Question

Why is DC not used for measuring resistance of ionic solution?

Hint:

DC $\rightarrow$ Electrolysis $\rightarrow$ Concentration change $\rightarrow$ Error. AC $\rightarrow$ No net electrolysis $\rightarrow$ Accurate measurement.

Answer 1

Step 1: Effect of DC on an ionic solution.
DC causes cations to migrate continuously to the cathode and anions to the anode, resulting in electrolysis. Ions are discharged at the electrodes.
Step 2: How electrolysis disrupts measurement.
The composition of the solution changes over time (ions are consumed), so conductance is not constant. The electrode products create a back-EMF opposing the applied voltage, causing the apparent resistance to differ from the true value.
Step 3: Why AC is preferred.
High-frequency AC reverses direction so rapidly that ions cannot migrate appreciably to the electrodes, preventing net electrolysis and back-EMF.
Step 4: Conclusion.
DC is not used because it causes electrolysis, alters solution composition, and generates back-EMF, all making resistance measurement unreliable. High-frequency AC is used instead.