The Viologen Indicators

The tabulation gives the normal potentials of the various indicators at 30 degrees C.; referred to the normal hydrogen electrode, the accuracy is estimated to be +/-0.002 volt. Normal potentials of the viologens at 30 degrees 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 = E(o) - 0.06 log See PDF for Equation where E(o) is the normal potential according to the above tabulation. This normal potential is independent of pH.