Measurement of the Conductivity of a Solution

A direct current cannot be used for measuring conductivity because the electrodes polarize which artificially changes the resistance to be measured. To avoid this problem an alternating current is employed with an appropriate ‘ Wheatstone’ type Bridge. A diagram of a Bridge that can be used for this purpose is shown in figure 1.

Figure 1. An AC Bridge Suitable for Measuring the Conductivity of  a Liquid.

The sample is placed in the conductivity cell and R₁, R₂, and R₃ adjusted until the potential across the bridge as measured by the meter is zero.

           Then if (C_R) is the resistance of the cell, R₁/R₂ =C_R/R₃

                     or,                                                                      (7)

If the measuring cell has a significant capacity the balance point of the bridge may not be very distinct. This situation can be improved by adjusting the capacity (C). There are many types of measurement cells and that shown in figure 2 is but one example. The electrodes are normally of platinum foil and their cross section is known and made equal and the distance apart is also carefully defined.

Figure 2.  A Cell for Measuring the Conductance of a Liquid.

As an electrolyte is diluted its equivalent conductance becomes greater and, thus, there is a limiting equivalent conductance called the equivalent conductance at infinite dilution (Λ_o). It can be taken as the conducting power of one equivalent of the electrolyte when completely ionized.