Zn2+ + 2e" = Zn


CNO" + H2O + 2e" = CN" + 2OH"


Na+ + e" = Na


Cr2O72" + 14 H3O+ + 6e" = 2 Cr3+ + 21 H2O

EOx2/Red2 = 1-330

The reaction is not complete at pH 4.0 but is substantially complete at pH 2.0, assuming that a concentration of 10"6 M represents completion.

In industrial and laboratory work, ORP cell potentials are measured, not against a SHE but against an Ag/AgCl,

4 M KCl reference, EOx/Red = +0.199 V, or a saturated calomel electrode (SCE) whose E° = 0.244 V. The fol lowing equation converts to the potential measured, designated as EAgQ ref

For example, the E° for the cell is:

Fe3+ + e" = Fe2+ EOx/Red = 0.770 vs SHE 7.7(6)

and the potential measured versus the silver-silver chloride reference is:

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