Why is direct current (DC) not used to measure the resistance of an ionic solution?
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Solution and Explanation
• Resistance measurement of solutions involves passing a current through the liquid between two electrodes.
• Ionic solutions contain mobile ions that participate in electrochemical processes.
• Using the wrong type of current can lead to chemical and physical changes in the sample.
Step 1: Effect of electrolysis
Direct current (DC) flows in one direction. This causes prolonged electrolysis of the solution. Ions migrate to the electrodes and undergo redox reactions, which changes the chemical composition of the solution.
Step 2: Change in concentration
Because of the chemical reactions at the electrodes, the concentration of the ions in the bulk solution changes. Since resistance depends directly on ion concentration, the measured value becomes inaccurate and non-reproducible.
Step 3: Polarization effects
DC current leads to the accumulation of products (gases or solids) at the electrode surfaces. This creates a "polarization" potential that opposes the applied voltage, leading to a false increase in the measured resistance.
Step 4: The solution
To avoid these issues, Alternating Current (AC) with a frequency in the audio range is used. AC reverses direction periodically, preventing net chemical change and polarization. The final answer is because DC causes electrolysis and changes the solution composition.
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