1905.] Catalysis of Hydrogen Peroxide ly Colloidal Platinum. 567 



In another part of his paper Sand discusses the observation of 

 Bredig that the reaction- velocity increases more rapidly than the 

 concentration of the catalysor, and concludes that this can only be 

 accounted for by increased convection owing to the brisker evolution 

 of oxygen when large amounts of catalysor are present. 



In the present paper it is shown that when K D , which is a measure" 

 of the maximum rate at which hydrogen peroxide can be supplied to* 

 the particles by diffusion, is great in comparison with the observed 

 velocity-constant K, the hydrogen peroxide concentration at fche ; 

 surface of the particles will be maintained by diffusion at a value 

 which does not differ appreciably from the average concentration in 

 the solution, so that increased convection will have practically no 

 effect on the observed reaction-velocity. From this it follows that if r 

 as Sand maintains, increased convection does modify the reaction- 

 velocity considerably, K D cannot be large in comparison with K, since- 

 convection can only modify the value of the former constant. 



The question here considered is somewhat analogous to that of a 

 slow chemical action succeeded by a rapid one. Under such circum- 

 stances it is the velocity of the slow action which is measured, and it is. 

 clear that an increase in the rate of the rapid reaction will have no- 

 effect on the observed velocity. It is not a priori evident that the 

 same considerations apply to a slow chemical action, accompanied by 

 rapid diffusion, since the constants of the two changes are not directly 

 comparable, though both have reference to the rate of transference o 

 hydrogen peroxide. 



If KV (as defined above) be n times as great as the velocity-constant NQ 

 of the chemical action at the surface of the colloidal particles, the concentra- 

 tion at the boundary will be maintained by diffusion at a value not less thorn 

 (n - I) /nth of the average concentration in the solution.. 



Let K be the observed velocity-constant and D, as defined below y 

 have a value which does not differ appreciably from the average con- 

 centration C of hydrogen peroxide in the solution, then we have, for 

 the rate of fall of concentration in the main bulk of liquid, 



-dC/dt = KD (1)- 



Further, let K be the velocity-constant of the chemical action, and 

 C r the concentration of peroxide close to the surface of any particle,, 

 then, if we assume that the reaction velocity is proportional to C r , 

 we find for the rate of fall of concentration at the boundary due to 

 chemical action (which is of course equal to the observed rate of fall 

 in the main bulk of liquid, since hydrogen peroxide is only removed at 

 the boundary), 



- dC/dt = 



If this loss is just compensated by diffusion inwards towards the 



2x2 



