USE OF INDICATORS 



25 



The electrode potential equation referring to the oxidation-reduction indicators is 



E, = E. + fiT- [0-1 



2F 



In 



so that at any pH and at 30°C. this becomes 



Eh = Eo + 0-03 log 



[Red.] 



[Ox.] 

 [Red.] 



It is obvious therefore that the useful range of a dye will depend on its Eo, 

 the potential at 50 per cent, reduction at the given pH. The useful range extends 

 about 0-05 volt above and below this potential. Thus, if the Eo of a dye at a given 

 pH is -f 0-1 volt, the dye will be about 98 per cent, oxidised at + 0-15 volt and 98 per 

 cent, reduced at + 0-05 volt. 



+ 2 - 



^0 

 < 



75% 50% 



OXIDATION 



Fig. 5 

 Relation between Ej, and Percentage Oxidation of Various Dyes 



AT pH 7-4 



I. o — Chlorophenol indophenol V. Methylene blue 



II. Thymol indophenol VI. Indigotetrasulphonate 



m. 1 — Naphthol— 2 — sulphonate VII. Indigotrisulphonate 



indophenol VIII. Indigodisulphonate 



IX. Indigomonosulphonate 



IV. Thionine 



