Conductance and conductivity:
When a field (electric) is put across an aqueous ionic solution by applying a voltage between two parallel plate electrodes, the cations are attracted towards to the negative plate (cathode) and the anion towards the positive plate (anode). This movement of ion in a field is called migration and the movement of charge results in a current in the electric circuit connecting the electrodes. The field between the plates is initially linear and produces a constant ion velocity at all points. If there is no redox reaction, cations and anions will separate and collect at the cathode and anode respectively with time, balancing the electrode charge and there will be fewer ions in the bulk of solution. This is a process known as concentration polarization which also leads to the modification of the original linear potential profile between the two electrodes and a variation of ion velocity between the plates. The current will also fall with time. Concentration polarization is therefore useful when separating ions is define, such as in electrophoresis and electro osmosis, but that is to be avoided when measuring fundamental ion motion.
Fig. 1. The effect of an AC voltage on the ion motion as electrode polarity is switched.
The ions then alternately migrate first to one plate and then the other during cycling, so that the ions oscillate around a fixed position, avoiding concentration polarization.