Indicator electrode is — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Define Reference and Indicator Electrodes * **Reference Electrode:** Maintains a stable, constant, reproducible potential independent of the analyte solution composition. * **Indicator Electrode:** Has a potential that depends directly on the concentration of a specific analyte species in the solution. Step 2 - Analyze Option (A): Standard Hydrogen Electrode (SHE) The SHE has a potential defined as exactly $0.000\text{ V}$ at all temperatures. It is the primary **reference electrode**, not an indicator electrode. Step 3 - Analyze Option (B): Calomel Electrode The calomel electrode (saturated $\ce{KCl}$, $E = +0.244\text{ V}$ vs. SHE) is a highly stable secondary **reference electrode**. Step 4 - Analyze Option (C): Silver/Silver Chloride Electrode The $\ce{Ag/AgCl}$ electrode with fixed $[\ce{Cl^-}]$ has constant potential. It is a secondary **reference electrode**. Step 5 - Analyze Option (D): Quinhydrone Electrode Quinhydrone is a 1:1 complex of quinone ($\ce{Q}$) and hydroquinone ($\ce{QH2}$). The redox half-reaction: $$\ce{Q + 2H^+ + 2e^- <=> QH2}$$ Applying Nernst equation with $[\ce{Q}] = [\ce{QH2}]$: $$E = E^\circ_{\ce{Q/QH2}} - 0.0591\text{ pH}$$ Since $E$ varies linearly with pH, this electrode acts as an **indicator electrode** for measuring pH. $$\text{Correct Option: } \boxed{\text{D}}$$