison. The a-particle G H is~ from Senvar this system is independent of intensity and C2H60H and Hart and is the Hz yield in pure deaerated H Z O . ~ ~ concentration, it should serve as a good measure of There is probably a small amount of back reaction with OH radicals tending to lower the H2 yield, GH. The effect of C2H50H concentration and 0 2 con- but this effect should be small, of the order of From the values of G(H202) in Table 111, we centration on G(H202) for other radiations is given in Table 111. C2H50H concentration has little or can calculate GH for the two oxygen concentrations no effect, but G(H202) increases slightly with 0 2 for each radiation quality. The effect of C2H60H concentration. This is to be expected since more on GH; and GH may be safely neglected in compariH atoms react with O2 producing HZOZa t the son to the effect of 0 2 , as the ratio of rate constants higher concentration rather than combine with for H reacting with 0 2 and C2H5OHis about 500 other radicals in the track. C&€,OH does not as may be determined from acetaldehyde proreact efficiently with H atoms, so it should not duction in these solution^.'^ Since the combined effect of two solutes on G,, is that produced by an have much effect on the yield. effective concentration which is a linear combination of the product of the solute concentrations by TABLE I11 their relative rate constants for reaction with H, Hz02 YIELDSIN SOLUTIONS CONTAINING ETHYLALCOHOL 10-2 M C&T60Hwould act as only a 10% increase AND OXYGEN in O2concentration in air-saturated solution, which G(HdM is negligible. GH is given in Table IV. 18 MeV. 32 MeV. 11 MeV. Solution D+ He++ He++ GH~O,(and consequently GOH)can be obtained 2.21 1.83 1.38 lo-* M EtOH, air-sat. from the yields in Br- solution when a relation 10-2 M EtOH, 0 2 - s t . 2.27 1.91 .. between q and bromide concentration has been 10-1 M EtOH, air-sat. 2.27 1.84 1.32 established. Schwarz finds that for Cow y-rays Br- a t a given concentration has the same perG(H202) for air-saturated lop2 M C2H60H with centage effect on GH,o~as NOz- a t three times this 18 MeV. deuterons is 2.21, which agrees with the (27) W. M. Garrison, U C K I -3653. extrapolated yield in the H2,Oz system (2.2). This (28) E. J. Hart, THISJ O U R N A L , 1 6 , 9312 (1954). is the only check possible between the two systems, (29) C. B. Senvar and E. J. Hart, Proc. Inlcrn. Conf. Pcaccful Uses Afomic Energy. (1958). but it is quite satisfactory. A sample HzOz-dose curve is given in Fig. 2 for 02-saturated, M C2H60H irradiated with 32 MeV. helium ions. In Fig. 4, there is a comparison of the beam current effect in the Ha, OZsystem and in M C2HsOH,02 saturated solutions for 18 MeV. deuterons. There is possibly a slight current effect a t lov8 ampere but certainly none in our usual working range. C2H50H,02 solutions are convenient to use as the C2H60H concentration can be varied over a wide range. G(H202) in air-saturated solution is shown as a function of C2H60H concentration in Fig. 5 for 32 MeV. helium ions. G(HZ02) is
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RADIOLYSIS OF NEUTRAL WATERBY CYCLOTRON PRODUCED DEUTERONS
April 20, 1050
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TABLE IV THEMOLECULAR AND RADICAL YIELDSFOR VARIOUSENERGIES AND SOLUTE CONCENTRATIONS (q) Radiation
Grcl
P
GHzOt
GH
GOH
p ; d 4
Cmdd
2 . 5 x 10-4 0.42 0 . 6i11 2. 7813 2.28 0.39 3.68 1.3 X lo-' .4OZ2 18 MeV. D + 2 . 5 x 10-4 .66 0.87 1.55 1.13 0.75 2.52 1.3 X IO-' .62 0.91 1.65 1.07 32 MeV. H e + + 2 . 5 X lo-' .90 0.94 0.93 0.85 1.01 1.5G 1.3 X lo-' .87 1.00 1.04 .78 11 Mev. H e + + 2 . 5 X lo-' 1.19 1.08 0.19 .41 -10-4 1.5" (1.4)" (0.2)" ( .4)" 1.66 3 . 4 MeV. a Estimated from results of Barr and Schuler assuming the product distribution to be the same in neutral solution.
