1905.] Catalysis of Hydrogen Peroxide by Colloidal Platinum, 569 



the chemical velocity-constant K is scarcely different from the 

 observed velocity-constant K. 



Just as in the case of successive inter-dependent chemical actions, 

 there are, therefore, three cases to distinguish : 



(1) If K is great in comparison with K D (Nernst's hypothesis), the 

 observed reaction-velocity depends only on K D , it is that with which 

 diffusion and convection bring the reacting material to the boundary. 



(2) If K and K D are comparable in value, the observed velocity will 

 depend on both, and will, consequently, be modified by alterations in 

 either. 



(3) If K is small in comparison with K D , the observed reaction- 

 velocity depends only on K , and will, consequently, not be affected 

 by such changes as stirring may produce in the value of K D . 



So far as we have gone, it would seem that the platinum catalysis of 

 hydrogen peroxide is an example of Case (3). 



The reaction-velocity increases much more rapidly tlwn the Catalysor 

 concentration. Bredig* has found that, on doubling the amount of his 

 catalysor, the reaction-velocity is trebled instead of merely being 

 doubled, as one would expect if the catalysor particles act independently 

 of one another, and Sand, apparently with good reason, has suggested 

 that this is due to convection currents set up by the rapid evolution of 

 oxygen bubbles in solutions containing large quantities of catalysor. 

 This view as to the great effect of convection currents cannot, however, 

 be easily reconciled with the conclusion arrived at above, that we are 

 dealing with a " chemical " reaction-velocity. We have seen that, in 

 the case considered, C r , with a minimum of stirring, is yf of the 

 average concentration in the solution. The effect of stirring would be 

 to shorten the diffusion-layer, and bring C r nearer to the average con- 

 centration ; but it is clear that the greatest possible alteration of the 

 reaction-velocity from this cause will only amount to ^ of its value, 

 which is quite insufficient to account for Bredig's results. 



In searching for an explanation of this difficulty, we must first con- 

 sider what evidence there is for the suggestion that the want of 

 proportionality between K and the catalysor concentration is due to 

 the disturbing effect of convection currents. If this is not the case if 

 an explanation on chemical grounds can be found for the deviation m 

 question then we must regard the observed velocity as in all proba- 

 bility a chemical velocity. If, on the other hand, the want of 

 proportionality is due to the effect of convection in modifying the 

 reaction-velocity, it is clear that K D cannot be great in comparison 

 with K, and we must search for a possible error in the assumptions 

 which have led to this conclusion. 



The Part Played by Convection Currents. As Sand has pointed out, 

 " the fact that K increases more rapidly than the catalysor concentra- 

 * Bredig and Miiller von Berneck, loc. cit., p. 312. 



