The potential difference produced between the electrode and the solution in a half cell. It is not possible to measure this directly since any measurement involves completing the circuit with the electrolyte, thereby introducing another half cell. Standard electrode potentials E⦵ are defined by measuring the potential relative to a standard hydrogen half cell using 1.0 molar solution at 25°C. The convention is to designate the cell so that the oxidized form is written first. For example,Pt(s)|H2(g)H+(aq)|Zn2+(aq)|Zn(s)The e.m.f. of this cell is –0.76 volt (i.e. the zinc electrode is negative). Thus the standard electrode potential of the Zn2+|Zn half cell is –0.76 V. Electrode potentials are also called reduction potentials. See also electromotive series.