Conductometry:

Methods which involve the charge transport by the ions in solution (migration) are termed conductometric methods. Within these conditions the resistance of the cell depends upon the velocities of the ions and on the number of migrating ions. It is observed that the conductance of all cells, that is the reciprocal of the resistance of the cell, is approximately proportional to the concentration of the electrolyte. The principle advantage of this technique is its simplicity and relatively good sensitivity, however the selectivity is poor since the conductance of a cell depends on the concentration of all the ionic species in solution.

The principle of the Wheatstone bridge can be used to measure the conductance of solutions. Therefore, the following considerations must also be kept in mind.

i)  Since a direct current would polarize the electrode in the conductivity cell by electrolyzing the solution therefore, to avoid polarization an alternating current usually ac voltages of 3-6 volts with frequency of 50-1000 Hz are used.

ii)  A suitable conductivity cell (with two plating electrodes dipped in the solution) is placed in one end of the bridge in place of an unknown resistor.

iii)  Since the cell is too acts such a small capacitor therefore to balance its capacitive reactance a variable capacitor must be inserted into the bridge.

In the above discussion you have seen that the conductance is an additive property of ions in a solution. This feature is the necessary basis for conductometric titrations, in which the change in conductance is related to concentration changes of the ionic species involved in the titration curve, varies from one type of titration to another.