THE PHOTOCHEMICAL DECOMPOSITION OF HYDROGEN PEROXIDE FIRST PAPER
BY J. HOWARD MATHEWS AND HARRY A. CURTIS
Introductory It has been known for a long time that light is a factor in the ordinary slow decomposition of hydrogen peroxide. In 1879, Downes and Blunt’ found that an 8 percent solution of hydrogen peroxide was entirely decomposed after a ten months’ exposure to sunlight, while a similar solution, kept in the dark, was much more stable. This accelerating influence of light was also noted by D’Arcy? in his investigation of the electrical phenomena accompanying the decomposition of hydrogen peroxide. The decomposition of hydrogen peroxide in the presence of blood serum salts was investigated by Kistiak~wsky,~ who came to the conclusion that the light, in this case, acts as an indirect catalyst through the formation of a “photo-colloid” from the blood serum salts, which then decomposes the peroxide. Supporting this view is the fact that the reaction continues at the same rate in the dark after the solution has been once illuminated. I n 1907, Thiele4 reported the decomposition of hydrogen peroxide by the light from a mercury arc, and three years later Tianj measured the rate of this reaction in ultraviolet light, finding it to be a reaction of the first order. During the summer of 1913, while the experiments which we describe below were in progress, two papers by Henri and Wurmser‘j on the photolysis of hydrogen peroxide appeared. The first of these deals with the decomposition in monochromatic light of various wave lengths, _________.
Nature, 20, 521 (r879). Phil. M a g . , [6] 3, 42 ( 1 9 0 2 ) . Zeit. phys. Chem., 35, 431 (1900). Zeit. angew. Chem., 22, 2472; Ber. chem. Ges. Berlin, 40, 4914 (1907). Ccmptes rendus, 151, 1040 (1910). h i d . , 157, 126, 284 (19x3).
Photochemical Decomposition of Hydrogen Peroxide
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the energy absorbed a t each wave length being measured by a thermopile and galvanometer. The second paper concerns the negative catalysis of the photochemical reaction by various substances. Source of Light As a source of light we have employed a “uviol lamp” (mercury arc in Schott & Gen. ‘‘ Uviol B ” glass) running on a I I O volt circuit. In series with the lamp were placed a rough rheostat, a small sliding-contact resistance coil, the usual inductance spool to compensate for small variations of current, and an ammeter. A volt-meter was connected across the terminals of the lamp. Since there is a considerable “kick” . from the inductance spool when the lamp goes out, and this sooner or later burns out the volt-meter, it was found best to include a key in the volt-meter circuit, making contact only for the instant required to read the instrument. In order to get constant illumination it is necessary to keep the potential drop across the lamp constant, and this is not easy to do if the lamp is exposed to the air. We found it much better to jacket the lamp in a uviol-glass tube and then keep the temperature constant by drawing air through the jacket. With this arrangement the resistance of the lamp becomes constant a few minutes after lighting it, and the regulating is very easy. The temperature inside the jacket may be kept considerably above room temperature-we use 80-goo-and the tendency for mercury globules to fog the sides of the lamp thereby is much lessened.
Reaction Chamber A tube of quartz, closed a t one end with paraffin, served as the reaction chamber. This tube was jacketed in a uviolglass tube and kept at constant temperature by pumping water from a thermostat through the jacket. The quartz tube carried a wooden pulley a t its upper end, and was rotated on its axis at constant speed by a small electric motor and pulley system. This rotation secures uniform illumination of all sides of the reaction tube, and, together with the mixing
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J . Howard Mathews and Harry A . Curtis
of the solution caused by the bubbles of oxygen rising from the decomposing peroxide, accomplished a very fair stirring. The reaction tube was placed a t a distance of 5 cm (nearest sides) from the lamp, and parallel to it. The arrangement of the apparatus is shown diagrammatically in Fig. I .
Fig.
I
A-Pulley for rotating quartz tube. B-antrance for water from thermostat. C-Quartz reaction tube. D-Uviol-glass jacket . E-Paraffin.
F-Cork. G-Brass spindle. H-Cork. )-Exit for water returning to thermostat. I
