245

T H E THERMAL DECOMPOSITION O F HYDROGEN PEROXIDE IN AQUEOUS SOLUTIONS.’

BY B. H. WILLIAMS.

Received 3rd December, I 9 2 7. Although the thermal decomposition of hydrogen peroxide in liquid

solutions has been extensively studied, it is only recently that suficient attention has been paid to the degree of purity of the peroxide.

1 Communicated by Professor W. C . M. Lewis.

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246 THERMAL DECOMPOSITION OF HYDROGEN PEROXIDE

Twelve years ago, Clayton,2 working in this Laboratory investigated the kinetics of the decomposition of hydrogen peroxide under conditions in which particular attention was paid to the purity of the water. Clayton concluded that the presence of organic colloidal matter, even in minute amounts, exerted an appreciable effect. Clayton, however, employed Merck’s perhydrol as a source of the reactant. I t is now known, particu- larly in the light of F. 0. Rice’s researches, that such material, which contains inhibitors, is an unsuitable source.

The most valuable contributions to the subject of hydrogen peroxide decomposition are those of F. 0. Rice,s Rice and Rieff.* Using hydrogen peroxide, free from preservative, this author has established the following points :-

(a) That the thermal decomposition of hydrogen peroxide, observed in aqueous solutions, as ordinarily prepared, is due in great part to the presence of dust in solution ; efficient removal of dust renders the material very stable;

(b) That the reaction proceeds more rapidly on surfaces subjected to cleaning treatments than on a freshly formed surface, e.6, freshly fused glass surface ;

(c) That the decomposition of an aqueous hydrogen peroxide solution prepared in such a manner as to be free from preservative, is a zero order reaction.

The writer has re-investigated the kinetics of decomposition, starting, with Merck’s perhydrol and also the decomposition of material free from preservative. The results of Rice which became available during the course of the present work are such as to render of little value a record of the measurements with the inhibitor-containing material. Consequently, these are omitted, and the account limited to the behaviour of pure hydrogen peroxide.

In general, the results obtained confirm the main conclusions arrived at by Rice; they are, however, in some respects, novel and serve to give prominence to certain aspects of the difficult problem regarding the true nature of the decomposition of hydrogen peroxide in aqueous solutions, notably the attempt which is here made to separate the contribution made by the dust in the solution itself from the contribution made by the surface of the vessel.

The Thermal Decomposition of Hydrogen Peroxide, Preservative Free, in Aqueous Solutions.

The results obtained are given under different sections as follows :- Section I deals with the decomposition of pure hydrogen peroxide, in

dust free water as solvent, and in freshly fused glass reaction vessels. Section 2 deals with the decomposition of pure hydrogen peroxide in

conductivity water as solvent, and in a vessel, the walls of which are wax- coated.

Section 3 deals with the decomposition of pure hydrogen peroxide in conductivity water as solvent, and with specially cleaned glass and silica surfaces as reaction surfaces.

2Trans. Favaday SOC., XI, 64, 1915. 3 y . Amer. Chem. Sot., 48, 2099, 1926. J. physical Chem., 31, 1352, 1927.

6 The writer desires to acknowledge with gratitude the information given personally by Professor Rice in connection with the preparation of dust-free and chemically pure hydrogen peroxide solutions.

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B. H. WILLIAMS 24 7

Section I .-Th ThrmaZ Decomposition of Pure Hydrogen Peroxide in Bust- Free Water as SoZvent.

The method of obtaining dust-free water is that described by Martin ti and employed by Rice. The pure hydrogen peroxide is prepared from pure sodium peroxide in the manner described by Rice.3 The bulbs, con- taining the dust-free water, are freshly blown from a hard glass, and to the dust-free water in the bulbs a small quantity of a strong solution ( 3 0 per cent. to 40 per cent. by weight) of hydrogen peroxide is added, the re- sulting system being a dilute solution of hydrogen peroxide in dust-free water. The bulb containing the solution is connected to a constant volume manometer and the reaction followed by reading the manometer.

Under these conditions, the system, as already shown by Rice, is very stable. Such decomposition as occurred gave variable results owing to the fact that a small quantity of dust was inevitably introduced, during experi- mentation, into the solution.

