Question

Why is direct current (DC) not used to measure the resistance of an ionic solution?

Hint:

In a Wheatstone bridge circuit for conductivity, an AC source and a "conductivity cell" with platinum-coated electrodes are used to ensure precision.

Answer 1

Step 1: Picture what happens if current only flows one way.
An ionic solution contains free ions that are only too willing to react at electrode surfaces. If we push direct current through it, the positive and negative ions keep marching towards the same electrodes continuously and keep discharging there, which is exactly what electrolysis is.
Step 2: See how this ruins the measurement.
As electrolysis proceeds, the ion concentration near and even within the bulk solution keeps changing with time. Since resistance of the solution depends directly on how many ions are actually present to carry charge, a resistance value obtained this way keeps drifting and is simply not trustworthy.
Step 3: Notice the extra problem of polarisation.
Products building up on the electrode surface (gas bubbles or deposited solid) set up a back voltage of their own, opposing the current we are trying to pass, which further distorts any reading.
Step 4: State the fix.
This is why an alternating current of audio frequency is used instead. It reverses direction fast enough that no net electrolysis or product build-up occurs, so the resistance reading stays genuine.
\[ \boxed{\text{DC causes continuous electrolysis and electrode polarisation, so AC is used instead}} \]