The redox potential is the voltage difference at an inert electrode immersed in a reversible oxidation-reduction system. Redox potential measures the affinity of a substance for electrons compared with that of hydrogen, whose redox potential is set at 0. Strong oxidizing agents have a positive redox potential; strong reducing agents have a negative redox potential. Electrons move spontaneously towards those compounds with the more positive redox potentials.
The standard for the whole scale is pure hydrogen gas at 1 atmosphere pressure in contact with a platinum electrode immersed in 1 molar acid. This combination is called a standard hydrogen electrode (SHE) and is defined to have a redox potential of zero.