Polarography and the Dropping Mercury Electrode: Reading a Polarogram

Why mercury, and why dropping

Polarography uses a dropping mercury electrode (DME) as the working electrode: mercury flows slowly through a fine capillary, forming a fresh spherical drop every few seconds that eventually falls and is replaced by an identical new one. This constant renewal is the entire point — a solid electrode's surface accumulates adsorbed species and reaction products over the course of a measurement, distorting the response, while the DME presents a fresh, reproducible, contamination-free surface for every single drop. Mercury also offers an unusually wide cathodic potential window (it resists hydrogen evolution at quite negative potentials compared with most solid electrodes), which is why polarography historically became the method of choice for reducible species such as many metal cations and organic functional groups that are hard to study at other electrode surfaces.

Reading the polarogram

A polarogram plots the current measured at the DME against the applied potential, scanned slowly across a range. For a simple reducible species, the curve rises from a flat residual-current baseline, climbs through an S-shaped wave, and levels off at a plateau once the reaction becomes diffusion-limited (every ion reaching the electrode surface is reduced immediately, so current can no longer increase with more negative potential — it is capped by how fast the species can diffuse to the electrode).

The Ilkovic equation: quantifying the diffusion current

where n is the number of electrons transferred, D is the diffusion coefficient of the species, m is the mercury flow rate, t is the drop time, and C is the bulk concentration. The practical takeaway, without getting lost in the constant: diffusion current is directly proportional to concentration, with everything else (n, D, m, t) held constant for a given species and apparatus — which is exactly what makes polarography useful for quantitative determination once the method is calibrated with known standards.

How this compares with cyclic voltammetry

Cyclic voltammetry, run at a stationary solid electrode with a triangular potential sweep, gives a peaked response and both forward and reverse scans, letting you probe reaction reversibility and mechanism directly from peak separation and the ratio of anodic to cathodic peak currents. Polarography's S-shaped, plateau-based wave at a constantly renewed electrode is better suited to routine, reproducible quantitative analysis of a known class of species, which is why the two techniques are typically presented as complementary rather than one simply superseding the other, despite CV being more common in modern research settings.

FAQs

Why is mercury used as the electrode in polarography rather than a solid metal?

The dropping mercury electrode constantly renews its surface with each new drop, avoiding the fouling and adsorption problems that build up on a fixed solid electrode, and mercury also offers a wide cathodic potential window useful for reducible species.

What does the half-wave potential tell you in a polarogram?

It identifies the electroactive species being reduced or oxidised, since it is characteristic of that species and largely independent of its concentration, unlike the diffusion current plateau which scales with concentration.

Is polarography still tested in CSIR-NET and GATE Chemistry?

Yes, as part of electroanalytical chemistry alongside cyclic voltammetry, typically through conceptual questions on the DME, half-wave potential, and the proportionality in the Ilkovic equation.