Cow y-rays
concentration has on G H , . ~ This suggests that q = sition should be constant, but this includes the 1.5 (Br-). G H ~ oyields ~ for Br- concentrations amount of water reformed in the track reactions, corresponding to our two values of q are given in especially reaction 3. The yield of (3), which we Table IV. Note that G I Iis~higher ~ a t q = 1.3 X will call GHOII, would increase with -dE/dx so This is a reflection of the that the observable yield, G-H,o should decrease. than a t 2.5 X 9% increase in GH,O~upon 0 2 saturation given in It does between Co6" y-rays and 11 hlev. helium Table 11. Actually, the 02-saturated data of Table ions as is seen from Table V. However, G-II,O inI1 cannot be extrapolated to the proper Br- con- creases again a t still higher - dE/dx. That GII for centrations by themselves, so it was assumed that 3.4 MeV. a-particles probably represents an upper the 9% increase observed for both deuterons and limit docs not affect this conclusion, since if GH M Br- applied a t other were zero, G - I ~ ~ owould still be 3.0. Radicalhelium ions a t 2 X Br- concentrations. This assumption cannot in- product reactions occurring in the spur, such as (5)) (9) and (lo), all have H20 as a product, tendtroduce more than a few per cent. error. GOH, found by material balance, is also given in ing to lower G-H,o, accentuating the problem. Table IV. G H ~ o ~GH , and GOH for 3.5 MeV. a- Except a t high - d E / d x , the behavior of G-11~0 particles have been estimated by assuming that with -dE/dx is qualitatively as expected. The ~ the same as has been observed exception will be discussed more fully later. their ratio ta G H is The total amount of water decomposing, which in studies of the B10(n,a)Li7 reaction in 0.8 N H2S04,*and using the yield of Senvar and Hart for should be independent of -dE/d.v, is G - H , ~ This calculation tends to overestimate GH GHOH. We cannot measure GHOII, but reaction 3 and GOH,but since they are small, it will not cause should proceed as easily as reactions 1 and 2. appreciable trouble. Lefort finds values of G H ~ On a purely statistical model, GHOH would equal H ~~G H , Obut ~ , GH, and are not equal. approximately 10% higher than Senvar and Hart.30 ~ G or No attempt will be made to compare the yields In lieu of any other information, we have assumed . G H ~ GirZor for 3.5 MeV. a-particles to calculated ones, but they that GHOH = G H ~ G H ~ o ~G-H~o are of interest in connection with the amount of is given in Table V and is seen t o be constant a t 4.62 rt 0.14 between Co60 y-rays and 11 Illev. water decomposed. By the above procedure we have developed yields helium ions, with a possible slight increase for 11 for solute concentrations reduced to a standard MeV. helium ions. (Again, it is much larger for solute. Before the yields in Table IV can be used 3.4 MeV. a-particles.) This constancy is excellent to represent those arising in a solution of a single support of the mechanism and suggests that water solute reacting with equal efficiency with H and OH, is re-formed by reaction 3 in amounts to be excertain questions concerning the effect of solutes pected by comparison with reactions 1 and 2 . reacting with one radical only on the yield of the TABLE V other radical should be considered. These quesW A T E R D E C O M P O S I T I O N Y I E L D S AND T H E 1:RACTION OF tions do not arise in the case of GH¶,as NO2- does TO FORMMOLECULAR PRODUCTS react with approximately equal efficiency with RADICALSCOMBINING (FROM TABLE IV) both radicals.6 The values of GH,o~are given for ?(GHZ + solutions containing Br- and 0 2 a t equivalent reG-njn Gmn2 duced concentrations, and there is no real problem ( = 2 G H % G-HrO + G - l l p O f + GHz + GI12 + 1 in considering this combination as equivalent to one Radiation P CH1Oz sy G11) CHiOz solute reacting with both radicals. GH, however, Cots y-rays 2 . 5 X lo-' 3.62 4.71 0 . 4 0 0.48 was determined in the presence of CzH50Hand 0 2 1 8 M e v . D + 1 . 3 X lo-' 2.89 4.42 .(i9 .67 only. 0 2 does not react with OH while C2H50H 2 . 5 X lo-' 2.87 4.10 .GO .71 does a t an unknown rate. Varying the C2HsOH 32 htev. 1 . 3 X lo-' 2.i8 4.6,; .go .X" concentration between and 10-l M does IIe++ 2.5 X 2.73 4.58 .80 .% not affect G H ~ GH, so that t o a first approxi- 11 hfev. mation, GH is independent of the concentration of 2.57 4.81 .94 .91 He++ 2 . 5 X IO-' solutes capable of reacting with OH. (3.2) (6.1) 3.4Mev. a G-H,o, the observable rate of water decomposition, equal to ~ G H , GH or 2GHZoa GOH,is given Comparison with the Model of Ganguly and in Table V. The total yield of water decompo- Magee.-Ganguly and Magee have tleveloped a model which predicts the magnitude of the de(30) M. Lefort, J . chim. phys., 61,351 (1954).
