1905.] Catalysis of Hydrogen Peroxide ly Colloidal Platinum. 573 



The cause of this increased activity must be, for the present, largely 

 a matter of speculation. It may possibly be due to the breaking up of 

 aggregates of particles and consequent increase of the reacting surface, 

 but a more probable explanation is that the surfaces are partly freed 

 from some impurity which prevents action on the peroxide. The 

 suggestion may be made that this surface impurity is platinous oxide. 

 It has been shown by Mond, Eamsay and Shields,* and by Wohler,t 

 that this oxide results when platinum is heated in air, and, having regard 

 to the method of preparation of Bredig's solutions, it is to be expected 

 that the platinum will be partly oxidised. This is supported by the 

 observation of Bredig that a trace of his colloidal platinum is dissolved 

 by sulphuric acid. Engler and WohlerJ have shown that platinous 

 oxide acts much less energetically on hydrogen peroxide than platinum 

 itself. 



It seems probable that, as many investigators have suggested, an 

 intermediate oxide of platinum is formed in the platinum catalysis of 

 hydrogen peroxide, but most likely a higher oxide, perhaps Pt02 

 which can react rapidly with the peroxide. This would correspond 

 with Manchot's explanation of the great accelerating effect of ferrous 

 salts on oxidations as compared with ferric salts. He has shown that 

 the action proceeds by intermediate formation of a higher oxide of iron, 

 and that the catalytic effect is much lessened when part of the iron has 

 changed to the ferric condition. 



Whether this suggestion applies or not, it is clear from what has been 

 given above that there is reason for supposing that the entire platinum 

 surface is not active under ordinary conditions, and this seems the most 

 probable explanation of the difference between K D as calculated by 

 Sand, and K the observed velocity-constant. 



Summary and Conclusion. 



It has been shown that the deviations from the simple logarithmic 

 formula in the catalytic decomposition of hydrogen peroxide by 

 colloidal platinum are probably due to disturbances caused by convec- 

 tion currents. It has further been shown that when the velocity- 

 constant calculated on Nernst's diffusion hypothesis is great compared 

 with the chemical velocity-constant, increased convection can produce 

 no appreciable effect on the observed reaction-velocity. 



In the case under consideration, therefore, since increased convection 

 modifies the observed reaction-velocity, there must be some error in the 

 assumptions which lead to the conclusion that the diffusion velocity- 



* ' Zeit. physikal. Chemie,' vol. 25, p. 685, 1898. 

 f ' Zeit. Anorg. Chemie,' vol. 36, p. 3475 (1903). 

 J ' Zeit. Anorg. Chemie/ vol. 29, p. 13, 1901. 

 < Annalen der Chemie,' vol. 325, p. 93, 1902. 



