B. C. VICKERY AND K. G. DENBIGH 81

THE TERMINATION REACTION IN THE PHOTOLYSIS OF HYDROGEN PEROXIDE IN

DILUTE AQUEOUS SOLUTIONS

BY (THE LATE) D. E. LEA * Received 19th May, 1948

The quantum yields for the decomposition of 0.01 M aqueous H,O, have been determined over a wide range of intensities of light absorbed. At low intensities the quantum yield ( y = molecules H,O, decomposed per quantum) is found to be proportional to I:$[H,O,] in confirmation of the results of other workers at higher concentrations ; but a t high intensities y reaches a limiting value = 1-39 f 0.11 independent of Iabs and the acidity. It is concluded that either or both the reactions,

HO + HO,+ H,O + 0, and 2HO,-t H,O, + O,,

and hence also the reaction, HO + H,O,-t H,O + HO,, must be occurring. The reactions, z H 0 - t H,O, or H,O + 0 are not excluded, but are considered unlikely.

There is ample evidence, both direct and from sector experiments, that the rate of photolysis of hydrogen peroxide in dilute aqueous solution,

* Dr. D. E. Lea died on 16th June, k947, and this note has been written by F. S . Dainton, of the Laboratorv of Physical Chemistry, Cambridge, after con- sulting Dr. Lea’s practical note-books and inspecting parts of Dr. Lea’s dismantled apparatus. Whilst I?. S . D. accepts all responsibility for this publication he wishes to state that the purpose of, and many of the conclusions drawn from these experiments, although not written in Dr. Lea’s notes, were explicitly stated by Dr. Lea a t a colloquium held towards the end of 1946 and also separately to F. S . D. a t a later date. The work described was only a part of Dr. Lea’s programme, but it was nevertheless deemed advisable to publish his results thus far obtained.

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82 PHOTOLYSIS O F HYDROGEN PEROXIDE is proportional to the square root of the intensity of the absorbed light 1 and the first power of the hydrogen peroxide concentration.2 Quantum yields much larger than unity are observed and the reaction is considered to be an unbranched chain reaction in which the initial photochemical act may be written

Confirmation for this view is afforded by the fact that hydrogen peroxide is a photosensitiser for the polymerisation of water-soluble vinyl com- pounds just as the electron-transfer reaction, Fe++ + H,O, + Fe+++ + OH, which is also a source of hydroxyl radicals, is a thermal sen~itiser.~ The propagation reactions are likely to be those of the Haber-Weiss scheme, viz. :

H2OZaq + hv zOH,,. . - (1)

OH + H,O, +HO, + H,O . - (2)

HO, + H,O, +- H,O + 0, + OH. . - (3) The termination reaction must involve mutual destruction of the

chain centres in pairs, and there are three possible paired combinations of HO and HO, in which this may be achieved :

20H + H,O, * (44 z O H - t H , O + O * . - (4b) 2H0, - tH,O, + 0, . ( 5 )

HO + HO, + H,O + 0, * (6) In principle it is possible to devise experiments to distinguish whether (4a), or (qb), or (5) and (6) are the termination reactions. The method is to conduct experiments at such high light intensities and low hydrogen peroxide concentrations that the reaction ceases to be a chain reaction and merely consists of the groups of consecutive reactions (I) + (4a), or (I) + (4b), or (I) + (2) + (5 ) , or (I) + (2) + (6) . This condition will be readily recognised since the reaction rate will then be directly propor- tional to the absorbed light intensity and correspondingly the quantum yield will have fallen to a steady value independent of the light intensity. I f K, is the quantum yield of the primary act (i.e. reaction (I)) the steady values of the observed quantum yield (molecules of H,O, destroyed), for the consecutive reaction corresponding to the above groups would be 0, R,, 2R,, 2k, respectively.

indicated that such a limiting value to the quantum yield might be attained by using a powerful light source, but very high H,O, concentrations were employed. In the present work the quantum yield has been measured over a 50, ooo-fold intensity variation for solutions of concentration 10

An earlier investigation made by Heidt

M.

