Calculation of cell potential activities or concentrations

Although the use of activities in the Nernst equation is undoubtedly correct, it is worth considering whether it is necessary and what is the difference between activities and concentrations in general. 

In the context of this book, a detailed discussion of activities and concentrations is not justified. However, it is clear that in relatively concentrated solutions there will be interionic interactions that do not 

It is often useful to employ concentrations instead of activities in electrochemical experiments for example, in preparing solutions we use masses and volumes, that is we determine the concentration of a solution. Thus, the Nernst equation, instead of being written as 

In this last equation, E is the formal potential. It is related to the standard electrode potential, K , by 

An example of the difference between values of FT0- and E is the values obtained in the potentiometric titration of Fe2+ with Ce4+ in 0.5mH2SO4. These are, relative to the normal hydrogen electrode (NHE)  

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