OXIDATION AND REDUCTION 583 



the other part into molecular oxygen, according to the relative amount of the two enzymes 

 present. Catalase might thus act as a regulator of the oxidation process. 



Peroxides are characterised by the presence of two atoms of oxygen, directly I 

 united together. If we take oxygen as quadrivalent, such a- substance as hydrogen ' 

 peroxide contains two atoms of oxygen united by three valencies, and when one 

 atom is split off in the active form, this atom possesses four free valencies to be I 

 satisfied by combination with an oxidisable substance. Or we may put it thus, I 

 in the formation of the peroxide, the valency of the oxygen is changed from two ^ 

 to four. A substance gives none of the reactions of a peroxide, however many 

 oxygen atoms it may contain, unless some are directly connected together. 

 Persulphuric acid, HSO 4 - O 4 SH, thus behaves as a peroxide, similarly other per- 

 acids and their salts. Since the atoms of molecular oxygen are united together, 

 it seems that, when it combines with an autoxidisable substance, the primary 

 product must be a peroxide, as is assumed in the equation given above (page 581). 

 At the same time, ordinary oxygen has no peroxide properties, -although 

 by the addition of another atom to make ozone, we obtain a powerful peroxide. 

 In a peroxide, therefore, the two atoms directly united must, apparently, be 

 themselves united to some other atom or group, which may be oxygen itself. 

 The meaning of this is not clear. These groups rnay, indeed, be either " electro- 

 negative," as in persulphuric acid, or "electro-positive," as in sodium peroxide, 

 NaO - ONa. 



Most peroxides are hydrolysed by water in two stages, thus : 



- (I) 



AO + HOOH - (2) 



Therefore, a peroxide, arising by autoxidation of an oxidisable substance produced 

 by a cell, gives rise as a rule to the formation of hydrogen peroxide. 



Catalytic Activation of Peroxides. Indigo blue is very slowly oxidised in air, 

 presumably with the usual production of a peroxide. Oil of turpentine is 

 oxidised in a similar manner at a considerable rate. In the presence of the latter, 

 the oxygen of its peroxide is transferred to the indigo, which thus undergoes a 

 rapid oxidation. 



Now Bach holds (1913, p. 148) that the oil of turpentine should be called a catalyst in this 

 reaction, since it does not appear as a constituent of the oxidised indigo, although it is not 

 itself in its original form at the end of the reaction. I think, however, that it tends to 

 confusion to speak of a catalytic process, where energy for a reaction is afforded by the agent 

 called catalyst, as in this case, and that it is better to call it a coupled reaction. At the same 

 time, it must be confessed that it is difficult to draw a very marked line of demarcation 

 between this kind of reaction and that of the acceleration of the action of hydrogen peroxide 

 on hydriodic acid by molybdic acid. The only essential difference is that in the latter, true 

 catalytic action, the catalyst is recovered as in the beginning. This must be considered to be 

 the real criterion, for even when the catalyst is not recovered intact, the change it has under- 

 gone is independent of the main reaction, merely incidental, whereas the oxidation of the oil 

 of turpentine is an essential part of the reaction. 



The cases where colloidal platinum and related metals act as catalysts are 

 regarded by Bach (1913, p. 149) as precisely similar to that of indigo and oil of 

 turpentine, since a peroxide like that of oil of turpentine is. supposed to be formed, 

 but, in this case, becoming metal again. It seems to me, however, that these 

 phenomena of heterogeneous catalysis are not, as yet, satisfactorily explained by 

 the hypothetical assumption of various oxides of the metal, for whose actual 

 existence there is not sufficient evidence. There is the fundamental difference, 

 also, that the energy needed to raise the oxidation potential of the system is not 

 afforded by chemical degradation of the platinum, as it is in the case of oil of 

 turpentine and similar cases of autoxidation. The bearing of this question on the 

 nature of Bach's " oxygenase " will be seen presently. 



The process of autoxidation results, then, in the production of a peroxide and 

 in the simultaneous oxidation of certain other substances present in the system, 



