measurement of an unknown resistance can be performed on a Wheatstone bridge. However, for measuring the resistance of an ionic solution we face two problems. Firstly, passing direct current (DC) changes the composition of the solution. Secondly, a solution cannot be connected to the bridge like a metallic wire or other solid conductor.
The first difficulty is resolved by using an alternating current (AC) source of power. The second problem is solved by using a specially designed vessel called conductivity cell. It is available in several designs and two simple ones are shown in Fig. .
. . . Measurement of the Conductivity of Ionic Solutions Connecting wires Platinized Pt electrodes Platinized Pt electrode Platinized Pt electrode Connecting wires Fig.
. Two different types of conductivity cells. Basically it consists of two platinum electrodes coated with platinum black (finely divided metallic Pt is deposited on the electrodes electrochemically). These have area of cross section equal to ‘ A ’ and are separated by distance ‘ l ’.
Therefore, solution confined between these electrodes is a column of length l and area of cross section A . The resistance of such a column of solution is then given by the equation: R = r l A = l A ( . ) Table . : Conductivity and Molar conductivity of KCl solutions at .15K mol L – mol m – S cm – S m – S cm mol – S m mol – .
.00141 . . . × – Concentration/Molarity Conductivity Molar Conductivity The quantity l / A is called cell constant denoted by the symbol, G *.
It depends on the distance between the electrodes and their area of cross-section and has the dimension of length – and can be calculated if we know l and A . Measurement of l and A is not only inconvenient but also unreliable. The cell constant is usually determined by measuring the resistance of the cell containing a solution whose conductivity is already known. For this purpose, we