4
$\begingroup$

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?

$\endgroup$
1
$\begingroup$

The Nernst Equation is $E = E^\circ - \frac{RT}{nF} \ln{Q_c}$. From that you can derive the approximation you gave. $Q_c$ is the product of the activities of the products over those of the reactants. This can be, for dilute solutions where activity approximates concentration, the typical [B]/[A] for the reaction A→B. So, the only thing you need to "explain" is why E° would be zero. Review the definition of E°. hint:What is ln(1/1)? See the discussion on concentration cells in: http://www.chem1.com/acad/webtext/elchem/ec4.html

$\endgroup$
  • $\begingroup$ If the concentration of the solutions is different, then their activities are also different (since concentration is a measure of activity). Why E° value is zero? (please correct me if I am wrong) $\endgroup$ – Aurges Sep 5 '16 at 3:48
  • $\begingroup$ $E^\circ$ only depends on the standard states, not on actual states. It thus doesn't really depend on the activities of anything. For most ion-sensitive electrode applications, there is no electrochemical cell across the ion-selective membrane. $\endgroup$ – Curt F. Dec 3 '16 at 21:41
  • $\begingroup$ Also there is a misplaced $nF$ in this answer. I'll edit to move it, but make sure you agree with this change please. $\endgroup$ – Curt F. Jan 3 '17 at 4:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.