340
W. FORST AND PAULA. G~GUERE
Indeed, in the most densely cross-linked material, there has been comparatively little reduction in line width a t temperatures well above the melting point of unirradiated polyethylene. One is impelled to seek a more detailed description of the molecular motions. As with unirradiated polyethylenes,2 it may be deduced here that chain motion not far above room temperature assumes a vigor (in samples which have received as much as 18 units of irradiation) greater than that which may be calculated for rotation of all individual chain segments. Hence there exists a eignificant amount of motion, in terms of the nuclear resonance experiment, which transcends rotation, and which therefore must involve limited translatory movement. At more drastic dosages, the motion is more restricted. If one assumes that the motion responsible for the line narrowing may be characterized by a single correlation frequency, even though in fact there is a spectrum of motions, then it is possible to relate this frequency t o the line width and to obtain an activation energy.2Js Though not a precise measure of the motional process, such an activation energy is a t least valuable for comparison among samples and among motional processes examined by different methods. Figure 8 shows the variation of the correlation frequency with reciprocal temperature for polyethylene irradiated to a dose of 18 units in the atomic pile. As with unirradiated polyethylene, there appear to be two motional processes, one a t low temperature characterized by an activation energy of 1.4 kcal./mole and one at high temperature with an activation energy of 7.4 kcal./mole. A similar situation exists with the other samples described in Fig. 7, giving energies of the order of 1-3
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kcal. and 6-8 kcal., with the exception of the sample irradiated a t 149 units. I n this extreme case only one motional process is identified in the temperature range studied (up t o 420°K.) ; it has an activation energy of 1.8 kcal./mole. As in earlier studies12the low-energy process, which in most samples is manifest only at low temperatures, is ascribed t o small-amplitude motion, presumably torsional in character. The process a t 6-8 kcal. is ascribed to large-amplitude torsional motion or to rotation and limited translation of chain segments. In the most highly irradiated sample, the molecular segments are so stiff that only the low-energy process can be realized. Indeed, very likely no more vigorous motion can be achieved without bond fracture. 5. Conclusion
It has been found, somewhat a t variance to previous s t ~ d i e s that , ~ ~high-energy ~ ~ ~ ~ ~ irradiation inflicts change concurrently upon the crystalline and the amorphous regions of polyethylene. The characteristic crystalline spacings become altered with increasing dosage, and lattice defects are introduced. Ultimately the crystalline structure merges with the amorphous, which itself changes with increasing irradiation. After the irradiation has destroyed the crystallinity, further bombardment causes increasing constraint to even small-scale motions of molecular segments. Acknowledgment.-We are grateful to Dr. A. Charlesby, formerly of the British Atomic Energy Resea,rch Establishment, Harwell, for the generous donation of some irradiated samples; and to Dr. W. L. Brown and Dr. V. L. Lanza of these laboratories for assistance with the electron irradiated samples.
INHIBITION BY HYDROGEN PEROXIDE OF THE SECOND EXPLOSION LIMIT OF THE HYDROGEN-OXYGEN REACTION’ BY W. FORST AND PAUL A. GIGUSRE Department of Chemistry, Laval Universitv, Quebec, Que., Canada Receiued October 26, 1967
The inhibiting action of hydrogen peroxide on the second limit of the hydrogen-oxygen reaction has been studied a t 447’ by the withdrawal technique as a function of the composition of the mixture. The pressure ratio pi/p2 of the inhibited limit to the normal second limit showed a quadratic dependence on the peroxide concentration. The sim lest scheme that O g = H20 HO2 accounts for the results is obtained by adding the two reactions HzO2 H = H 2 0 OH and H202 t o the generally accepted mechanism for the second limit.
+
During a recent investigation of the homogeneous decompGition of hydrogen peroxide vapor carried Out in this Laboratory2 it was discovered that hydrogen peroxide acts as a strong Inhibitor of the hydrogen-oxygen reaction a t the second limit.3 I n view of the important r81e played by hydrogen peroxide in that reaction a detailed study of the novel effect seemed to be called for.
inciden-
(1) This work waB supported i n part by the Office of Scientific Research of the U.s. Air Force under Contract ivo. AF 18(800)-492. (2) P. A. Giguere and I. D. Liu, Can. J. Chem., 36, 283 (1957). (3) P. A . Giguere and I. D. Liu, J. A m . Chem. SOC.,79,5073 (1957).
