I am using the resistive pulse technique to measure the size of nanoparticles. To do so, a nanopore of known size and shape is placed in an electrolytic solution (PBS, which is mostly NaCl). An Ag/AgCl electrode (silver wire pasted with a Ag/AgCl paste) is placed in the solution on each side of the nanopore. A DC voltage is applied and the current is measured (see attached figure).

From my understanding Ag/AgCl, like any other metal/metal-salt, electrodes are used because they do not involve changes in the concentrations of ions in and electrolyte containing Cl- ions. Also a Ag/AgCl electrode is non-polarizable, so keeps its potential regardless of the current flowing through it. The reversible redox-reaction that is happening in one direction on the anode in the other direction on the cathode is the following: AgCl + e- = Ag + Cl-

So far so good, but what actually happens to the Na+ ions? Obviously they will migrate in the electric field towards the negative electrode.

But what happens then?

Are the Na+ ions getting reduced at the anode and build a layer on top of it?

If so, what's the corresponding oxidative reaction on the cathode to close the circuit?

The actual current I measure in such an experiment agrees very well with the theoretical value by using the measured conductivity of the electrolyte (PBS, mostly NaCl) and then calculating the resistance of the system from the known nanopore geometry. So the current must be transported through the pore by both ion species, which then has to be converted from an ionic current to an electric current through redox reactions, as far as I understand. The conductivity was measured with a conductivity meter with a two electrodes sensor (stainless steel electrodes). I believe a conductivity meter uses low voltage AC to determine the conductivity of the sample by comparing it to a standard solution of known conductivity. In this case, no redox reactions actually happen on the electrodes as the voltage is much too low. So it's a quite different system from my setup. What would happen if I would use stainless steel electrodes in my setup?

As a side question: How do I know that my electrodes are still working properly? They will degrade over time I assume. Could I use a commercial Ag/AgCl electrode and measure the potential between it and one of my electrodes with a potentiostat. If the measured voltage is close to 0mV then I know my electrodes are still working fine, right?

Apologies in advance for the lengthy and maybe trivial question. I am quite new to electrochemistry and appreciate any sort of input and help. Thank you!