I am very new to electrochemistry and ion selective electrodes. The membrane potential of an ion selective electrode is given by $$E=\frac{RT}{nF}\ln{\frac{c_2}{c_1}}$$ Where R is universal gas constant, T temperature in kelvin, n the number of electrons transfered, F Faraday of electricity, $c_2$ the concentration of internal standard solution and $c_1$ is the concentration of the ion to be calculated.
But according to the Nernst equation $$E= E^\circ -\frac{RT}{nF}\ln k$$ Can someone explain how to come up with the membrane potential equation using the Nernst equation?