The slowest rate of decomposition, obtained by proceeding in the manner just described, was given by a 0.25 per cent. H202 solution, which at 60" C. and after a time interval of 10,000 mins. was found to have decomposed by less than 0.1 per cent.

With increase in the initial concentration of the H,Oz in the solution, it was found that the rate of decomposition increased owing no doubt to the fact that, since it is impossible to free the peroxide itself from dust, a solution containing a high percentage of hydrogen peroxide will contain a corresponding amount of dust, causing an increased rate of decomposition.

The slowest rate of decomposition found with a 1.0 per cent. H20z solution is o*ro x 10 - grams Hz02 per min. per C.C. of solution.

If no attempt be made to rid the solvent (water) from dust, then the rate of decomposition found, with a 1-0 per cent. H202 solution, and with a freshly fused glass surface as reaction surface, is 1-20 x 10 - grams H202 per min. per C.C. of solution. So that even with the partial removal of the dust content of a I -0 per cent. H202 solution, the rate of decomposition has decreased twelve times. This statement refers to a freshly fused:glass vessel. With a glass surface treated with cleaning agents and consequently etched, the contribution made by the glass as is shown later in the Paper becomes quite comparable with that made by the dust.

Section 2 .-The Therma 2 Decomposition of Pure Hydrogen Peroxide in Condwfivity Water as SoZvent and in VesseZs fh WaZZs of which are Coated with Wax.

In general, the hydrogen peroxide suffers decomposition in virtue of its molecules being adsorbed on a suitable surface comprising (a) the surface wall of the reaction vessel, (6) the surface of the dust particles inevitably present in conductivity water. This view has already been expressed by other workers.

By coating the reaction vessels with wax, the decomposition due to the adsorption of the hydrogen peroxide on the surface of the reaction vessel is largely eliminated, so that the decomposition taking place in wax-coated reaction vessels may be attributed almost entirely to the presence of dust in solution.

Small, freshly blown bulbs of 150 C.C. capacity were thoroughly cleaned and dried. The inside of these bulbs was coated with a high-melting

'jf. Physical Chem., 24, 478, 1920.

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248 THERMAL DECOMPOSITION OF HYDROGEN PEROXIDE

paraffin wax7 which softened at 60" C. The wax was slightly unsaturated, so that before use, it was thoroughly washed with warm water, and was

Concentration of HzOz Solution

in Per Cent. (Grams per 100 c.c.)

0.60

1'00

1-50

2'00

2.50

3-00

3-50

4-00

4'50

5-00

_____

Temp. c.

5 0 45 50 45 50 45 50 45 50 45 50 45 50 45 50 45 50 45 50 45

TABLE I.

Rate of Decomposition in Grams of HzOs per Min.

per C.C. of Solution. ( x I0 + 6).

0'75 0.605 0.975 0.790 1.270

1'535

1.800 1.400 2.050 1-600 2.250 1.800 2.400 1.925 2'550

2'650

1'000

1'200

2'100

2'200

Temp. Coefficient for 5' C.

1-14

1-23

1-27

1-25

1-28

1-25

1-25

1-24

1-22

1'20

CalcuIated Rate of De. :omposition at 60' C. in Grams H202 per Min.

per C.C. of Solution. ( x I 0 + G).

1-05

1-55

2'00

2-40

2-95

3'35

3-50

3-60

3'75

3-90

heated to 100' C. with a concentrated hydrogen peroxide solution, in order to ensure that the wax was as inactive as possible towards the hvdro-

1.0 2'0 3'0 4-0 5'0 6'0 7.0 Concentration in gms. per IOO C.C.

( I ) Dust effect at 60' C. (2) Dust effect at 50' C. (3) Dust + silica surface effect at 60" C.

FIG. I.

Sen peroxide. The iodine value of the wax, before treat- ment, was 5-6.

In the wax- coated bulbs were placed about 60 C.C. of hydrogen peroxide solutions of various concen- trations. Experi- ments were carried out at 45' C. and 50' C. (at these temperatures t h e wax was found to remain quite hard for a considerable period of time), the rate of decomposi- tion being deter- mined by the titra- tion method.

7 The writer is indebted to Professor F. Francis of Bristol for information regarding the wax most suitable for the present series of experiments.

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B. H. WILLIAMS 249

The values of a-x were plotted against the value of t (mins.), and by drawing tangents to the curves obtained the rate of decomposition corre- sponding to any particular bulk concentration of the HzOz is easily determined, both at 45" C. and 50" C.

