The tabulation gives the normal potentials of the various indicators at 30°C.; referred to the normal hydrogen electrode, the accuracy is estimated to be ±0.002 volt.
Normal potentials of the viologens at 30°C.:
Methyl viologen –0.446 volts
Ethyl viologen –0.449 volts
Betaine viologen –0.444 volts
Benzyl viologen –0.359 volts
Supposing some solution brings about a coloration of one of these indicators to the extent of A per cent of the maximum color, the oxidation-reduction potential of this solution is E = Eo – 0.06 log
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where Eo is the normal potential according to the above tabulation. This normal potential is independent of pH.