3
$\begingroup$

An electrolyte can be electrolysed by a voltage higher than the reverse reaction’s cell potential, while conducting a current in the process. But what happens at the electrodes when the applied voltage is below this cell potential, such as below 1.23 V for water or 4.07 V for NaCl? Does the reverse reaction occur simultaneously, or is the electrolyte unable to conduct a current with such a low voltage at all?

$\endgroup$
1
$\begingroup$

Electrolysing water is NOT required for it to conduct electricity. Water ionizes into $\ce{H+}$ and $\ce{OH-}$. Pure water will contain $10^{-7}\ \mathrm{M}$ of either ion at room temperature at equilibrium. This will result in $18\ \mathrm{M\Omega\ cm}$ of resistivity. If and when potentials above the electrolysis potential is applied, the local pH around the electrodes will change and more ions will be created.

$\ce{NaCl}$ dissociates as soon as it is dissolved in water. The $\ce{Na+}$ and $\ce{Cl-}$ ions created in the solution carry current and increase the conductivity.

$\endgroup$
  • $\begingroup$ I think the OP also wants to know what will happen to the water or NaCl outside the window. In short, in water, you'll evolve $\ce{H2}$ or $\ce{O2}$ at the electrodes, or $\ce{Na(s)}$ or $\ce{Cl2}$ with $\ce{NaCl}$ depending on oxidizing or reducing conditions. $\endgroup$ – Geoff Hutchison Apr 30 '15 at 15:56
  • $\begingroup$ What happens when these ions meet the electrode? How does the current continue from the ions to the electrode if the potential is not enough to add/remove electrons from the ions? $\endgroup$ – kadabrium May 3 '15 at 23:38
  • $\begingroup$ The concept appears related to chemistry.stackexchange.com/questions/278/… , but neither did answers under that question fully explain the very nature of current in electrolyte and particularly at the metal-electrolyte boundary. $\endgroup$ – kadabrium May 3 '15 at 23:44
  • $\begingroup$ @kadabrium in that case the system will act as a pure capacitor. It will charge until the voltage is fully screened and then the current will drop to zero asymptotically. At DC voltage, there will not be a sustained current without redox reactions $\endgroup$ – Burak Ulgut May 4 '15 at 5:23

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.