Hofmann voltameter

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Hoffmann voltameter is an apparatus for electrolyzing water, invented by August Wilhelm von Hofmann (1818–1892). It consists of three joined upright cylinders, usually glass. The inner cylinder is open at the top to allow addition of water and an ionic compound to improve conductivity, such as a small amount of sulfuric acid. A platinum electrode is placed inside the bottom of each of the two side cylinders, connected to the positive and negative terminals of a source of electricity. When current is run through the hofmann voltameter, gaseous oxygen forms at the anode and gaseous hydrogen at the cathode. Each gas displaces water and collects at the top of the two outer tubes.

The presence of gas in the collecting tube can be detected with a smoldering match or "glowing wood splint"; oxygen will cause the match immediately burst into bright white flame and will burn vigorously, while the presence of hydrogen gas will merely cause the match to glow a bit more brightly.

A glowing wood splint, placed over Hofmann's collecting tube to test if oxygen is present.

A brightly glowing ember, indicating oxygen is detected in the collecting tube.

The name 'voltameter' is a historical artifact - voltameters are no longer used as measuring devices. However, before the invention of the ammeter, voltameters were often used to measure direct current, since current running through a voltameter with iron or copper electrodes electroplates the cathode with an amount of metal from the anode directly proportional to the total current (Faraday's law of electrolysis).

The amount of electricity that has passed through the system can then be determined by weighing the cathode. Thomas Edison used voltameters as electricity meters. (A Hofmann voltameter cannot be used to weigh electric current in this fashion, as the platinum electrodes are too inert for plating.) A Hofmann voltameter is often used as a demonstration of stoichiometric principles, as the two-to-one ration of the volumes of hydrogen and oxygen gas produced by the apparatus illustrates the chemical formula of water, .

[edit] References

• von Hofmann, A. W. Introduction to Modern Chemistry: Experimental and Theoretic; Embodying Twelve Lectures Delivered in the Royal College of Chemistry, London. Walton and Maberly, London, 1866. [1]