Question

In polarography, when limiting current is achieved, which of the following processes takes place?

• A. The rate of electron transfer just matches the rate of mass transfer
• B. The rate of electron transfer is slower than the rate of mass transfer
• C. The rate of electron transfer becomes independent of the rate of mass transfer
• D. The rate of electron transfer far exceeds the rate of mass transfer

Hint:

At the limiting current plateau, the chemical reaction rate is incredibly fast. The electrode acts like a perfect sink, meaning the overall current is entirely bottlenecks by physical transport (diffusion control).

Answer 1

The Correct Option is D

Concept: Polarography is an electrochemical technique that measures current changes flowing through a dropping mercury electrode (DME) as an applied potential voltage is varied. The total current profile is governed by two key continuous dynamics: the rate of the chemical electron transfer occurring at the electrode interface and the physical rate of mass transport (diffusion) of analyte ions moving from the bulk solution toward the surface.

Step 1: Understand the polarization curve regions As the applied negative potential is systematically increased, the rate of electrolytic reduction escalates.
• At low potentials, the overall system kinetics are limited by the activation energy required for electron transfer.
• As the potential becomes sufficiently negative, the activation barrier drops, causing the rate of electron transfer to step up drastically until it far exceeds the rate of mass transfer.

Step 2: Define the state of limiting current When the electron transfer rate becomes exceptionally fast, every target analyte ion arriving at the electrode surface is instantaneously reduced. At this stage, the concentration of electroactive species directly at the electrode surface drops to zero. The overall current can no longer increase with higher voltage because it is strictly limited by how fast new ions can physically diffuse across the concentration gradient from the bulk solution. This plateau value is the limiting current, and because it is entirely diffusion-controlled, it forms the basis for quantitative polarographic analysis.