Conductimetric measurements

The basic instrumentation for conductimetric measurements is a Wheatstone bridge arrangement (Figure 4.17). An alternating rather than direct current is used in order to prevent polarization of the solution (i.e. anions moving to the anode and cations moving to the cathode) and any electrolysis that might result from this polarization.

With fixed resistances, Rt and R2, the variable resistance Rv is adjusted until no current flows through the galvanometer. Under these conditions:

Figure 4.17 A Wheatstone bridge

Figure 4.17 A Wheatstone bridge

-o r\j a

This gives a value for resistance of the cell which can be converted to conductance by calculating the reciprocal.

The electrodes of conductivity cells are usually made of platinum coated with platinum black with a known area. Although in many cells the distance between the electrodes is adjustable, for any series of experiments it must be held constant and for many calculations the precise value is required. The cells must be thermostatically controlled because any changes in temperature will cause significant alteration of conductivity values.

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