From the data so obtained, the rate of decomposition corresponding to any particular bulk concentration of the HzOz solution at 60" C. can be calculated by means of the Arrhenius equation. The results obtained by following this method of treatment are !given in Table 1. Taking the average value of the temperature coefficient of the reaction between 45O C. and 50" C. as 1-24, the value of the critical increment of the reaction is 9,000 cals. The values of the rate of decomposition at 60" C. give an upper limit to the decomposition which takes place on the surface of the dust alone at 60" C.

The values of the rate of de- composition at 60" C. are given in curve I, the values at 50" C. being given in curve 2 of the same figure. I t will be observed that at low concentrations there is roughly a linear relation between the rate of de- composition and concentration. At higher concentrations (above 3.0 per cent.) the rate falls off markedly.

The values are plotted in Fig. I.

Section 3.-The Thermal Decomposition of P u r e Hydrogen Peroxide in Cond2dvity Water as SoZven t.

(A) SiZica Rash as Reaction YesseZ.-The flask was cleaned before use (a) by treatment with cold chromic acid solution, (6) by washing with cold conductivity water, (c) by a thorough steaming for 10 minutes. The water employed as solvent had an average specific conductivity of 3 x 10 - reciprocal ohms.

The reaction was followed by withdrawing aliquot portions of the solution from the reaction vessel and titrating with permanganate solution in the presence of sulphuric acid.

The concentration of the hydrogen peroxide solution can be varied at will, but it is important that the initial volume of the solution be kept constant from one experiment to another.

To economise space, the tables of data obtained are not quoted. hstead, the results are shown by means of the graphs of Fig. 2. Each point of any one graph represents at least two concordant series of measurements.

I n Fig. 2, the graphs numbered I, 2, 3, 4 refer to different initial concentrations of H202, the values being 2.j5, 3.6, 5 - 0 and 9.5 7 grams HzOa per IOO C.C. of solution respectively. From these curves the mean rate of decomposition, corresponding to any particular bulk concentration of the hydrogen peroxide, at 60" C., can be obtained by drawing tangents to the above-mentioned curves. Thus curve 3 of Fig. I gives the mean rate of decomposition in conductivity water in a silica flask corresponding to any bulk concentration of H202 solution, at 60" C., between the range of Concentration of 0.25 per cent. and 5 . 2 per cent. H2OP

We have already seen that curve I of Fig. I gives the rate of decom- position, corresponding to any particular bulk concentration of H202, at 6c" C., such rate being due to adsorption of the hydrogen peroxide molecules on the surface of the dust. Hence by comparing curve I of Fig. I , and curve 3 of Fig. I, the values of the rate of decomposition, corresponding to any bulk concentration of HZO2, at 60" C., due to adsorp- tion on fhe surface of the siZica reaction vesseZ, may be determined.

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250 THERMAL DECOMPOSITION OF HYDROGEN PEROXIDE

gives the rate of decom- position corresponding to oz% bulk concentrations of 22 ''O

H202 solutions above 5-0 & 2 per cent. The values of .;*: 6'0

the rates of decomposition, g 3 corresponding to bulk 15% concentrations of H202 #j 4'0

above 5.0 per cent., when o 5 plotted, yield the dotted 2 2.0

portion of curve 3, Fig. I . ?.j Hence the behaviour X E

.

m 1 s . I)

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B. H. WILLIAMS 25 1

If the concentration of the hydrogen peroxide solution be below I -7 per cent., then the silica surface is not saturated with adsorbed HzOz molecules, and the number of hydrogen peroxide molecules suffering adsorption will be directly proportional to the concentration of the hydrogen peroxide in the bulk of the solution.

As the concentration of the hydrogen peroxide in the bulk of the solu- tion is increased the number of hydrogen peroxide molecules adsorbed increases, until when the concentration of the hydrogen peroxide in the bulk of the solution reaches 1 - 7 per cent., the silica surface becomes saturated with adsorbed HzOz molecules.

