Redox Titrations and Oxidation Numbers

The oxidation of an element is characterized by an increase in its oxidation number, and its reduction is represented by a decrease. An oxidant is a species whose oxidation number decreases during the course of a redox reaction, and a reductant is a species whose oxidation number increases. The coupling of two half-redox reactions for interpreting a redox reaction can be extended in terms of oxidation numbers. An example is provided by the reaction of the reduction of carbon dioxide by dihydrogen to give carbon monoxide  

We can see that in the carbon dioxide, the carbon exhibits the oxidation number -hIV, whereas in the carbon monoxide it does it in the oxidation state +11. During the course of the reaction, hydrogen goes from state 0 to state +1. The two fictitious half-redox couples involved are 

The last sentence points out that an ambiguity can exist in the case of the oxidation or reduction of a polyatomic species without any supplementary information. Indeed, the element that undergoes oxidation or reduction must be named. Two interesting examples are provided by the couples of water. The first one corresponds to the half-redox reaction 

Usually, we speak of water oxidation, but, actually, it is the oxidation of the oxygen element, which in water exhibits the state —II and which is in the state 0 in the 

No ambiguity exists in the first half-redox equation The redox couple under study is H+/H2(g). However, according to the second half-reaction, there is an ambiguity when the reduction of water is involved. It would be more appropriate to speak of the reduction of the hydrogen element in water. 

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