while using nerst equation, some time concentration of analyte is expressed in terms of concentration, and some time in terms of activity, how to guess , under which conditions which one should be used.
In general activities should be used in the Nernst equation, however calculating activities can be tedious as the activity coefficients can only be approximated or are unknown. Therefore the conditions under which the electrode potentials are determined should be specified. If your problem for example involves a redox couple then you can use the formal potential to evaluate the ratio of activity coefficients. To do this set up a half cell were the concentrations of Ox and Red are equal such that their ratio is unity. In this case the measured potential will be equal to the standard potential and the nernstian term involving only the activity coefficients. Or better the factors which are responsible for the deviation from using only concentration. Indeed solvent interaction, complexation etc may cause far greater deviations than ionic strength. Therefore it's important to write the full reaction equilibrium including as much as is known about products and reactants.
Best regards, Mike
Activity represent effective concentration of a substance in non ideal solutions (eg. concentrated solution). Whereas concentration used in case of dilute solutions.
https://www.quora.com/What-is-the-difference-between-activity-and-concentration-of-a-substance
I studied the relationship between the activity and concentration using MD simulation before, for more details, please go to my paper "Investigation of concentration-dependence of thermodynamic properties of lanthanum, yttrium, scandium and terbium in eutectic LiCl-KCl molten salt".
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