Questions: Potentiometry and Ion-Selective Electrodes
3 questions to test your understanding
Score: 0 / 3
Question 1 Multiple Choice
According to the Nernst equation, what does the cell potential measured in potentiometry directly reflect?
AThe current flowing through the cell
BThe activity of the analyte ion
CThe concentration of the reference electrode solution
DThe resistance of the ion-selective membrane
The Nernst equation E = E° − (RT/nF)ln(Q) relates cell potential to the activity (not concentration) of the ions involved. Potentiometry exploits this by measuring potential at zero current, so the measured voltage directly encodes the analyte's activity in solution.
Question 2 True / False
A glass pH electrode measures the concentration of H⁺ ions in solution.
TTrue
FFalse
Answer: False
The glass electrode responds to H⁺ activity, not concentration. Activity and concentration differ especially at high ionic strengths, where activity coefficients deviate significantly from 1. This is why accurate pH measurements require ionic strength adjustment or calibration in solutions that closely match the sample matrix.
Question 3 Short Answer
Why is a stable reference electrode essential in a potentiometric measurement?
Think about your answer, then reveal below.
Model answer: A reference electrode provides a fixed, known potential so that any change in the measured cell voltage can be attributed entirely to the indicator electrode responding to the analyte. Without a stable reference, you cannot isolate the signal from the analyte.
Potentiometry measures the potential difference between two electrodes. If the reference electrode potential drifts, it is impossible to determine whether a voltage change reflects a change in analyte concentration or a change in the reference. Common reference electrodes (Ag/AgCl, saturated calomel) maintain constant potential through well-buffered redox couples.