The conductivity of an electrolytic solution is measured with a conductivity cell, which has two electrodes a fixed distance l apart, each with an area of cross-section A. The resistance R of the cell is given by R = ρl/A = l/(κA), where ρ is the resistivity and κ the conductivity of the solution.
The quantity l/A is constant for a given cell; it is denoted by G* and is called the cell constant. It is found by measuring the resistance of the cell filled with a KCl solution whose conductivity is known accurately at various concentrations: G* = l/A = Rκ.
The molar conductivity of a solution, Λm, is its conductivity divided by its molar concentration (Λm = κ/c). Its SI unit is S m2 mol−1 when c is expressed in mol m−3. Conductivity always decreases with decrease in concentration, for both strong and weak electrolytes, because the number of ions per unit volume that carry the current in the solution decreases.
Consider the following statements about the measurement of the resistance of an ionic solution.
(A) Alternating current is not used for measuring the resistance of an ionic solution, because it changes the composition of the solution.
(B) The conductivity cell used for the measurement has two platinum electrodes coated with platinum black.
(C) In the Wheatstone bridge set-up for the measurement, the bridge is balanced when the maximum current passes through the detector.
(D) In the Wheatstone bridge set-up for the measurement, the bridge is fed by an oscillator, a source of a.c. power in the audio frequency range.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (B) and (D) only
The resistance of an ionic solution is measured on a Wheatstone bridge, but with two changes: an a.c. source is used, and the solution is held in a conductivity cell.
(A) Alternating current is not used for measuring the resistance of an ionic solution, because it changes the composition of the solution. — False. It is direct current (DC) that changes the composition of the solution, through electrochemical reactions at the electrodes. This difficulty is removed by using an alternating current (AC) source.
(B) The conductivity cell used for the measurement has two platinum electrodes coated with platinum black. — True. The electrodes are platinised: finely divided platinum is deposited on them electrochemically.
(C) In the Wheatstone bridge set-up for the measurement, the bridge is balanced when the maximum current passes through the detector. — False. The bridge is balanced when no current passes through the detector (a headphone or other electronic device).
(D) In the Wheatstone bridge set-up for the measurement, the bridge is fed by an oscillator, a source of a.c. power in the audio frequency range. — True. The oscillator is the a.c. source that feeds the bridge.
NCERT (2026-27 edition):
2.4.1 Measurement of the Conductivity of Ionic Solutions – “…However, for measuring the resistance of an ionic solution we face two problems. Firstly, passing direct current (DC) changes the composition of the solution. Secondly, a solution cannot be connected to the bridge like a metallic wire or other solid conductor. The first difficulty is resolved by using an alternating current (AC) source of power.”
“Basically it consists of two platinum electrodes coated with platinum black (finely divided metallic Pt is deposited on the electrodes electrochemically).”
“…The Wheatstone bridge is fed by an oscillator O (a source of a.c. power in the audio frequency range 550 to 5000 cycles per second). P is a suitable detector (a headphone or other electronic device) and the bridge is balanced when no current passes through the detector.”