A polarographic experiment measures Cd²⁺ in 1 M KCl, finding E₁/₂ = −0.40 V. The supporting electrolyte is then changed to 0.1 M HNO₃. What should the experimenter expect?
AE₁/₂ remains at −0.40 V because it is a fixed property of cadmium
BE₁/₂ shifts because the half-wave potential depends on the analyte–electrolyte system, not the analyte alone
CE₁/₂ disappears entirely because cadmium cannot be reduced in nitric acid
DThe diffusion current changes but E₁/₂ stays constant
E₁/₂ is characteristic of the analyte in a specific supporting electrolyte, not of the analyte alone. Complexation equilibria, activity coefficients, and pH all affect the formal reduction potential in solution, shifting E₁/₂. This is why reference tables specify both the analyte and the supporting electrolyte. Option A is the classic misconception — treating E₁/₂ as an intrinsic property of the element rather than a system-dependent parameter.
Question 2 Multiple Choice
In a DC polarogram, why does the current plateau at the limiting diffusion current (id) rather than continuing to rise as potential becomes more negative?
AThe electrode becomes fully coated with reduced metal, blocking further reaction
BEvery analyte molecule arriving at the electrode surface is immediately reduced, so the rate is set by diffusion, not electrode kinetics
CThe mercury drop falls off before higher currents can develop
DCapacitive charging current cancels the faradaic current at very negative potentials
At potentials on the limiting-current plateau, the reduction is so thermodynamically favorable that every analyte ion reaching the electrode surface reacts instantly. The current is no longer limited by electrode kinetics but by the rate at which analyte can diffuse from the bulk solution to the surface — a mass-transport limit. Because diffusion rate is fixed by concentration gradient and the diffusion coefficient, the current stops rising even as potential becomes more negative. The Ilkovic equation quantifies this diffusion-limited rate, which is why id is proportional to concentration.
Question 3 True / False
The primary analytical advantage of the dropping mercury electrode is that each new drop provides a fresh, uncontaminated surface, eliminating memory effects from previous measurements.
TTrue
FFalse
Answer: True
Surface renewal is the DME's defining advantage. Solid electrodes accumulate adsorbed products, oxide films, and surface contamination that alter their behavior over time, requiring frequent reconditioning. The DME circumvents this by discarding the old drop every few seconds and growing a pristine mercury surface. This gives exceptional reproducibility from drop to drop and explains why polarography remains the reference method for certain trace metal analyses despite mercury's toxicity.
Question 4 True / False
The half-wave potential measured for cadmium in 1 M KCl is an intrinsic property of cadmium ions and remains the same regardless of the supporting electrolyte used.
TTrue
FFalse
Answer: False
E₁/₂ depends on the complete analyte–electrolyte system. The supporting electrolyte affects the formal reduction potential through complexation (Cd²⁺ forms chloro-complexes in KCl), ionic strength, activity coefficients, and pH. Changing from 1 M KCl to 1 M NH₄OH, for example, would shift E₁/₂ substantially because cadmium forms amine complexes in ammonia. This system dependence is why half-wave potential tables always specify the electrolyte conditions.
Question 5 Short Answer
Why does mercury provide a wider cathodic potential window than solid platinum or carbon electrodes?
Think about your answer, then reveal below.
Model answer: Mercury has an exceptionally high overpotential for hydrogen evolution — it requires a much more negative potential to reduce H⁺ to H₂ than platinum or carbon do. This means you can scan to approximately −2.0 V versus SCE in many supporting electrolytes without the background current from hydrogen evolution interfering. Platinum and carbon catalyze hydrogen evolution more readily, so they produce interfering background currents at much less negative potentials, limiting how far into the cathodic range you can probe.
The cathodic window is determined by the competing reduction of the solvent or supporting electrolyte. For aqueous solutions, this is usually hydrogen evolution (2H⁺ + 2e⁻ → H₂). Mercury's high hydrogen evolution overpotential is a kinetic barrier — the reaction is thermodynamically favorable but kinetically sluggish on mercury surfaces, suppressing the background current and allowing measurement of analyte reductions at very negative potentials. This unique property makes polarography ideal for reducing metal ions like Zn²⁺, Cd²⁺, and Pb²⁺ that require strongly negative potentials.