electrogravimetry
/ ih-lek-troh-grav-ih-MET-ree /
Imagine using electricity to plate a thin coat of metal onto a spoon, then weighing the spoon before and after to learn exactly how much metal you laid down. Electrogravimetry is that idea turned into a measurement.
Electrogravimetry determines an analyte by depositing it electrically as a solid onto a pre-weighed electrode and then weighing the electrode again. The increase in mass is the mass of analyte that plated out, usually a metal reduced from its ions onto a cathode.
It matters because the deposit can be extremely pure and the method needs no precipitating agent or filtration. The catch is that you must fully deposit the analyte and nothing else; controlling the electrode potential is what keeps unwanted metals from plating alongside it.
Copper in a solution is determined by plating it all onto a weighed platinum gauze cathode; the gauze is dried and reweighed, and the gain in mass equals the copper deposited.
Plate the analyte onto an electrode, then weigh what you deposited.
Electrogravimetry sits at the border of gravimetry and electrochemistry. Controlled-potential operation is essential for selectivity: it lets you deposit one metal while leaving others, with a less negative deposition potential, still in solution.