Hofmann voltameter
A Hofmann voltameter is a simple device for electrolysing water, invented by August Wilhelm von Hofmann in 1866. It has three joined glass tubes: two outer tubes where the gases collect, and a middle tube that holds water with a small amount of acid to improve conductivity. Each outer tube has a platinum electrode at its bottom connected to the positive or negative terminal of a power source.
When electricity passes through, water splits into hydrogen at the cathode and oxygen at the anode. The gases bubble up and push the water out of the outer tubes, collecting at the tops. The outer tubes are graduated so you can measure the gas volumes.
This setup helps show that water is H2O, since hydrogen and oxygen pop out in a 2:1 volume ratio under normal conditions (assuming both gases are diatomic). Historically, voltameters were used to measure direct current before ammeters existed: the amount of metal deposited on the cathode was proportional to the total charge transferred. Today Hofmann voltameters are mainly used as demonstrations of electrolysis and stoichiometry, and the device is sometimes called an electrochemical coulometer. The correct spelling is Hofmann (one 'f'); the term voltameter was coined by Daniell.
This page was last edited on 2 February 2026, at 17:09 (CET).