+
+
+
Experimental The apparatus was essentially the Name as described previously2 exceptfor the addition of storage. flasks for the various gases, and a flowmeter for controllmg the rate of evacuation of &e reaction vessel through insertion of capillaries of suitable size. The pressure was recorded by means of a transducer of the absolute type (Statham, Model PA 24 TCa-2-350), with a range of 0-100 mm., in combination with a Lee& and Northrup “Speedomax” recording Dotentiometer. The i n w t voltage of the transducer (about volts) was measured-on Le& and Northrup TYPe K potentiometer. Calibration of the transducer against a mercury manometer showed its response to be linear within experimental error over the entire range at the Working
,
March, 1958
THESECOND EXPLOSION LIMITOF
THE
341
HYDROGEN-OXYGEN REACTION
temperature (80" t o prevent condensation of peroxide and water vapor). All the experiments were carried out a t 447". The temperature remained constant to within a few hundredths of a degree during a given run, and never changed by more than 0.5" from one run to the other. The reaction vessel was a 2liter Pyrex flask cleaned with hot fuming sulfuric acid and conditioned by treatment with hydrogen peroxide vapor until reproducible rates were obtained for the homogeneous decomposition. The various gases were taken directly from commercial cylinders without purification, and were transferred to their storage vessels through a liquid-air trap. Argon was used as a diluent . The explosion limits were determined by the withdrawal method. Hydrogen, argon and oxygen were first admitted into the reaction vessel, in that order, except in a few runs where the order had to be changed to oxygen-argon-hydrogen, due t o the position of the limit. Then, a prepared mixture of hydrogen peroxide vapor with oxygen and/or I2 I6 20 rnrn.Hg argon was added quickly and the whole mixture immediately withdrawn. Total pressure prior to withdrawal was 65 P ~ R T I A LPRESSURE 02. mm. in all experiments. Fig. 1.-Second limit of hydrogen-oxygen mixtures a t The gaseous mixtures containing the inhibitor were prepared by evaporating some liquid hydrogen peroxide (99 yo) 447' in terms of the partial pressures of the two gases a t the a t 65" into a 5-liter Pyrex flask kept a t 120', and then add- explosion boundary. ing oxygen and/or argon to bring the total pressure up to 100 mm. After allowing 5 minutes for mixing, the concentration of peroxide was determined by following its complete decomposition in the reaction vessel a t 447 A similar test at the end of a series of experiments on explosion limits 32made it possible to estimate by interpolation the concenI tration of peroxide a t the beginning of each run, assuming I the peroxide vapor in the storage vessel to decompose linearly W with time. This was found to be true, within experimental 4 error, the rate being of the order of 0.1% per minute a t 120". VI 24Presumably due to some alteration in the surface activity W E of liquid hydrogen peroxide reservoit, it was difficult to prepare mixtures containing the desired concentration of in8 16hibitor within close limits. In the final compilation of data only those runs were considered in which the initial W peroxide Concentration was between 85 and 90% of the nominal value. 8-
4t
oo
.
0
2 -
Results A first series of experiments served to establish the explosion limit of hydrogen-oxygen mixtures containing no inhibitor or diluent. The results are plotted in Fig. 1 in terms of partial pressures of each gas a t the explosion boundary. The curves in Fig. 2 show the dependence of the explosion limit on composition for various amounts of inhibitor, and for a fixed concentration (in mole fraction) of either hydrogen or oxygen. The concentration of hydrogen peroxide a t the explosion limit, a matter of considerable importance here, is subject to some uncertainty as the amount of hydrogen peroxide decomposed during withdrawal could not be estimated directly. It was found that if the "withdrawal time," that is the time elapsed between introduction of one-half of the inhibitor mixture and the explosion, was less than 25 sec., the limit was essentially independent of the rate of withdrawal, an indication that little peroxide had decomposed under these conditions. With slower pumping speeds the limit tended to increase appreciably. Therefore the "withdrawal time'' was kept close to 20 sec. in all the runs reported here. The rate of decomposition of hydrogen peroxide was measured in several preliminary tests, in the presence of inert gas and hydrogen at total pressures of about 65 mm. Neglecting the heterogeneous decomposition, which was unimportant a t 447°,2 a very liberal estimate based on these tests indicated that the decomposition suffered by
0
I 01
I
02
I OJ
I 04
fo,
I
05
I 08
o
I 0.1
I a2
I
I
I
03
04
a5
I 0.6
fH*,
Fig. 2.-Inhibiting effect of hydrogen peroxide on the second explosion limit of hy di ogen-oxygen mixtures of various compositions.
f H202 v
f%
.
