26 



OXIDATION-REDUCTION POTENTIALS 



The choice of suitable indicators for any given range of potentials may be made 

 after consulting tables 3a and 3b, which are drawn up for pH 7-0, but can equally 

 well be used for other pH values, by using the data given in table 1. The range of 

 Ej, covered by each dye is about 0-1 v. ; that is, from 98 per cent, oxidation (and 

 colour (down to about 2 per cent., the greatest sensitivity being in the centre of 

 the range. Thus in the centre of the range a 10 per cent, difference in oxidation is 

 effected by a change of 0-006 v., whereas to pass from 89 per cent, to 99 per cent. 

 oxidation an increase of over 0-031 v. is necessary. Having chosen a dye covering 

 the required range, however, dyes that are either totally reduced or totally oxidised 

 are useful checks. 



The relation between the percentage oxidation and the electrode potential of 

 various oxidation-reduction potential indicator dyes is represented graphically in 

 fig. 5. 



It may be noted that the curves relating the percentage oxidation and reduction 

 of a system to the oxidation-reduction potential is of the same S-type as the 

 polarographic curves mentioned in Chapter VIII. 



In the polarographic curves the electrode potential applied to the dropping 

 electrode is related to the current flowing in the circuit. In both sets of curves, the 

 half-way point of inflection corresponds to the electrode potential of the half-oxidised, 

 half-reduced system, but in the oxidation-reduction potential curves, we are dealing 

 with the 'proportion of the oxidised and reduced forms, whereas, in polarographic 

 measurements the total concentration of the system determines the scale of the curve. 



TABLE 5 



Oxidation-reduction Indicator Dyes 



Eo values (potentials of half reduced dyes) at pH 7-0 



