22 



OXIDATION-REDUCTION POTENTIALS 



TABLE 3 



Vabiation of Potential with Percentage Oxidation, at Constant pH (30° C.) of Dyes 



Eh = Ei + k 



In table 2 are given the EJ, values of a number of indicators at different pH. 

 At any given pH therefore the EJ of the indicator can be immediately found. From 

 the usual equation the change of potential at various degrees of oxidation or 

 reduction can be calculated ; these values are smnmarised in table 3. The applica- 

 tion of these data is simple. Let us suppose that to a system at pH 7-8 there be 

 added a solution of indigo tetra-sulphonate, and colorimetric comparison of the 

 depth of colour indicates that the dye is 75 per cent, reduced {i.e., the dye has only 

 one-quarter of its full colour). From table 2 the E,^, value is — 0-077 volt at pH 7-8, 

 and since the dye is 75 per cent, reduced the potential is 0-014 volt lower (more 

 negative) than the E,^ value (from table 3), therefore the E^ of the system is 

 — 0-077 —0-014 = — 0-091 volt. From the tables it can be ascertained that 

 indigo tri-sulphonate should be 15 per cent, reduced at this level of potential {i.e., 

 the dye should have 85 per cent, of its full colour when added to the system). The 

 result obtained with the tetra-sulphonate can, therefore, be checked by observation 

 of the behaviour of the tri-sulphonate. 