Experimental (i) Optical Arrangement.-The light source used was a high-voltage

gas-filled mercury-vapour lamp, consisting of 30 cm. quartz tubing of diameter 0.8 cm. wound in a close spiral of about 3.3 cm. diameter. The pitch of the spiral was about 1-2 cm. and there were three complete windings. At 10 cm. distance

l Allmand and Style, J . Chem. Soc., 1930, 596 ; Kornfeld, 2. physik. Chem. B, 1935. 29, 205 ; Dain and Pusenkin, J . Physic. Chem. Russ., 1938, 4, 478; Qureshi and Rahman, J . Physic. Chem., 1932, 36, 664.

2 Kornfeld, Zoc. cit., Henri and Wurmser, Compt. vend., 1913, 157, 126 ; Taylor and Anderson, J . Amer. Chem. SOC., 1923, 45, 654.

Dainton, J . Physic. Chem., 1948, 52, 490, and unpublished data. Baxendale, Evans, et al., Trans. Faraday Soc., 1946, 42, 155, 668, 675.

5 Haber and Weiss, Proc. Roy. SOC. A , 1934, 147, 333. 6 Heidt, J. Amer. Chem. Sot . , 1932, 54, 2840. * This reaction was not considered by Dr. Lea.

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D. E. LEA 83 along a radius, the intensity of the full light emitted by the lamp when operating a t a current of 3 mA, was found from thermopile measurements to be about 104 ergs cm.-2 sec.-l. In the centre of the lamp the intensity was eighty times this value. The reaction vessels consisted of small, clear, quartz test-tubes of internal diameter 1.22 to 1-29 cm., external diameter 1-48 to 1'58 cm., and overall length 1.6 cm.

(p) Procedure.-Samples of B.D.H. A.R. hydrogen peroxide, freed from stabiliser, were redistilled in a Pyrex apparatus under reduced pressure and collected in a quartz receiver in which they were diluted with carefully distilled water as required. The molar extinction coefficient over the whole wavelength range emitted by the mercury lamp was determined by measurements on such samples placed in a standard quartz cell of length 1.0 cm. From the known intensity distribution inside and outside the lamp helix, the geometry of the reaction cells and the average molar extinction coefficient, the number of ergs of light energy absorbed per cm.s per sec. could be calculated.

In each run 1-2 ml. standardised 10-* M hydrogen peroxide solution were placed in a quartz test-tube provided with a glass dust cover. This cell was then clamped a t the desired position relative to the lamp. After suitable time intervals samples of about 0-02 ml. volume were withdrawn by a micro-pipette and transferred to a small glass absorptiometer cell containing a weighed amount of about 0.5 ml. titanous sulphate in N HBSO,. After reweighing, the solution was thoroughly nixed by sucking up and down several times through a dry pipette. The percentage transmission of blue light (Ilford Violet filter No. 601) permitted by the resulting yellow-coloured solution was then measured in a simple photo-electric absorptiometer. By comparison with a calibration curve the amount of hydrogen peroxide added could then readily be determined. For each run log,, (H,O,) was plotted against time and found to give a satisfactory straight line over periods of up to 5 hr. a t the higher intensities ; thus confirming the apparent first order of the reaction.

Some further experiments were carried out using a I-in-4 rotating sector driven through a gear system by an A.C. synchronous motor.

Results The quantum yields obtained under the two conditions of constant illumina-

In Fig. I these values of the quantum yield were plotted The data for intermittent illumination

tion are given in Table I. against log,, Isbs - log,, (6.03 x 1 0 ~ ~ ) . are given in Table 11.

TABLE I

Lamp Operating

Conditions Position of

Reacton Cell

cm. from helix 80 40 80 40 20 I1 I 0 5 5

2 0 I1 I 0 5 5

inside helix ,, 3 1

# #

Molar Concen tration of

HaOe Acidity

none added I ,

I *

# J

PH"3.5 none added

I J # I

S f , I

) S 3'5 ii 3'1

none added

> # 1'0 PH 3'5

4'4 2.4 x I O ~ 1-27 7.06 2.47 x rol 6-05 x rol 7'9 x I01 2'12 x I02

2-63 x 102 5.65 X 102 7-8 x I O ~ 2'13 x 103

5-63 x 103

5'8 x 104

, I

I 1

, I

,,

Quantum Yields

(mol. H202 decomposed/

quan tum) - --

48'4 20.7 33'6 8.94 5-11 3'53 3'38 1-57 1-78 1-26 1-46

1-15 1-40 1-56 1-48 1-49 1-37

1'20

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84 PHOTOLYSIS OF HYDROGEN PEROXIDE

TABLE I1 [H,O,J = 0-01 M ; pH = 5 ; cell at 20 cm. from lamp running at 3 mA on D.C.

