In a electrochemical cell, why does the anode (where oxidation occurs) have a lower concentration than the cathode?

I know that under standard conditions, the concentration in each half-cell is 1.0 M. However when the cell is not under standard conditions, let's say solution A is 0.1 M and solution B is 1 M. Solution A (0.1 M) will be the anode. Why is this?




Such a rule applies in concentration cell scenario, where the half-cells are the same but concentration.


The half cell (the first kind like M/M+, not the second kind like the reference silver chloride electrode) with lower metal ion concentration has lower potential(approximating activity by concentration):


$$E = E^{\circ} + \frac{RT}{nF} \ln{c}$$


therefore is anode, as oxidation occurs there.


For different electrodes, the one with lower concentration can be an anode or a cathode - case specific, depending on their potentials.


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