20 OXIDATION-REDUCTION POTENTIALS 



per cent, in the oxidised form and 9 per cent, in the reduced form. Conversely — and 

 this is of importance at the present stage — given a certain degree of oxidation the 

 potential corresponding to this can be calculated. If Xq be a coloured dye which is 

 colourless when reduced, the amount of colour in the dye solution indicates the 

 percentage of oxidation and hence the potential. Therefore, if the dye is added to 

 an oxidation-reduction system, and the colour of the dye indicates 50 per cent, 

 reduction, the system has Eh -f 0.1 volt. Hence, with a series of dyes changing 

 colour at different ranges of Ejj , a scheme of Eh determination is possible, directly 

 comparable to pH determination by the use of indicators. 



The dyes used have each a range of about 0.1 volt covering the change of the 

 coloured to the colourless form. Clark and his collaborators (1923-1926) have 

 studied a considerable number of dyes which behave as oxidation-reduction indicators 

 in this way. The majority of these dyes are indophenols or derivatives of indigo and 

 the colour of the oxidised form may be traced to the quinonoid structure of the 

 benzene ring. To take the simple case of hydroquinone : quinone 



ionisation oxidation 



CeH^ (0H)2 ^=^ CeH.O^e© ^==^ CsH.O^ 

 Hydroquinone ^ anion ^ quinone. 



In the diagram below each single valency bond is represented by a pair of 

 electrons and the mechanism of the oxidation is represented as involving the 

 intermediate formation of the secondary anion (single lines in the formulae represent 

 electrons and not valency bonds) : — 



H 



II II 



I I S 



H = c^ C=H H=C^ C=H H=-C, C=H 



c "c c 



II li (III 



m ? f2J ? f3J ^ 



H 



Ftg. 4 



Hydroquinone System 



(1) Hydroquinone, 34 electrons, colourless 



(2) Anion, 34 electrons, colourless 



(3) Quinone, 32 electrons, coloured 



A similar mechanism may be postulated for the oxidation-reduction indicators ; 

 in such cases as that of methylene blue nitrogen plays a similar part to that of oxygen 

 in the above scheme. The theories of the origin of colour and the formulation of 

 the electronic changes involved in the oxidation and reduction of these dyes are, 

 however, outside the scope of this treatise, and their application to the determination 

 of Eh is of greater importance. 