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HAROLD A. SCHWARZ, JAMES M. CAFFREY, JR., AND GEORGE SCHOLES
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H OH 3 HzO. However, GH, and GH~o,should be proportional to 1 - S. For y-rays, the proportionality factors and relationship between p and solute concentration were taken to give the best fit with the data, giving GH, = 0.865(1 - S ) , p = 0.5(N02-), GH,o~= 1.39(1 - S ) and p = 1.5(Br-). For the other radiations, the expressions for p were kept constant, and the normalizing factors for 1 - S were evaluated a t 211 for HZyields from (NOz-) and a t 2 X M for HzOz yields from Br- solutions, giving GH, = 0.923(1 - S ) and 1.06(1 - 5) for 18 MeV. deuterons and 32 MeV. helium ions, respectively, and GH,o* = 1.24(1-S), 1.13(I - S ) ,and 1.22(1 - S ) for 18 MeV. deuterons, 32 MeV. and 11 MeV. helium ions, respectively. Thus, R+R+Rz k for each product, n curves are specified by n 1 R S+ RS ks parameters, which is reasonable. where S is any scavenger present. The parameters The agreement with the data is excellent for involved in their theory are D,the diffusion coef- GH, and poor for GH,~,. Several things might acficient of the R radical; to, expressing the size of the count for the poor agreement of the HzOz curves. spur (equal to the time it would require for the spur First, in a two radical system, the parameters D, to expand from a point to its initial distribution) ; t o and k should be different for each radical, so that k, the rate constant of the combination reaction; values which fit G H z well should not be the best -dE/dx and p, which is proportional to the solute choice for GH~o,. It is difficult to predict whether concentration, as noted earlier. p is equal to a reasonable change in parameters would make an k ~ t o ( C swhere ) ks is the rate constant for the reaction appreciable differexice. Secondly, radical-product with solute and Cs is the solute concentration. reactions, in particular H HtOz HzO OH, They calculate S, the fraction of the radicals that occurring in the track would tend to cause a direact with the solute. (1 - S ) of the radicals re- vergence between experiment and theory. Anact to form Rz. other possible cause of the discrepancy is solute Some of the assumptions made in their treatment depletion. The model assumes that the solute are rather drastic, but their effect can be mini- concentration is unaffected by the occurrence of mized by evaluating the parameters such that they the reaction R S + RS. Actually, this reaction agree with experiment a t one point. Apparently will depress the solute concentration in the neighthey did this for Co6*y-rays, assuming R to repre- borhood of the spur, forming a sort of “negative” sent all the radicals produced, both H and OH. of the radical concentration. As the spurs become We have made one change in their calculations more closely spaced at high -dE/dx, this depresconcerning the method by which -dE/dx is intro- sion becomes more severe. I n the helium ion duced. They find that S is a function of the inte- tracks, the initial radical concentration averages about 1 M , so that it is obvious that solute degral (1/zI2) dp where R is the range of the pletion should be a serious effect. The last check between our experiments and the particle, R - p is the residual range and 21 is the model of Ganguly and Magee is the comparison average spacing of spurs at p. Ganguly and Magee the calculated 1 - S with the observed fraction evaluate this integral explicitly by noting that (dn/ of dp) = (l/Z1) where n is the number of spurs in the of radicals recombining. Since 1 - S includes retrack, and using the empirical range-energy rela- action 3, we evaluate it as tionship, ( R - p)/R = (Ep/Eo)v where q is a noi-tooGH, G H ~ , GEOH (1 - S ) e x p t l = sensitive function of the energy. With this approxiG-EM GHOH mation (q treated as constant) they evaluate the above integral as q 2 / [( 2 - n)vR]. This expression is not exact and leads to large errors a t low energies. We have evaluated this integral directly from using the same expression for GHoH as before. 