Understanding the Concept:
In potentiometric titrations, an electrochemical cell is constructed using two electrodes: a reference electrode, which maintains a constant potential, and an indicator electrode, whose potential changes in response to changes in the activity or concentration of a specific analyte ion.
Step 1: Analyze the requirements for Acid-Base Potentiometry
An acid-base titration involves monitoring changes in hydrogen ion activity (\(\text{a}_{\text{H}^+}\)) or pH. The indicator electrode must generate an electrical potential that responds linearly to variations in \(\text{H}^+\) concentration according to the Nernst equation:
\[
E = E^{\circ} + \frac{2.303 \cdot R \cdot T}{F} \cdot \log(\text{a}_{\text{H}^+}) = E^{\circ} - 0.0591 \cdot \text{pH} \quad (\text{at } 25^\circ\text{C})
\]
Step 2: Evaluate the Glass Electrode configuration
The glass electrode is an ion-selective electrode specifically sensitive to hydrogen ions. It features a thin, specialized hydrated glass membrane container enclosing an internal reference solution of constant acidity. When immersed in an external test solution, an ion-exchange process occurs at the outer hydrated gel layer of the glass membrane, generating a phase-boundary potential that is directly proportional to the pH of the solution.
Step 3: Distinguish alternative choices
• Calomel Electrode (\(\text{Hg}/\text{Hg}_2\text{Cl}_2\)): Functions exclusively as a constant-potential *reference electrode*, not an indicator electrode.
• Platinum Electrode: An inert electron-donor/acceptor electrode utilized as an indicator in *redox titrations*.
• Silver Electrode: Functions as an indicator electrode in argentometric precipitation titrations to monitor halide ions.
Thus, the glass electrode is the correct indicator electrode choice for monitoring acid-base systems.