OXIDASES, PEROXIDASES, AND CATALASE 93 



the reaction through radicals is the main point of the concept, while 

 the propagation of reaction chains through such radicals, although 

 an interesting phenomenon, is of secondary importance. In fact, the 

 two features are not necessarily associated with each other. The 

 detailed study of the kinetics of the decomposition of hydrogen 

 peroxide by ferrous and ferric salts suggests strongly that under 

 certain conditions the radicals may give rise directly to the formation 

 of the end products, water and oxygen, without initiating a chain 

 by further reacting with hydrogen peroxide and thereby repro- 

 ducing themselves. Thus the catalytic breakdown of hydrogen per- 

 oxide by ferri ions in acid medium does not, in general, represent 

 a chain reaction; the oxygen is released here by the process 



(3) Fe^^^ + Ha = Fe*" + H^ + O2 



Upon slightly changing the experimental conditions, e.g., by 

 increasing the hydrogen peroxide concentration or decreasing the 

 Fe'^^'^, reaction 2 is favored, with the result that chains appear (18). 



The second question relates to the way in which the central 

 process, consisting of steps 1 and 2, is initiated by the various 

 catalytic agents. The radicals OH and/or HO, can be created only 

 by a monovalent attack on hydrogen peroxide. It is the monovalent 

 character of the primary reaction of the catalyst with the substrate 

 which, according to Haber and Weiss (18, 78), is the common feature 

 of the chemical, photochemical, and electrochemical primary proc- 

 esses. Thus the radicals may arise by the transformation of a metal 

 ion into the one of next higher valency, as in the instance of ferrous 

 ions, 



(4) Fe"" + n^O. = Fe"^^ + OH- + OH 



or by the reduction of a higher valent state, as in the case of ferric 

 ions: 



(5) Fe^*^ + HO=- = Fe^" + Ma 



Hydrogen peroxide may also be split into two OH radicals by 

 ultraviolet light: 



(6) H202 + hv = 2 OH 



In heterogeneous systems, involving metal surfaces, the radical OH 

 is beUeved to result from a simple electron transfer, 



(7) H,02 + electron („,etai) = OH- + OH 



While reaction 7 is an expression of the oxidizing action of H2O2, 

 the reducing properties of the molecule are attributed to its anion 

 (HO2"), in accordance with the equation 



