polarography
/ poh-luh-ROG-ruh-fee /
Imagine doing a delicate measurement with a tool that constantly refreshes itself — a faucet of mercury releasing one clean drop after another, so each tiny droplet gives you a brand-new, untainted surface to measure on. Polarography is that idea: it is the original form of voltammetry, carried out at a dripping mercury electrode whose surface is renewed with every falling drop.
Formally, polarography is voltammetry performed at a dropping mercury electrode (or a related mercury electrode), in which the potential is scanned and the current recorded as each species is reduced at its own characteristic potential, giving a step-shaped wave. The voltage at the wave's midpoint, the half-wave potential, identifies the substance, and the height of the wave (the limiting current) is proportional to its concentration.
It matters historically: invented by Jaroslav Heyrovský, polarography won the 1959 Nobel Prize in Chemistry and founded the whole field of voltammetric analysis. The honest caveat is that classic dropping-mercury polarography has fallen out of routine use because of mercury's toxicity; its principles live on in modern voltammetry done at safer solid and film electrodes, often with pulse techniques for far better sensitivity.
On a classic polarogram, a solution of a single reducible metal ion gives one S-shaped wave; its half-wave potential names the metal, and the height of the step tells how concentrated it is. Add a second metal and a second step appears at a different potential.
Half-wave potential identifies; wave height quantifies — at a renewing mercury drop.
Polarography is the historical, mercury-drop subtype of voltammetry. The word is sometimes used loosely for any voltammetry, but strictly it refers to measurements at a dropping (or similar) mercury electrode.