The flow of electrons can be measured and is called redox potential or electromotive force . An element is found to be in its highest degree of oxidation when being in the compound that is poorest in energy. To depict redox reactions consistently is a common standard needed, whose potential has arbitrarily been defined as zero. All other potentials are then referred to this standard. The redox potential of any redox pair can now be measured and related to the standard reduction potential.
Positive signs characterize systems that have the tendency to take up electrons.
Biochemical reactions are usually not measured at pH 0, but at the more physiological pH of 7 instead. The values obtained under these altered conditions are marked by the symbol E' 0 .