CHAPTER II 



PRACTICAL METHODS OF E^ MEASUREMENT 



Naturally the most reliable method of measuring E^ is the direct electrometric 

 determination of the potential and this will be dealt with in the latter part of this 

 chapter. The universal application of pH measurements is undoubtedly due in 

 large measure to the simplicity of indicator methods and the first part of this chapter 

 will deal with the present position in the use of dyes as oxidation-reduction potential 

 indicators, 



USE OF INDICATORS 



Let Xr be the reduced form and Xq the oxidised form of the reversible oxidation- 

 reduction system, 



oxidation 

 Xr ^ =^ Xo+2 electrons 



reduction 



The pH is kept constant throughout so that the general electrode potential 

 equation is 



If the temperature throughout is kept constant at 30°C. the expression is simplified 



[Xo] 



Eh = Eo + 0.03 log 



[XJ 



If the potential of the 50 per cent, oxidised system {i.e., [Xq] == [XJ) be + 0.1 volt, 

 then Eo = +0.1 volt, and 



Eh = -f 0.1 + 0.03 log ^ 



From this we can calculate directly the potential at different stages of oxidation of X. 

 Thus, when 98 per cent, oxidised (2 per cent, reduced) the potential will be (in volts) 



98 

 Eh = 0.1 + 0.03 log ^ = + 0.15 volt. 



In this way the following table can be constructed : — 



Practically colourless 



50 per cent, coloured 



Practically fuU colour 



The oxidation-reduction condition of the system can be read off from a deter- 

 mination of the potential. Thus, if the Eh were + 0.13 volt, the system would be 91 



