When an oxidant accepts electrons, the species formed is in principle capable of donating electrons and acting as a reductant.
The oxidant/reductant pair are known as a redox couple.
The electrode potential indicates the tendency of the oxidant of a redox couple to accept electrons and the tendency of the reductant of the couple to donate electrons. E°(oxidant/reductant) is the electrode potential relative to the potential of the standard hydrogen electrode .
E° is more positive if the oxidant is stronger
E° is more negative if the reductant is stronger
When comparing the strengths of the oxidants or the reductants of two couples:
The oxidant/reductant pair are known as a redox couple.
Fe2+ and Fe3+ are a redox couple as are Fe2+ and Fe.
Note that some species such as Fe2+ can be the oxidant in one couple and the reductant in another. This is because these species are in the intermediate oxidation state for that element.
| Fe3+ | + | e– |  | Fe2+ |
| oxidant | reductant | |||
| Fe2+ | + | 2e– | | Fe |
| oxidant | reductant |
Note that some species such as Fe2+ can be the oxidant in one couple and the reductant in another. This is because these species are in the intermediate oxidation state for that element.
The electrode potential indicates the tendency of the oxidant of a redox couple to accept electrons and the tendency of the reductant of the couple to donate electrons. E°(oxidant/reductant) is the electrode potential relative to the potential of the standard hydrogen electrode .
The couple to which the potential applies is quoted in brackets. The oxidant is given first because the electrode potentials are defined in terms of the reaction in which the oxidant is the reactant.
E°(Fe2+/Fe) = –0.41 V
E°(H+/H2) = 0.00 V
E°(Fe3+/Fe2+) = +0.76 V
E°(Fe2+/Fe) = –0.41 V
E°(H+/H2) = 0.00 V
E°(Fe3+/Fe2+) = +0.76 V
E° is more positive if the oxidant is stronger
(and the tendency for the forward reaction above is greater).
Fe3+ is a stronger oxidant than H+.
Fe2+ is a weaker oxidant that H+.
Fe3+ is a stronger oxidant than H+.
Fe2+ is a weaker oxidant that H+.
E° is more negative if the reductant is stronger
(and the tendency for the reverse of the reaction above is greater).
H2 is a stronger reductant than Fe2+.
Fe is a stronger reductant than H2.
H2 is a stronger reductant than Fe2+.
Fe is a stronger reductant than H2.
When comparing the strengths of the oxidants or the reductants of two couples:
The reductant of the couple with the more negative E° is the stronger reductant.
The oxidant of the couple with the more positive E° is the stronger oxidant.
The oxidant of the couple with the more positive E° is the stronger oxidant.