Fig. 3.-Dependence of the ratio P i n h i h i t e d l i m i t / P s e o o n d on the concentration of hydrogen peroxide.
limit
the peroxide during the 20 sec. of withdrawal amounted t o no more than about 5%, so that the uncertainty i? the concentration of peroxide could not affect seriously the shape of the experimental curves nor the conclusions of this investigation. Discussion The plot in Fig. 1is typical of a BzOs-typesurface4 (4) A. Egerton and D. R. Warren, Proc. Roy. Soc. (London),204A, 465 (1951).
W. FORST AND PAUL A. G I G U ~ R E
342
and can be represented by the equation
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the reactions of hydrogen peroxide with the chain particles
+ 0.33~0,= 11.24 + 5.09(poZ)-‘h
PHZ
(I) HzOz H = H01 Hz (9) The ($02) -’h term, which is generally interpreted HzOz + H = H20 + OH (10) to represent the extent of second-order branching14z5 HzOz 2H = 2Hz0 (11) is responsible for the curvature of the plot. The HzOz OH = HzO + HOz (12) linear portion, with an intercept of 13.5 on the HzOz 0 = OH HOz (13) pH2 axis, corresponds to a region where secondHzOz 0 HzO 0 2 (14) order branching is unimportant; that is, where the term 5.09(p0,)-’/~ does not exceed the value 13.5 From what is known of boric acid-type s u r f a ~ e s , ~ ~ ~ ~ ~ ~ - 11.24 = 2.26. This happens for PO, > 5. and of the decomposition of hydrogen peroxide I n the presence of an inert diluent, such as argon, itself12the above reactions may be assumed essena PA term multiplied by a constant appears on the tially homogeneous. All of these reactions, first left-hand side of equation I; however, the right- one by one, then two a t a time in all possible comhand side remains the same since the rate of binations, were added to the mechanism for the branching is unaffected, and therefore the condition uninhibited limit and the theoretical expressions for negligible second-order branching is still given for pi/p2 derived. It was found that only the by PO, > 5. Inspection of Fig. 2 shows that this sequence (1) - (4),(10) and (12) could account for requirement is satisfied for all values of f H z (at fo, = the observed relationship between pi/pt and ~ H ~ ol,3. 0,441, and for fo,> 0,25 (at f ~ =, 0.44). These This sequence is most significant in that it repredata are replotted in Fig. 3 as pi/p2 (the ratio of the sents the simplest possible mechanism for the inhibited limit to the ordinary second limit) against inhibiting action of hydrogen peroxide. Addition fHPqI. Thus, the dependence of the uninhibited of three or more of reactions (9)-(14) to the (1)-(4) limit on the mixture composition is eliminated. scheme might also explain the experimental results, The curves show a quadratic or, more precisely, but would lead to very complex expressions. o ~found , by hyperbolic dependence of pi/p2 on ~ H ~ as From the above sequence the inhibited limit is selecting any five points on a curve and substituting given by the equation their coordinates into the general equation of a conic. According to the generally accepted mechanism,“ll the simplest reaction scheme to account Since the concentration of peroxide is presumably for the essential features of the uninhibited second too small to affect the k4 term, it follows from (II), explosion limit, in the range of oxygen partial after some reductions pressures of interest here, may be represented by the sequence H+O*=OH+O O+Hz=OH+H OH Hz = HzO H H 02 M = HOz M
+ + +
+
+
+ + + + +
(1) (2) (3)
(4)
which leads to the condition for explosion Pz
= 2ki/k4
Now, an explosion limit is met when the rate of chain breaking equals the rate of branching. Provided both processes are first order in chain carriers, the limit is independent of the rate of chain initiation, so that the OH radicals released into the gas phase by the decomposing peroxide2 cannot affect the second limit. Therefore, the observed inhibition must be due t o some other causes. Because of its low concentration, hydrogen peroxide cannot affect appreciably reaction 4 as a third body. Other reactions, notably with molecular hydrogen, may be ignored on the ground that they are not fast enough to be of importance here. The only remaining possibility seems to be one or more of ( 5 ) G. Dixon-Lewis, J. W . Linnett and D. F. Heath, Trans. Faraday Soc., 49,766 (1953).
