PHOTO-CHEMICAL DECOMPOSITIOS O F HYDROGEK P E R O X I D E I N AQUEOUS SOLUTION I N P R E S E S C E O F SODIITbI NITRO-PRUSSIDE. I BY M. QURESHI
As is well known, an aqueous solution of hydrogen peroxide, free from dust particles and catalytic influences due to the surface of the containing vessel, is insensitive to the visible light. This is, however, not the case with a solution of hydrogen peroxide, to which a relatively small quantity of a mixture of potassium ferrocyanide and ferricyanide has been added. I n the latter case, hydrogen peroxide suffers decomposition in the visible light with a brisk evolution of oxygen. This was discovered by Kistiakowsky,' who also found that the decomposition follows the unimolecular law and continues with undiminished velocity after the illumination ceases. According to the same author, an insolation of one minute in sunlight is sufficient to produce the maximum rate of decomposition. I t has been furt,her shown by Xeigert' that the reaction takes place in the dark as well, when the solution of potassium ferrocyanide is first illuminated alone and afterwards added to the solution of hydrogen peroxide. Obviously, in the case studied by Kistiakowsky potassium ferrocyanide or ferricyanide, on illumination, gives rise to some catalyst, which remains effective in the dark. But the exact nature of this 'dark' catalyst is not, yet definitely known. It may be of a colloidal nature, as suggested by Kistiakowsky, or it may be simply Fe"' ions, that are known to act catalytically in the thermal decomposition of hydrogen peroxide. There is a further possibility of both the catalysts appearing and working simultaneously. Any decision on this point must await the results of further experiments. I n the meantime, it may be of interest to report another case of a similar nature, which throws some light on the nature of the catalyst, responsible for the after-effect in the photo-chemical decomposition of hydrogen peroxide. While observing the decomposition of sodium nitro-prusside in the visible light, it occurred to the present writer to try the effect of nitro-prusside compounds on the photo-chemical decomposition of hydrogen peroxide. As the nitro-prusside compounds are decomposed by light with the formation of Prussian blue, it was thought very likely that an aqueous solution of hydrogen peroxide, t o which a few drops of sodium nitro-prusside have been added, would be sensitive to the visible light, owing t o the colloidal Prussian blue, formed as a result of the photo-chemical decomposition of the latter compound. These expectations came out to be true. ;In aqueous solution of hydrogen peroxide, to which a few drops of a freshly prepared solution of sodium nitro-prusside had been added, was fairly stable in the dark for long Z. physik. Chein., 35, 431 (1901) Ann. Physik, (4) 24, 261 (1907).
PHOTO-CHEMICAL DECOMPOSITION O F HPDROGEX PEROXIDE
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intervals of time. When the same solution was exposed in glass vessels t o the light of a metal filament lamp or a quartz mercury vapour lamp, decomposition set in after a short time and continued with a brisk evolution of oxygen after the light was shut out. A few experiments are described below. Experiment No. 1 One C.C. of a solution of sodium nitro-prusside (lI,'io), prepared in the dark, was added to one C.C. of 3 3 ' 3 hydrogen peroxide, and the mixture made up t o I O O C.C.by the addition of dust-free conductivity water. The process of mixing, which was carried out in a semi-dark room, did not take more than a minute. One portion of this mixture was transfered to a small, thoroughly cleaned, Erlenmeyer flask, placed in a thermostat, and exposed to the light of a quartz mercury lamp. The flask was closed by a cork, carrying a valve and an arrangement for pipetting out one C.C. of the mixture. The remaining portion of the mixture was transferred to a second flask, similar to the first, but painted black from outside for comparitive experiments in the dark. After an exposure of I O minutes, the light was shut, out and the course of the reaction followed in both the flasks by withdrawing one cc. of the mixture at noted intervals of time and titrating against potassium permanganate. The results are stated below. The values of the velocity constant (K X 109, given in the last column of the table, have been calculated on the basis of the uni-molecular formula: K = I,/t logi,(a,'(a-x)).
TABLE I (a) Rate of decomposition after exposure Concentration of hydrogen peroxide = 0.1molar approx). Concentration of sodium nitro-prusside = 0.001molar. Time of exposure = I O minutes. Temperature = 30'. Time (in minutes)
Permanganate titer (c.c.)
0
20.2
'9
19.3
38 69
18.4 16.7
117
Velocity constant
(k x
105)
14.7
(b) Rate of decomposition in the dark
-
0
20.2
15
20.2
34 57 80
20.1
I1
20.I
-
20 0
9
Experiment No. 2 I n this experiment, the concentration of hydrogen peroxide was nearly half of that in the first experiment, but the time of exposure was increased to
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15 minutes. Other conditions were the same as in the first experiment. For titrations, a burette, graduated to I ' j o C.C.with a certificate of K.P. L. was used.
TABLE I1 (a) Rate of decomposition after exposure Concentration of hydrogen peroxide = 0.0 j molar (approx.) Concentration of sodium nitro-prusside = 0.001molar. Time of exposure = I j minutes. Temperature = 30'. Time (in minutes)
Permanganate titer (in c.c.)
0
8.58 8.02 7.26 6.85 6.32
21
45 65 9'
Velocity constant (k X io6)
I39 I 80 126 I34
(b) Rate of decomposition in the dark 0
9.28
-
9.20 9.14 9.02
I1
7 9
Experiment No. 3 The mixtwe, exposed to light, appeared to possess a pale blue colour and showed Tyndall effect. The mixture, kept in the dark, however, neither developed blue colour nor showed Tyndall effect, but on standing for twentyfour hours deposited a precipitate of ferric hydroxide. Experiment No. 4 On adding a few drops of a solution of nitro-prusside, that had been separately illuminated for half an hour, to a n aqueous solution of hydrogen peroxide, evolution of oxygen took place in the dark. The course of this reaction was not followed quantitatively. The above experiments, though preliminary in character, are sufficient to establish the following conclusions : ( I ) An aqueous solution of hydrogen peroxide, to which a few drops of a solution of sodium nitro-prusside are added, is sensitive to the visible light. (2) The decomposition of hydrogen peroxide, under the above conditions, continues after the illumination ceases. (photo-chemical after-effect). (3) During the illumination, very probably colloidal Prussian blue is formed, which acts as a catalyst in the dark reaction. Further experiments on the effect of the variation of intensity of illumination and temperature on this reaction are in progress and will be reported in due course. Department of Chemistry, Osmania University College, Hyderabad, Deccan, India.
