Half-reactions relative oxidizing abilities

All electrochemical cells involve two half-reactions an oxidation half-reaction in which electrons are released, and a reduction half-reaction in which electrons are taken up. The net voltage of the cell, the only quantity that can be measured experimentally, is the algebraic sum of the potentials for the two half-reactions. Each potential is a measure of the relative ability of a given half-reaction to occur. However, since the potential for a half-reaction cannot be measured directly, numerical values for half-reaction potentials are arbitrary and must be based on a reference potential. The hydrogen half-reaction serves as the reference for all electrochemical potentials. 

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