IS) C . N. Hinshelwood and A. T. Williamson, “The Reaction between Hydrogen and Oxygen,” Oxford University Press, 1934. (7) G . von Elbe and B. Lewis, J . Chem. Phgs., 10, 366 (1942). ( 8 ) A. H. Willbourn and C. N. Hinshelwood, Proc. Roy. SOC.(London), 1868, 353,369,376 (1946). (9) C. E”. Cullis and C. N. Hinshelwood, i b i d . , 1868, 462, 469 (1946). (10) C. N. Hinshelwood, i b i d . , 188A, 1 (1946). (11) B. Lewis and G . yon Elbe, “Combustion, Flames a n d Explosions,” Academic Press, New York, N. Y.,1951.
+
+ +
where a = Ic10/2kl and b = lcI2/k3. Upon fitting the experimental results into (111) the value of “a” falls in the range from 50 to 80, and that of “b,” from 4.8 to 5.7. However, these figures should be viewed with caution as they are quite sensitive to the mechanism assumed for the second explosion limit. For instance, addition of reactions 5, 6 and 7 to that mechanism, increases the value of “a” about fivefold, and that of “b,” twenty-fold. These findings receive some support from other sources. Thus Geib,14 who studied briefly the reaction of hydrogen atoms with hydrogen peroxide, concluded that it takes place via step 10 rather than (9) or (11). His arguments were based on the relative abundance of the final products and on certain energy considerations. The latter may be wort,h restating here in terms of the latest thermochemical data, Using the fairly well established value D(H - 02) = 47 k ~ a I . , it * ~turns out that reaction 10 is more exothermic (AH = -68 kcal.) than (9) by about 55 kcal. Also, the 0-H bond (12) D. R. Warren, Trans. Faraday Soc., 68, 199 (1957). (18) This is true also if the steps
HOz
+
HO2 ‘/2H2Oz ‘/~OZ HzOz = HzO 0 2 OH H202 = 20H
+
+ +
(5) (6)
(7)
are added t o the scheme for the uninhibited limit. (14) K.H. Geib, Z.physik. Chem., 1698, 161 (1934). (15) 8. N. F o n e r a n d R . L. Hudson, J . Chem. Phys., 23, 1364 (1955)
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March, 1958
CATALYSIS OF DECOMPOSITION OF CARO’S ACID
which must be broken in step 9, is twice as strong as the 0-0 bond involved in (10).I6 A recent reinvestigation of the reaction of atomic hydrogen with hydrogen peroxide has again been interpreted in favor of Geib’s mechanism.17 On the assumption that his measurements referred to reaction 10, Geib calculated the collision yield and, taking a steric factor of 0.1, he estimated the activation energy to be about 4.5 kcal. A rough calculation of Elo also may be attempted from the present results by obtaining first an estimate of k~ a t the temperature of our measurements. Lewis and von Elbe7have given an expression for kl but there are reasons to believe that it is in error by a factor of lo3 a t 520’. According to Baldwin and Walshl*a more plausible value would be em. molecules-’ sec.-l a t that temperature. WarrenlShas measured the temperature dependence of the second explosion limit, and obtained E,, = (El - Ed) = 20 1 kcal. Now, Hoare and WalshZ0have proposed for reaction 4 a negative activation energy of about -4 kcal., which makes E1 cr: 16 kcal. in good agreement with the endothermicity of that reaction, 17 kcal. a t 700°K.21 These data lead to kl N 3 X C M . molecules-’ ~ set.-' a t 447’. Taking a = 65 one gets klo = 2akl N 4 X ~ m molecules-l . ~ sec.-l a t the same temperature. Finally, using Geib’s figures for the collision diameters of H and HzO2 the collision yield may be calculated, leading to EloN 7 kca1.22with a steric factor of 0.1. Considering the
*
(16) T. L. Cottrell, “The Strengths of Chemical Bonds,’’ Butterworths, London, 1954. (17) J. S. Batzold, C. Luner and C. A . Winkler, Can. J . Chem., 31, 262 (1953). (18) R. R . Baldwin and A. D. Walsh, Disc. Faraday Soc., 17, 97 (1954). (19) D.R. Warren, Proc. Roy. SOC.(London), !4llA, 96 (1952). (20) D. E. Hoare and A. D. Walsh, Trans. Faraday Soc., 53, 1102 (1957). (21) F. D. Rossini, et al., “Selected Values of Chemical Therrnodynamic Properties,” Vol. 111, Circular 500, Washington, D. C., 1952.
