Polarography

Polarography is an electrochemical analytical technique in which current is measured as a function of applied potential using a renewable mercury electrode as the working surface. It was developed by Jaroslav Heyrovský in the 1920s and became a foundational form of voltammetry. The method relies on a dropping mercury electrode, which forms new tiny droplets as a potential sweeps, providing a clean surface and maintaining diffusion-controlled conditions.

In a polarographic experiment, a three-electrode cell is used: a dropping mercury electrode as the working electrode, a reference electrode (such as Ag/AgCl or saturated calomel), and a counter electrode (usually platinum). A supporting electrolyte minimizes migration of ions. As the potential is scanned, reducible species in the solution migrate to the mercury surface and undergo reduction, producing a current. The resulting current–potential plot, or polarogram, features waves corresponding to individual reducible species. Each wave has a half-wave potential (E1/2) characteristic of the species, and a limiting current (IL) proportional to its concentration, under diffusion-controlled conditions.

The technique was widely used for trace metal analysis and qualitative identification of electroactive species, with applications in chemistry, environmental monitoring, and qualitative analysis of organic compounds. Advantages include a renewable, smooth electrode surface and straightforward interpretation of diffusion-controlled currents. Limitations include the toxicity and environmental concerns associated with mercury, surface fouling by certain species, and a limited potential window. Although many mercury-based methods have declined in favor of mercury-free electrodes, polarography played a crucial historical role in electroanalytical chemistry and contributed to the development of modern voltammetry.