Question

Why alternating current is used in place of direct current in measuring the electrolytic conductance?

Answer 1

Using DC causes polarisation of the electrodes and electrolyte which leads to error in measured conductance values over a period of time.

In AC there is no polarisation due to non-continuous flow of current, hence,AC is used.

Answer 2

Since in AC current, equal current flows in both directions over a given amount of time(larger enough than time period), reaction takes place in both the directions. On an average, no chemical change in composition would be observed.(Of course, that doesn't mean no reaction has taken place.) Therefore the conductivity(or conductance) you measure will be the conductivity you wanted to measure.