Oxidation-reduction reactions and speciation

Oxidation-reduction reactions, often called redox reactions, occur because many elements can occur in more than one oxidation state, and can, therefore, influence chemical speciation. At normal temperatures and pressures, the number of elements involved in such reactions in relatively small. 

Oxidation reactions involve the loss of electrons reduction the gain of electrons. The species accepting the electron is termed the oxidant and the species donating the electron the reductant. The oxidation state of an element is increased after oxidation and lowered after reduction. 

For the reduction of ferric iron, Fe3+, to ferrous iron, Fe2+, one electron is donated by Fe2+, the reductant, and accepted by Fe3+, the oxidant  

A general equation for a redox reaction is Oxidant + e Reductant 

The standard electrode potential E° of a redox reaction is a measure of the potential that would be developed if both reductants and oxidants were in their standard states at equal concentrations and with unit activities. The units of E° are volts and ° can be calculated from the Gibbs free energy change (AG ) of the redox reaction from the relationships 

---