The ability of a half-reaction in a
redox reaction to lose
electrons and so change from the less oxidised to the more oxidised form, a more negative value meaning a greater ability. When measured under
standard conditions, it is referred to as the standard electrode potential.
In the ionic reaction:
Cu2+ + Zn = Cu + Zn2+,
there are two half-reactions: copper/copper(II), and zinc/zinc(II). Zinc is better at being oxidised to zinc(II) than copper is at being oxidised to copper(II), so the system zinc/zinc(II) is said to have a more negative electrode potential.
Such a half-reaction, which can be used to form a half-cell in a battery, does not necessarily consist of a metal and its ions. The two parts of the half-reaction could be non-metallic, could both be in solution, and could
consist of two rather than one species.
Electrode potentials are given the sign E, and are quoted in volts. They cannot be measured absolutely, but instead are taken relative to that of hydrogen, which is given an electrode potential of 0 V. The electrode potential of hydrogen is measured by a reference electrode, a half-cell consisting of hydrogen gas (the less oxidised species) being bubbled over a platinum black surface into a 1 M solution of hydrochloric acid, whose H+ ions are the more oxidised species.
The difference between the electrode potentials of the two half-reactions in a full electrochemical cell is equal to the EMF generated by the cell. Thus the most powerful cells use one half-reaction with a highly positive value for E, and one with a highly negative value.
Bear in mind that, since hydrogen is an arbitrary choice for "0 V", there is no qualitative difference between positive and negative values. The EMF is determined simply by the difference between the values of each half-cell.