Oxidation and Reduction Sign Conventions

There is obviously a slight problem with the scheme outlined above, and that is that if we add the voltages in (18.11) and (18.12) we do not get the cell voltage of the zinc-copper cell (f ceii + 4ii) because the hydrogen electrode voltage does not cancel but is doubled. That s why we qualified the presentation with if we arrange things properly . 

The lUPAC recipe for half-cell manipulations s first, half-cells are always written and tabulated as reductions when combined with the SHE and second, calculate the cell voltage as the right (reduction) half-cell voltage minus the left (oxidation) voltage, whether using the SHE or not. This is the opposite of the way we wrote equations (18.7) and (18.8) for the zinc-hydrogen cell, so we rewrite them (according to the first rule) as 

Considering only standard conditions for now, if is the standard zinc halfcell potential and is the SHE potential, then by the second rule above, the cell potential is 

This cell has a measured potential of —0.763 volts (i.e., the standard zinc electrode is 0.763 volts more negative than the SHE) so 

Substituting Cu for Zn we have the copper-hydrogen cell, with 

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