1047 



result that tlie homogeneous chemical reaction in the adsorbed layer 

 is the real process of which we determine the temperature coefficient. 



Now it is interesting to obseive that Brkdig and Tkletoff (Zeit. 

 Elektrochem. 1906, 12, 583) have calculated the thickness of the 

 diffusion layer from the Nkrnst diffusion expression (Zeit. Phys. Oliem. 

 1888, 2, 634), utilising the data obtained from the decomposition 

 of hydrogen peroxide in presence of colloidal platinum and fonnd 

 the thickness to be 0.05 mm. i.e. of the same order as Bkunnkr 

 found in the case of the dissolution of benzoic acid in water. This 

 seems to show that in spite of the Brownian movement the dilfusion 

 layer remains unchanged. If this is true, the above explanation of 

 the high temperature coefficient in the case of colloids and enzymes 

 breaks down. Moreover, on this point of view, the high values of 

 the temperature coefficient obtained in the oxidations of oxalic acid 

 and phenylthiocarbamide in presence of the solid catalyst blood- 

 charcoal, remain entirely unexplained. 



Looking at the whole problem, it seems probable that in some 

 cases the slow chemical change affects the velocity of the total 

 reaction, whilst in otlier cases, diffusion plays the most important 

 role and it is desirable to investigate fully the kinetics of each 

 individual case. 



(-ertainly much light would be thrown on the whole cpiestion if 

 we can study the kinetics and temperature coefficients of one and 

 the same reaction without any catalyst and in presence of both 

 homogeneous and heterogeneous catalysts. The velocity of decom- 

 position of hydrogen peroxide, for example, may be investigated at 

 various temperatures (1) without any catalyst and (2) in presence 

 of iodides or any other substance soluble in water (compare Bredig 

 and Walton. Zeit. Phys. Ohem. 1904, 47. 185) and (3) in presence 

 of colloidal metals, MnO,, charcoal, solid metals etc. 



Lahoratoire de Chimie Minerale, Sorbonne, Paris. 

 Imperial Colleye of Science. London S. W. 7. 



68 

 Proceedings Royal Acad. Amsterdam. Vol. XXI. 



