24 BACTERIOLOGICAL CHEMISTRY 



systems are compared at half complete oxidation or 

 reduction they can be arranged in order of their oxidising 

 or reducing intensities. A system having a certain value 

 of Eo will oxidise (and be reduced by) all systems having 

 a negative or less positive potential, and in its turn will 

 reduce (or be oxidised by) all systems having a more 

 positive or less negative potential. Alternatively, if we 

 know the value of Eh and of Eo for any system it is possible 

 to calculate its state of oxidation or reduction, i.e., the 

 proportion of oxidised and reduced constituents in it. 



Besides the direct electrical method of measuring the 

 electrode potential use is often made of the simpler but 

 less accurate method of employing oxidation-reduction 

 indicators. A number of organic dyes are capable of 

 existence in the oxidised and reduced conditions, the two 

 states being characterised by different colours. The 

 obvious example is methylene blue, which in the oxidised 

 state has a blue colour, but which in the reduced state, 

 known as leuco -methylene blue, is colourless. A consider- 

 able number of other dyes, mostly of the indophenol series, 

 are known which undergo a colour change on conversion 

 from the oxidised to the reduced form. These dyes, like 

 other oxidation-reduction systems, have a characteristic 

 range of Eh from the oxidised to the reduced state, the 

 particular Eh value depending on the proportion of 

 oxidised to reduced dye present. The colour, too, will 

 depend on the ratio of oxidised and reduced components, 

 so that an observation of the colour of such a dye will 

 give information of the degree of oxidation or reduction 

 which it has undergone, and consequently of the Eh 

 value of the mixture. The addition of a small quantity 

 of an appropriate dye to an oxidation-reduction system 

 will serve as an indicator of the Eh value obtaining in the 

 system. A range of dyes which cover Eh changes from 

 about +0-4 volts to —0-01 volts is available, so that by 

 using a suitable dye an estimate of the oxidising or 

 reducing intensity of a system can be made very readily. 