Jabs for continuous illumination at this distance = 24.7 (quanta 1. -I sec. -I x (6.03 x 10~~)) and quantum yield under these conditions = 5-11. Interpolated values of y at 0.25 this value of Iabs = 9-7 : I in 4 sector used throughout.

I I Sector speed rev. sec. -I . log1,, (time of I rev.) . Quantum yield ( y ) .

38

3-42

9'4

-100

-5.0

9.0

Table I and Fig. I show clearly that for 0.01 M H,O, solutions irradi- ated with the unfiltered light of a gas-filled mercury-vapour lamp, the quantum yield decreases to a limiting value as the intensity is increased. This relation appears to be unaffected by the presence of added sulphuric acid within the pH range 1-6. The averaged limiting value of the quantum yield is 1.39 & 0.11, and obtains a t intensity values greater than 2.4 x 1 0 ~ 7

quanta 1.-1 sec.-1. At lower intensities the quantum yield is propor- tional to labs as is shown by the fact that the equation of the full curve drawn in Fig. I is ydlz = 6.12 x 108 molecules quanta-& 1.) set.-*, and also by the sector experiments. In the latter the value of the quantum yield at high sector speeds is almost 8, i.e. 1-8 times the value of the quantum yield for continuous illumination with this incident intensity,

FIG. I.

whereas a two-fold increase in quantum yield would have been expected for n = That the rate depends on a power of the light intensity less than unity is sub- stantiated by the decrease of quantum yield with falling sector speed

in the expression, rate = k I&,, using a I-in-4 sector.

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D. E. LEA 85 shown in Table 11. This decrease begins a t a sector revolution time of about 0.5 sec. ; but i t will be observed that even at the longest times employed (23 sec.) the quantum yield has not fallen to the limiting value of 5 anticipated theoretically on the basis of a l/llrbe relation. This is in agreement with Allmand and Style’s results a t higher concentrations.

In all the experiments carried out at high intensities appreciable amounts of H 2 0 2 were decomposed and the concentration decreased ac- cording to a first-order law, as would be expected if rate = K I a b e and

= k l I,[H202], where I, is the incident light intensity. At low intensities the data for the decay of [H20,] fit an order 1-5 better than unity. This suggests that the expression for the rate in this region is K(Iab,)*[H20,], In agreement with this, the quantum yields appropriate to 0-034 M H 2 0 2 are respectively 4 - 0 and 3-85 times the values a t the lower concentrations for the same value of l a b s .

The conclusions to be drawn from these experiments are as follows. (u) Aqueous solutions of hydrogen peroxide of 0.01 M concentration

undergo photolysis by a chain mechanism provided the intensity of light absorbed is less than - 1 0 1 7 quanta 1.-l sec.-l. Under these con- ditions the rate is found to be proportional to I,)b [H,O,] in accordance with the results of previous workers and the Haber-Weiss mechanism.

(b) At intensities greater than N 3 x 1017 quanta 1.-l sec.-l the rate is proportional to Iabe only and the reaction is no longer a chain reaction. The limiting value of the quantum yield under these conditions is 1-39 f 0.11, i.e. greater than unity, and therefore excludes the possi- bility that either reaction (4a) or (qb), followed by 0 --f 90,, is occurring. It follows that some HO, radicals are formed, presumably by reaction (2), and that some if not all the radicals are removed by reaction (5) or (6) .

The results do not exclude the possibility that the quantum yield of the primary act (= k,) is less than unity and hence that (4b) is not occurring at all.

(c) The fact that the values of the quantum yield appear to be inde- pendent of PH suggests that the various reactions take place as written in (I), ( z ) , and (3), and do not involve H0,- or 0,- ions, the concentration of which must have been very different in those experiments carried out in the presence of sulphuric acid from those with no added acid present.

Physical Chemistry Labovatory, The University,

Cambridge.

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