Chemistry
How Polarography Measures Trace Metal Concentrations in Aqueous Samples
Quick fact
Polarography was invented by Jaroslav Heyrovský in 1922 and earned him the Nobel Prize in Chemistry in 1959.
Why this is interesting
Imagine being able to detect a single drop of lead in an Olympic-sized swimming pool—that's the sensitivity of polarography. But how does simply applying a voltage to water reveal such tiny amounts of metal?
Read the full explanation
Understanding How Polarography Measures Trace Metal Concentrations in Aqueous Samples
Think of polarography like a precision metal detector that works in liquids. You have two electrodes: a tiny mercury drop that continuously falls from a capillary, and a reference electrode. As you slowly increase the voltage (more negative here), the metal ions in the water are attracted to the mercury drop. At a specific voltage—unique for each metal—the metal ions gain electrons and become metal atoms, dissolving into the mercury. This electron transfer creates a tiny electric current, which shows up as a peak on a graph. The height of the peak tells you how much metal is present, and the position of the peak identifies which metal it is.
A deeper explanation
The core of polarography is the relationship between voltage and current. Each metal has a characteristic half-wave potential (E1/2), which is the voltage at which its reduction current is half of the maximum. This is governed by the Nernst equation, linking the electrode potential to the concentration of oxidized and reduced species. As the voltage becomes more negative, more metal ions are reduced, and the current increases until it reaches a limiting value controlled by the rate at which ions diffuse to the electrode surface. The limiting current is proportional to the concentration of the metal ion, according to the Ilkovic equation: Id = 607 n D1/2 m2/3 t1/6 C. Here, n is the number of electrons, D is the diffusion coefficient, m is the mercury flow rate, t is the drop time, and C is the concentration. The continuously dropping mercury electrode provides a fresh, clean surface, ensuring reproducible measurements and avoiding contamination. This technique's sensitivity and selectivity make it indispensable for monitoring trace metals in environmental and industrial samples.