The electrochemical reduction of a species by Single Electron Transfer from an electrode can be assigned a voltage with respect to a standard hydrogen electrode. During a reduction half reaction a species gains one or more electrons and these appear of the left hand side of the equation. By convention, redox half reactions are compared as the reduction half reaction. For more information look in the Chemogenesis
webbook sections on redox
chemistry and substitution,
transfer, abstraction, displacement (STAD).