A further increase in the concentration of the bulk of the solution causes no further increase in the concentration of the adsorbed Hz02 molecules, so that, above concentrations of 1.7 per I0'O

cent. in the bulk of the solution, the reaction, for 9'O its primary portion, will be a zero order reaction, 8.0 changing as the reaction proceeds into one of a *S - 1 7.0

When, however, the % concentration of the hy- < 6.0 drogen peroxide in the " bulk of the solution

5'0 reaches a value above 5.0 a per cent., the rate of de- composition begins again 5 4.O to increase with increas- ing concentration in the & 3.0 bulk of the solution. .-

This latter pheno- 2 2.0

menon may possibly be explained by the assump- tion that when the con- 1'0

centration of the hydrogen peroxide exceeds 5-0 per cent. other factors such as secondary adsorption Time in minutes. films enter the reaction. Alternatively it may be Temperature = 60' C. explained by assuming that at sufficient concentration the HzOa reacts with the silica surface to produce new active points.

Experiments were also carried out at 50" C. with the silica flask as reaction vessel, and the above method of analysis is applied to the results so obtained. Knowing the rate of decomposition, due to adsorption on the surface of the silica alone, at 50" C. and 60" C. respectively, the critical increment of the silica surface process is calculated to be 17,500 calories.

(B) Glass FZask as Reaction YesseL-The flask was cleaned before use, (a) by treatment with cold chromic acid solution, (6) by washing with cold conductivity water, (c) by a thorough steaming for 10 minutes. The water employed as solvent had an average specific conductivity of 2 x I C - ~ reciprocal ohms.

unimolecular nature. 5:

c

0

2000 4000 6000 8000 10,000 12,000

FIG. +-Reaction Vessel : Glass Flask.

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2 5 2 THERMAL DECOMPOSITION O F HYDROGEN PEROXIDE

c ; 17'5 -2 !i CYA x": 12.5

i x w; 10'0 .5 .z 23 :z fs ,.5 k% g :

15.0

g L.. 5'0 a 0

d 2 ' 5 . &

.- .- U

s o

zero order reaction, which

portion into one of a uni-

In this respect, the decomposition on a glass surface resembles the de- composition on a silica surface, for it has been shown that if the initial concentration of the hy- drogen peroxide solution be above 1 . 7 per cent., then the decomposition, for its primary portion, will be a zero order reaction, changing, during its latter

changes during its latter r

molecular nature.

/*(I)

'

,

/+ +/o ,

,+'

L L - . portion, into one of a uni-

i /t' /'*

* *dn

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B. H. WILLIAMS

7'54 3'8 2'3 1'5 5'24

253

13'500 17*000

g.000 23.000 I 7.500

glass surface alone, is directly proportima2 to the initial cancelitration of the hydrogen peroxide so Zution .

Furthermore, it must once more be emphasised that this initial rate of decomposition can be shown to remain constant for a considerable period of time, in any one reaction, despite the diminution in the con- centration of the hydrogen peroxide solution in the bulk.

These results may possibly be explained in the following manner :- The hydrogen peroxide acts on the glass surface, in some manner,

producing active catalyticpoints, which when once formed at the beginning of the reaction remain active throughout the particular experiment.

At the beginning of any one reaction, the active points so formed are saturated with adsorbed hydrogen peroxide molecules, so that the rate of decomposition will remain constant for a period, until when the concen- tration of the hydrogen peroxide in the bulk of the solution reaches a critical value (the concentration of the hydrogen peroxide suffering diminu- tion as the reaction proceeds) the molecules of adsorbed hydrogen peroxide which suffer decomposition are no longer immediately replaced by hydrogen peroxide molecules from the bulk of the solution.

When this happens, the number of molecules of hydrogen peroxide which suffer adsorption on the active points becomes proportional to the concentration of the hydrogen peroxide in the bulk of. the solution. Hence, the reaction will assume a unimolecular nature. Furthermore, the number of active points formed at the beginning of an experiment is proportional to the initial concentration of the hydrogen peroxide. On the other hand, silica behaves as though it possessed a certain number of preformed active points which remain unchanged in number over a wide range of bulk concentration of H,O, (up to 5 per cent.). As already pointed out, with concentrated H2-02 the silica surface effect increases in magnitude either due to a change in type of adsorption or to the formation by the H202 of fresh points.

The active points on the glass surface are of an unstable nature, for when the reaction vessels are treated, after an experiment, with cold chromic acid solution the active catalytic points are destroyed. The behaviour noted indicates that glass is chemically less resistant to H202 than is silica.

