OXIDATION-REDUCTION POTENTIALS 



Our problem, then, is to find a method of quantitative study of reversible 

 oxidation-reduction systems ; and, fortunately, in electrode potential* measurements 

 we have an exact and useful method. As an example, let us suppose that we have a 

 solution of iron salts partly oxidised and partly reduced, say a mixture of ferrous and 

 ferric chlorides. If an unattackable electrode be immersed in the solution an 

 electrical potential difference will be set up between the electrode and the solution. 

 This potential difference will depend on the relative proportion of ferric and ferrous 

 chlorides. In solutions containing the same proportion of the two salts the potential 

 difference will always be the same ; but if there is more oxidised (ferric) salt the 

 potential difference will be greater, and if more reduced (ferrous) salt is present the 

 potential difference will be less. It is easy to see that after a few preUminary 

 measurements with mixtures of ferric and ferrous chlorides of known composition 

 we have a method of determining the composition of any unknown mixture of iron 

 chlorides. This method can, of course, be applied to other reversible oxidation- 

 reduction systems in the same way, but in order to remove the empirical nature of 

 the observations and to place the method on a sound theoretical basis we must 

 define our terms more closely and apply some physico-chemical algebra. 



The terms oxidation and reduction themselves are not free from ambiguity. 



The term oxidation immediately suggests the addition of oxygen to the oxidisable 

 substance, for example the oxidation of formaldehyde to formic acid, or ferrous to 

 ferric oxide : — 



HCHO + -> H.COOH 



2 FeO + -^ FeaOj 



But such reactions as the conversion of hydro quinone to quinone are obviously 

 oxidations although the oxidant contains no more oxygen than the reductant. 



It is evident therefore that the definition of oxidation must be extended to cover 

 the removal of hydrogen. This extension, however, does not cover all cases ; for 

 example, the conversion of ferrous to ferric chloride must be regarded as oxidation 

 just as much as the conversion of ferrous to ferric oxide, although oxygen and hydro- 

 gen are not concerned in the reaction. 



FeCla -f CI -^ FeClg 



To gain a general \dew of oxidation-reduction processes it is necessary to consider 

 the electronic concept of atomic constitution. Atoms are composed of a nucleus 

 positively charged with electricity and a number of electrons each bearing a unit 

 negative charge. The positive charge on the nucleus is numerically equal, although 

 opposite in sign to the sum of the unit charges on the electrons, so that the atom 

 taken as a whole is electrically neutral. There is one electron in the hydrogen atom 

 so that there is a single positive charge on the hydrogen nucleus to neutralise it. The 

 carbon nucleus possesses six positive charges and the carbon atom contains six 

 electrons. Chemical reactions involve interchanges and re-arrangements of certain 



* The terras oxidation-reduction potential, reduction potential, redox potential and electrode 

 potential, which are used by diiTerent authors, may be taken generally as being synonymous. As far 

 as possible, the expression electrode potential, or Eh, is used throughout this text. 



