What is the effect of concentration on \(E^o\) ?
Consider the following standard electrochemical cell potential
where [Cu^{2+}] = [Ag^{+}] = 1 M.
If [Ag^{+}] > 1 M, the equilibrium will shift to the right according to Le Chatelier’s principle. This implies that the reaction has become more spontaneous from left to right, resulting in a more positive E^{o}. Conversely, if [Ag^{+}] < 1 M, the equilibrium will shift to the left, giving a less positive E^{o}. In general, the effects of concentration of chemical species on the electrochemical cell potential is given by the Nernst equation:
where

 E is the potential of the cell under nonstandard conditions, e.g. when T is not 298 K or concentrations of species are not at 1 M and 1 atm.
 E^{o} is the overall standard electrode potential of the cell.
 R is the gas constant
 T is temperature
 F is the Faraday constant
 Q is the reaction quotient, which for the above reaction is:
For example, if [Cu^{2+}] = 1.8 M and [Ag^{+}] = 0.02 M at rtp,
It is important to note that the Nernst equation applies to situations where no current flows, i.e. it allows the potential of a cell to be calculated under open circuit situations. This means that the calculated value, E, for the above example, is the predicted potential of the cell (containing[ Cu^{2+}] = 1.8 M and [Ag^{+}] = 0.02 M at rtp), which is connected to an external emf that exactly balances this calculated potential.
Question
What If we want to calculate the potential of a cell where a current flows, e.g. an electrolytic cell ?
Answer
We will have to account for changes in electrode potentials due to a phenomenon called polarisation. Please see this article in the advanced section for details.