Experiments were carried out with the glass flask, as reaction vessel, at 50" C. The results, obtained at this temperature, were of the same type as the results obtained at 60" C.

If the analysis, already outlined for the results obtained at 60" C., be carried out for the results obtained at 50" C., with the glass flask as re-

TABLE II.-INITIAL RATES OF DECOMPOSITION OF H,O, AT 60" C.

Conditions Determining the Chemical Change.

Dust surface + silica surface Dust surface + glass surface Dust surface alone : whence Glass surface alone, and Silica surface alone

Observed Initial Velocity in grams, Hz02 per min. per c .c of Solution

( x 1 0 9

Initial Concentration of HzOs = I Per Cent.

4'425 2-05 1-30 0'75 3.125

Initial Concentration of H-z,Oz= z Per Cent.

Critical Increment or Energy of Activation

in Calories.

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254 THERMAL DECOMPOSITION OF HYDROGEN PEROXIDE

action vessel, then the critical increment of the decomposition of hydrogen peroxide molecules due to adsorption on the active glass surface can be calculated. The value of this critical increment is found to be 23,000 calories.

I t will be convenient at this point to summarise and illustrate the kind of results which we have been considering by collecting in tabular form the values ascribed to vessel wall and to dust respectively in two typical instances, namely I per cent. and 2 per cent. H202 solutions. The results are given in Table 11. The table likewise contains the corresponding critical increments. These are obtained from the observed individual velocities at two temperatures.

Summary.

(I) In glass and silica reaction vessels, the thermal decomposition of pure hydrogen peroxide solutions is a zero order resction, for the first portion of the decomposition, for all initial concentrations in the case of a glass surface, and above a limiting concentration in the case of the silica surface, the decomposition being due (a) to adsorption of the H202 mole- cules on the walls of the reaction vessel, (b) to adsorption of the H202 molecules on the surface of the dust present in solution.

(2) Am upper limit to the magnitude of the decomposition, due to adsorption on the surface of the dust present in solution has been deter- mined by using a wax vessel. The critical increment of the decomposition of this process has been found to be 9,000 calories.

(3) The adsorption of the H202 molecules on the silica surface is directly proportional to the concentration of the hydrogen peroxide in the bulk of the solution, below a limiting concentration of 1.7 per cent. Between this limiting concentration of 1.7 per cent. and 5.0 per cent. HzOz concentration, the adsorption on the silica surface is independent of the concentration of the Hz02 in the bulk of the solution, this being accounted for on the assumption that the surface of the silica is saturated with a unimolecular film of adsorbed H2Q molecules. This implies that the silica surface possesses a certain number of preformed active points. The critical increment of the decomposition, due to adsorption on the silica surface alone, is I 7,000 calories. For still higher concentrations (above 5 .0 per cent.) in the bulk of the solution, other factors are brought into play, such as secondary adsorption films.

(4) On a glass surface, any one decomposition is, for its first portion, a zero order reaction, which changes into one of a unimolecular nature, as the concentration of the hydrogen peroxide in the bulk of the solution decreases. This is due to the fact that the effective catalytic area of the glass surface and dust surface is saturated with adsorbed H202 molecules for the first period of the reaction, any molecules suffering decomposition being immediately replaced by other molecules from the bulk of the solu- tion. As the concentration of the HzO2, in the bulk of the solution, decreases, a point is reached when the molecules, suffering decomposition, are no longer immediately replaced by molecules from the bulk of the solution, and now the concentration of the adsorbed H202 molecules becomes proportional to the concentration of the HaOz in the bulk. The critical increment of the process involving adsorption on the glass surface alone is 23,030 calories. Again, the initial activity of the glass surface is proportional to the initial concentration of the H202 solution. This is accounted for on the assumption that the hydrogen peroxide creates centres

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B. H. m’ILLIAMS 2 5 5

of activity on the glass surface, these active points when once formed, remaining active throughout any one decomposition. There is, therefore, a sharp contrast between the glass surface and the silica surface in regard to active points.

The author wishes to tender his thanks to the National Kitchener Memorial Fund for the grant which enabled this work to be pursued, and to Messrs. Brunner Mond & Co. for a grant to the Department of Physical Chemistry of this University.

Muspratt L d o r a tory of Physical and EZectro- Chmis fry, The universip of Liverpool.

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