coulometry
/ koo-LOM-uh-tree /
Imagine paying for groceries by handing over exactly enough coins to cover the bill — count the coins you spent and you know the bill. Coulometry counts electrons instead of coins: it drives the analyte's reaction completely to the end and tallies up the total electric charge it took to do so. Because each molecule needs a fixed number of electrons, counting the charge counts the molecules.
Formally, coulometry determines the amount of analyte from the total charge (in coulombs) consumed to oxidize or reduce it completely, using Faraday's law of electrolysis to convert charge into moles. In controlled-potential coulometry the voltage is held fixed and the charge is integrated until the current dies away; in coulometric titration a constant current generates a titrant in situ, and the time it takes gives the charge.
It matters because, done right, coulometry is an absolute method: it needs no calibration curve and no standard solution, since the charge-to-moles conversion rests on the fundamental Faraday constant. The honest caveat is that it only works if every electron goes to the intended reaction — current efficiency must be essentially one hundred percent, so side reactions, including reduction of dissolved oxygen, must be ruthlessly excluded.
The Karl Fischer coulometric titrator measures tiny traces of water by electrically generating iodine just as fast as the water consumes it; the instrument counts the total charge used and reports the water content directly, with no titrant solution to standardize.
Count the charge, convert by Faraday's law, get the amount.
Coulometry and amperometry both measure current, but their goals differ: amperometry reads the instantaneous current as a concentration signal, while coulometry integrates current over time (charge) to count the total amount that reacted.