1905.] Catalysis of Hydrogen Peroxide ly Colloidal Platinum. 571 



In solutions of hydrogen peroxide so dilute that the oxygen evolved 

 would be insufficient to saturate the solution, we would expect K to be 

 proportional to the catalysor concentration, and independent of the 

 peroxide concentration. This has not been directly determined for 

 platinum, but Sand,* by collating results in different parts of Bredig's 

 paper, has shown that in solutions containing very little platinum, K is 

 approximately proportional to the platinum concentration, 'it is 

 strictly true of the hsemase catalysis ; f only when the evolved oxygen 

 is more than sufficient to saturate the solution do deviations from the 

 simple logarithmic formula appear. 



The fact that all the observations in the platinum and hsemase 

 catalysis of hydrogen peroxide are thus satisfactorily accounted for on 

 the hypothesis of convection currents, seems a strong argument in 

 favour of the view that the latter play a considerable part in the action. 



Possible erroi's in the assumptions used in calculating K D . If the above 

 views as to the part played by convection currents are accepted, we 

 must investigate the assumptions which have led to the conclusion that 

 the value of K D is great in comparison with that of K. Sand 

 arrived at this result by substitution of probable minimum values for 

 k and v, and a maximum value for L, the diameter of a particle, in 

 Equation (5), K D = 12%/IA 



There would seem to be at least three possible sources of error in 

 the calculation of a numerical value for K D . (1) The value 

 (10~ 5 cm. 2 per sec.) assumed for k, the coefficient of diffusion of 

 hydrogen peroxide, may be too large. (2) The average diameter L of 

 the particles may be greater than the maximum value assumed by 

 Sand (0*5ju). (3) Only part of the platinum surface may be active 

 towards hydrogen peroxide ; the value taken for v may thus be too 

 large. 



These three possibilities will now be shortly considered : 



(1) Although the diffusion coefficient of hydrogen peroxide has not 

 been determined, there can be very little doubt, from the values obtained 

 with substances of similar molecular weight, that the value assumed for 

 k is not too large, so that an error from this cause is very unlikely. 



(2) The size of the particles. Since, according to Equation (7) K D varies 

 inversely as the square of the diameter of the particles, the velocity 

 calculated on Nernst's hypothesis would correspond with the velocity 

 actually observed if the average diameter of the particles be four times 

 the maximum value (O'Sju) assumed by Bredig and Sand. BredigJ 

 mentions that his solutions showed no suspended particles when 

 examined under a microscope capable of detecting objects of the 

 diameter 0'2/*. Although it ought to be borne in mind that particles 



* Loc. cit. 



f Senter, ' Zeit. physikal. Chemie,' vol. 44, p. 257, 1903. 



1 ' Anorganische Fermente,' Leipzig, 1901, p. 21. 



