Redox couples and electrode potentials
Redox couples are two species that contain the same atom in different oxidation states.
The principles below are illustrated using the HSO4–/H2SO3 couple.
The standard electrode potentials (E°) of two different couples can be used to compare the strength of the two reductants and the two oxidants.
Redox reactions are product-favoured if
The diagram of the oxidation states of sulfur shows several redox couples.
| S(+6) HSO4- | oxidant  | |
| S(+4) H2SO3 | reductant | oxidant |
| S(0) | reductant | oxidant |
| S(-2) H2S | reductant |
The principles below are illustrated using the HSO4–/H2SO3 couple.
The species containing the atom in the higher oxidation state is the oxidant of the couple (HSO4-).
The species containing the atom in the lower oxidation state is the reductant of the couple (H2SO3)
The species containing the atom in the lower oxidation state is the reductant of the couple (H2SO3)
The standard electrode potentials (E°) of two different couples can be used to compare the strength of the two reductants and the two oxidants.
The oxidant in the couple with the more positive potential is stronger.
The reductant in the couple with the more negative potential is stronger.
The reductant in the couple with the more negative potential is stronger.
Redox reactions are product-favoured if
- one reactant is an oxidant and the other reactant is a reductant and
- the electrode potential of the couple containing the oxidant reactant is more positive than the electrode potential of the couple containing the reductant reactant