[Dolphinsiu]

It is well known that conductivity cell is used to measure the conductivity of unknown solution.

Alternative current is used rather than direct current because this avoids polarization and electrolysis - modification of composition in electrolyte that is in contact with electrode. Then, actually do you have some examples or experimental data to support this 'modification'? Also, what's mearnt by polarization?

[allanf]

Polarization is just a general word for when the electrochemical cell deviates from Ohm's law, usually when there is a decrease in current across the cell from what would be expected.

Imagine one has two electrodes in a solution, a conductivity cell, and a potential is applied across them.  Immediately there will be a charging current as electrons accumulate on one electrode and are stripped away from the other.  Leading to one electrode with a net positive and the other a net negative charge.  Ionic species in solution will migrate towards the appropriate electrode (positive goes to negative and vice-versa) creating an electrical double layer, or an accumulation of charge in the solution immediately adjacent to the electrode.  But there is no carrying of charge across the solution between the two electrodes and current would stop entirely.

In this way the solution would come to resemble a capacitor, the potential applied would charge up the electrodes until they were saturated and that's it.

In an electrochemical cell there would be redox chemistry driving a flow of ions through solution, something that doesn't occur in the conductivity cell.

[Dolphinsiu]

I see. Thank you very much!