Hoffman voltameters are used for demonstrating the chemical composition of water by volume. The apparatus is filled with dilute sulphuric acid (approximately 1N) through the central funnel. The two stopcocks are opened briefly to remove the air from the graduated tubes. A DC current (6-12 volts works well) is passed through the electrodes. The cathode evolves oxygen and the anode hydrogen. Theoretically twice the volume of hydrogen is evolved than oxygen. However oxygen is much more soluble in water than hydrogen, so the measured amount tends to be lower than expected.

After 10 to 20ml of gas has been collected the stopcock can be opened to collect the gas in a small test tube. Each gas can be tested to show they are different: The hydrogen should burn with a squeaky pop when a lighted splint is applied to it, and the oxygen should relight a glowing splint.

It is important that there is a reasonable surface area of platinum to evolve gas at a rapid rate when a voltage is applied. Otherwise the collected volumes are small and the gasses are difficult to test for.