343
entirely different approach of the two methods, the agreement with Geib’s estimate is gratifying. It is of course no proof for the value of the activation energy. The alternative interpretation of Geib’s results proposed by Lewis and von Elbe7 would thus appear doubtful. As yet no quantitative measurements have been reported on the reactions of OH radicals. The mass spectrometric investigations of Foner and Hudson15 point to a very high rate for reaction 12. From the value obtained here for constant “b,” this reaction should be faster than 3 by a factor of a t least five. I t may also be noted that reaction 12 is twice as exothermic ( A H = -32 kcal.) as reaction 3. An interesting question raised by the present findings concerns the possible influence of hydrogen peroxide on the second explosion limit under conditions of predominant second-order branching, e.g., with a clean Pyrex surface, such as used here, but at higher temperatures. I n Egerton and Warren’s4 derivation of equation I the coefficient of the ( p ~ , ) - ’ / ~term is proportionaI t o the half-power of the rate of chain initiation. This has prompted Warren19 to suggest that explosion sensitizers may expand the limits by affecting the rate of initiation. It is entirely conceivable that under appropriate conditions the increased chain-initiating effect of added hydrogen peroxide would far outweigh its chain-breaking effect through steps 10 and 12. Thus, a t a high enough temperature, hydrogen peroxide might become an explosion sensitizer instead of an inhibitor. Such a conclusion seems all the more plausible as reactions 10 and 12 very likely have low activation energies, making their rates almost temperature independent, whereas the second-order branching term in equation I involves a large activation energy, of the order of 35 k ~ a l . ~ The authors are grateful t o the National Research Council of Canada for financial assistance. (22) A value of 300 for “a” would make E M N 5 kcsl. other things being equal.
THE CATALYSIS OF THE DECOMPOSITION OF CARO’S ACID1 BY DONALD L. BALLAND JOHN 0. EDWARDS Metcalf Chemical Laboratories of Brown University, Providence I d , R. I . Received October SO, 1967
Evidence has been found for the catalysis of the decomposition of Caro’s acid (peroxymonosulfuric acid) by specific substances in aqueous phosphate buffer. Cob?lt (11)and molybdenum(V1) are especially effective, although some ot,her metal ions also act as catalysts for the decomposition. The observed catalytic decompositions are first order in the concentration of Caro’s acid with the exception of the cobalt-catalyzed decomposition which is second order in Caro’s acid. The order in catalyst concentration could not be determined except in the case of molybdate wherein an order of one-half and an induction period were found.
Introduction The decomposition of Caro’s acid (Le., peroxymonosulfuric acid, H2S06, to form oxygen and sulfuric acid) in aqueous buffer solutions is susceptible to catalysis by trace amounts of impurities.l Apparently these catalytic paths are eliminated by the (1) The Kinetics and Mechanism of the Decomposition of Caro’s Acid. 11; prior paper, J. Am. Chem. floc., 78, 1125 (1056).
addition of sinal1 amounts of ethylenediaminetetraacetic acid (EDTA). I n the absence of EDTA, the observed rate of decomposition was dependent on the sample of phosphate used in preparing the buffer solutions; therefore the observed effect has been ascribed to the nature and amounts of the cat,alytic substances present as impurities in the phosphate. The fact2 that dipicolinic acid stabilizes
