CHAPTER XX 

 OXIDATION AND REDUCTION 



IN the preceding chapter we have seen how, by the aid of light energy, the 

 chemically stable system of CO 2 + H 2 O is converted into one of higher potential 

 energy, carbohydrate and oxygen. In reconversion to its original state, the energy 

 of this system is utilised by living organisms for various purposes. But, although 

 the system possesses considerable potential energy, it is, chemically, a stable one. 

 This is, indeed, necessary, in order that its energy should not be given off 

 spontaneously at all times, but only when required. Molecular oxygen is unable i 

 to oxidise carbohydrate, except at an extremely slow rate ; although there are ( 

 certain substances, such as simple aldehydes and those compounds called "unsatur- 

 ated," which are "autoxidisable," that is, capable of oxidation by molecular 

 oxygen. In this process, moreover, by a mechanism which will require discussion 

 later, other substances, not themselves oxidisable by molecular oxygen, undergo 

 simultaneous oxidation ; we have a " coupled reaction." 



This mechanism alone, however, will not satisfy the requirements of the case. ( 

 We have, accordingly, a catalytic mechanism in addition, which has the effect \ 

 itself of " activating " oxygen. 



It may be remarked here that the processes of oxidation and reduction are not merely of 

 use for the purposes of obtaining energy by complete combustion. Intermediate stages result 

 in the formation of substances required for use in chemical reactions of importance for other 

 purposes. The monograph by Dakin (1912) will serve to show the numerous cases of interest 

 in this respect. 



In the discussion of the question it must not be forgotten that the oxidation 

 of one substance is always accompanied by the reduction of another. As Hardy 

 points out (note appended to Drury's paper, 1914, p. 175), the place where oxida- 

 tion takes place in a cell may also be a reduction place, if a different zero of 

 oxidation potential be taken. A convenient one is that of atmospheric oxygen. A 

 region of such a chemical potential would be a reduction place for compounds 

 whose oxygen potential is higher than that of atmospheric oxygen, but an oxidation 

 place for substances in which it is less than that of atmospheric oxygen. The 

 absence of agreement as to the ZCFO may lead to confusion. 



ACTIVE OXYGEN 



The first question to be investigated is the nature of the state into which 

 oxygen is put, so as to be able to oxidise substances upon which it has no action 

 in its ordinary molecular state. 



It is sometimes stated that it is in the " atomic " state, but this suggestion 

 does not really help much, since we do not actually know what the difference 

 between the atomic and molecular state is. 



Again, the active state is sometimes spoken of as the " nascent " condition. 

 It is a matter of experience that chemical elements or groupings are more ready 

 to enter into combination at the moment of their liberation from previous combina- 

 tion. It appears that, in the process, chemical energy is made use of before it . 

 has become degraded into heat, hence more free energy is available. 



The most probable view seems to be that it is in the process of changing its I 

 valency, or electric charge, that oxygen is in the active state. At all events, I 



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