1 1 curves of -dE/dx vs. E.31 Noting the Z, = ; These ratios are given in Table V along with 1 - S calculated by Ganguly and Magee. The agreewhere e is the energy released per spur ment is remarkably good. Note that Ganguly and Magee predict a larger value of 1 - S at lower (100 e.v.), we have (dp = dx) solute concentration (more combination of radicals) which is not apparent in the experiments. This discrepancy is more apparent than real and is due which is a convenient form for evaluation. to assuming GHOH = G H ~ GH,o~. The exWe will first check the solute dependence of the planation for the increase in GFI,~,with 0 2 conmolecular yields. Ganguly and Magee calculate centration (a decrease in the reaction H HzOz 1 - S, the fraction recombining, as a function of HzO OH and an increase in H HOz HzOz) solute concentration. This includes reaction 3, would not predict an increase in GHOH. If GHOR were evaluated as being proportional to (31) R . H. Schuler and A. 0. Allen, unpublished calculations, and G H the ~ discrepancy would disappear. H . A. Bethe, Revs. Modern Phgs., 44,213 (1950). pendence of the radical and molecular yields on -dE/dx and solute con~entration.~In this model, one type of radical, R, is formed with a yield of 6 per 100 e.v. Initially, each spur contains 6 of these radicals in a gaussian distribution. The spurs are spaced randomly along a linear track, the average spacing being inversely proportional to -dE/dx. In order to make the model amenable to mathematical analysis, they invoke the method of “prescribed diffusion,” i.e., as the radicals diffuse, each spur maintains the gaussian distribution. (If there were no bimolecular reactions occurring, this would be exact.) The only reactions considered during the expansion of the track are
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RADIOLYSIS OF NEUTRAL WATERBY CYCLOTRON PRODUCED DEUTERONS
April 20, 1959
In general, the agreement between the calculations of Ganguly and Magee and our data is excellent, considering that theirs is a one radical model with several approximations. The various reactions proposed to account for the deficiency in calculating G ~ I ~would o ~ require the introduction of several new parameters into the model which might require much more effort than it is worth. Certainly, the over-all mechanism is well substantiated. Radical Distribution in the Track.-In their calculations, Ganguly and Magee assume that all of the radicals are formed in spurs of about 10 8. radius. In order to support this, Samuel and Magee proposed that all electrons produced in the solution were brought rapidly to thermal energies and were recaptured by their parent ions,3 &us eliminating the possible formation of H atoms at large distances from the track. Platzman has calculated the rate a t which the secondary electrons lose their last few volts of energy32and concludes that they come to thermal energies in the solution only a t large distances from the parent ion (of the order of hundreds of A.). These electrons never return to *their parent ion but presumably form H atoms or react as H atoms in a very diffuse distribution, approaching uniformity. If some radicals were formed in an essentially uniform distribution, Ganguly and Magee’s calculations would be in error, principally a t high -dE/dx. The experimental value of S would equal the S calculated by Ganguly and Magee plus a constant. Even apart from the calculations, there is no evidence of background of radicals a t the larger values of -dE/dx (see Table IV). It may be argued that at the high ion density associated with high -dE/dx the electrons do begin to return to the ion column formed, but the excellent agreement between experiment and the calculations of Ganguly and Magee strongly supports the conclusion of Samuel and Magee. The increase in G-H~o and G-H~O G H ~ GH,O~at high -dE/dx can be explained in terms of the radical distribution in the track. First, GHOHshould be greater than GH, GH¶o%, since an appreciable number of the radicals are produced in isolated pairs of H and OH which can recombine to HzO but cannot form HP or Hz01.3 At least 40y0 of the spurs contain only a single pair of radicals. At high -dE/dx, these pairs are no longer isolated but are part of a track, so that there is a probability for combination to form H2 and H202 and consequently a decreased probability for formation of HzO. This would lead to an increase in G-H,O GH, GH~o,. However, if the radicals in these pairs are separated by 5 8.or so, the probability3 of their recombining is small, of the order of 20%. This effect alone cannot account for the observed increase in yield which is 300/0. If all of the radicals including the isolated pairs are produced by decomposition of excited water molecules, as suggested by Samuel and Magee, the original pairs of H and OH should be closer than 5 k. If they are separated by 1 to
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(32) H. Fr6lich and R. L. Platzman, Phys. Reo., 93, 1152 (1953); R . L. Platzman, R a d i o f w n Research. I,1 (1955).
