29) Lets first write a general half reaction: Red --> Ox + ne(-). We can now write the expression for E:
E = E0 - RTln m(Ox)/m(Red)
We can see that when E = E0, RTln m(Ox)/m(Red) = 0 so the ratio of the molalities of the oxidized and reduced species is 1. E0 is called the midpoint potential because it is the potential in the middle of a titration between entirely oxidized compound and entirely reduced compound. It is the point where the concentrations of the oxidized and reduced species are equal.