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Hoffman Voltameter
The Hoffman voltameter splits a liquid by electrolysis and collects the gas from each electrode in its own graduated limb. Two borosilicate glass limbs, each marked for 50 cm³ and closed by a stopcock at the top, rise beside a central reservoir with a funnel-shaped bulb. Platinum electrodes serve for water, carbon electrodes for hydrochloric acid or ammonia. A stand is needed separately.
Hoffman voltameter specifications
| Parameter | Detail |
|---|---|
| Material | Borosilicate glass |
| Limbs | Two connected limbs, graduated for 50 cm³, each with a stopcock at the top |
| Reservoir | Integral reservoir tube with funnel-shaped bulb, capacity 200 cm³ |
| Electrodes supplied | One pair platinum, one pair carbon |
| Platinum electrodes | For electrolysis of water |
| Carbon electrodes | For electrolysis of ammonia or hydrochloric acid |
| Overall size (height x width) | 700 x 85 mm |
| Stand | Not included |
| Product code | SLE/CH/59 |
| Minimum order | 10 units |
| Dispatch | Usually within 7 to 10 working days |
| Manufacturer | ScienceLab (Jain Scientific Equipments Pvt Ltd), Ambala, India |
Electrolysis experiments with the Hoffman voltameter
Decomposition of water. Fit the platinum electrodes and clamp the apparatus upright in a Hoffman voltameter stand. Fill it with water made conducting by a little dilute sulfuric acid and connect a low-voltage DC source such as a smoothed power supply. Hydrogen gathers above the negative electrode and oxygen above the positive one, with roughly twice as much hydrogen.
Electrochemical equivalent of hydrogen. Put a digital ammeter in series and hold the current steady with a rheostat. Time the run with a digital stopwatch and read the hydrogen volume at the end. Convert that volume to a mass using the room temperature and pressure, then divide by the charge passed, which is current multiplied by time.
Composition by electrolysis. Swap in the carbon electrodes for hydrochloric acid. Hydrogen forms at the cathode and chlorine at the anode, and the volumes become about equal once the solution is saturated with chlorine. With ammonia solution, nitrogen and hydrogen collect in about one volume to three. Chlorine is toxic, so run this in a fume cupboard.
Frequently Asked Questions About the Hoffman Voltameter
Why is acid added to the water before electrolysis?
Pure water contains very few ions, so it carries almost no current. A little dilute sulfuric acid supplies ions to carry the charge. The acid is not used up: only water is broken down, so the acid simply becomes slightly more concentrated as the run goes on.
Which gas collects in which limb?
Hydrogen collects above the electrode joined to the negative terminal, where hydrogen ions gain electrons. Oxygen collects above the positive electrode, where water is oxidised. The hydrogen limb fills about twice as fast, which matches the formula of water.
Why is the hydrogen volume not exactly twice the oxygen volume?
Oxygen dissolves in water more than hydrogen does, so some of it stays in the liquid at first. Bubbles also cling to the electrodes. Let the apparatus run for a while, release the first gas through the stopcocks, refill the limbs and then start measuring.
How are the two gases tested?
For hydrogen, hold an inverted test tube over its stopcock, open the tap briefly, then bring a lighted splint to the tube: the gas burns with a squeaky pop. Oxygen is denser than air, so hold a glowing splint right at the other stopcock as it opens: the splint relights. Keep other flames away from the hydrogen limb.
Why must the supply be DC and not AC?
With direct current each electrode keeps one polarity, so hydrogen and oxygen collect in separate limbs. Alternating current switches the polarity many times a second, so both gases form at both electrodes and each limb fills with a mixture. That mixture is explosive and gives no useful volume ratio.
How do you fill the voltameter without trapping air?
Clamp it upright and open both stopcocks. Pour the electrolyte slowly into the funnel bulb until it rises to the top of each limb and air has escaped, then close the stopcocks. Leave room in the reservoir, because the liquid pushed out of the limbs by the gas rises there.
Stand, supply, ammeter and other voltameters
- Hoffman Voltameter Stand
- Power Supply, Smoothed
- Digital Ammeter
- Voltameter
- Water Electrolysis Apparatus (Hoffman Voltameter)
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Last updated: 14 November 2026