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2 ti., as seems likely,33 recombination will be of the order of SOYo or more. We may conclude that a t .ow -dE/dx (z.e., for y-rays) the radical distribution is not random. I n any given spur, the initial separation between dissociated water molecules is 5 to 10 8., while the distaFce between any pair produced from HzO is 1 to 2 A. There would be a strong tendency for the original pair to recombine relative to combination with the other radicals. At high -dE/dx the distance between spur centers is less than the average radius of a spur. For 3.5 MeV. a-particles, the average radical concentration along the track is about 5 ill, a factor of 2 higher than the concentration in an isolated spur. Consequently the correlation of the radical pairs is less noticeable, and there will be more random combination leading to increased HZand HPOZyields with respect to GHOH. A corollary of this model is that the total yield of water decomposition is considerably greater than the figure 4.6 indicated in Table V. A large value is quite reasonable, since many other non-aromatic systems have decomposition yields between 5 and 10. Firestone has found G-H~o = 11.7 in the gas phase.3 4 Comparison with Yields in Acid Solution.-In Table IV we have given G H ~and GH in 0.8 N HzS04 (Schuler and Allen)4 for comparison with our results. G-H~ois considerably greater in acid solution, principally because GH is larger. This increase in yield is caused by the hydronium ion. Na+ does not produce the effect,35and it is independent of the anion.36 The yields in acid solution are amenable to the same treatment as above in comparison with Ganguly and Magee’s 1 - S, but the agreement is not quite as good. There are few concentration dependence data available except for y - r a y ~ , ~ , ” J ~ but these studies indicate that GH, is more sensitive to solute concentration in acid than in neutral solution, while the GH,o¶ sensitivity is about the same. For y-rays, G H ~extrapolates to the same yield a t infinite dilution in both acid and neutral solutions.34 However, as -dE/dx increases, GH, in acid solution becomes larger than G H in ~ neutral solution, as can be seen in Table IV. One possible explanation of the pH effect is that acid destroys the correlation between H and OH somewhat, increasing the over-all yield of radicals but also increasing the dimensions of the spur. For y-rays, these two effects would have to balance to keep the combination yield constant. The increased spur size would also explain the increased scavenger efficiency, since the parameter p is proportional to the square of the average radius of the spur. A possible mechanism for this destruction of the correlation is the occurrence of the reaction H30+ e +H H2O instead of the electron returning to the parent ion. UPTON,L. I., N. Y.
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(33) L. Monchick, J . Chcm. Phqs.. 14, 381 (1933). JOURNAL, 79, 5593 (1Y57). (34) R. F . Pirestone, THIS (35) A. 0. Allen, V. D. Hogan and W. G. Rothschild, Radiofiorr Research, 7 , GO3 (1957). (36) H. A. Schwarz. THIS J O U R N A L , 79, 534 (lY57). (37) H. A. Schwarz and J. M . Hritz, ibid., 80, 5636 (lY58).
