Any oxidation-reduction (redox) reaction can be divided into two half-reactions: one in which a chemical species undergoes oxidation and one in which another chemical species undergoes reduction. If a half-reaction is written as a reduction, the driving force is the reduction potential. If the half-reaction is written as oxidation, the driving force is the oxidation potential related to the reduction potential by a sign change. So the redox potential is the reduction/oxidation potential of a compound measured under standards conditions against a standard reference half-cell. In biological systems the standard redox potential is defined at pH – 7.0 versus the hydrogen electrode and partial pressure of hydrogen = 1 bar.