20 BACTERIOLOGICAL CHEMISTRY 



oxidation being such a reaction as the combustion of 

 carbon to yield carbon dioxide, 



C + O2 > CO., 



or the combustion of methane to form carbon dioxide 

 and water, 



CH4 + 202 >C0o+2H20. 



Besides this type of oxidation by direct addition of 

 oxygen, there is another in which the proportion of 

 oxygen in a compound is increased by removal of some 

 other element such as hydrogen, a typical example being 

 the oxidation of ethyl alcohol to acetaldehyde, 



CH3.CH2OH — > CH3CHO + Ho, 



or the conversion of hydroquinone to quinone, 



OH 



I 1.1 



+ H.,. 



I II -- > II II 



\/ \/ 



I II 



OH 



Even this does not go far enough, however, and we now 

 recognise that certain reactions in which no oxygen at all 

 is involved are still of the nature of oxidations. This is 

 particularly true where such metals as copper or iron, 

 which have more than one valency, are involved. Thus 

 we regard trivalent ferric salts as more highly oxidised 

 than divalent ferrous salts, even though they may contain 

 no oxygen at all, as is the case with the chlorides. The 

 conversion of ferrous chloride to ferric chloride is an 

 oxidation : — 



FeCl^ + CI ^=^ FeClg. 



Now these reactions, like all chemical reactions, involve 

 the transfer of electrons from one atom to another. When 

 liydrogen is oxidised to water the sino^le electron whicli 



