Hofmann voltameter
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213050196
2008-05-17T15:09:07Z
59.165.15.69
[[Image:209px-Hofmann voltameter.fr.version.svg.png|thumb|150px|right|Hofmann voltameter schema]]
[[Image:Hofmann voltameter 1866.jpg|thumb|right|150px|Figure of the apparatus from Hofmann's 1866 book.]]
'''Hofmann voltameter''' is an apparatus for [[electrolysis of water|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 [[sulphuric 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 Hofmann's 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 to immediately burst into a bright white flame and will burn vigorously, while the presence of [[hydrogen]] gas will merely cause the match to glow a bit more brightly.
<gallery>
Image:Elektrolyse3.jpg|A glowing wood splint, placed over Hofmann's collecting tube to test if hydrogen is present.
Image:Elektrolyse4.jpg|A brightly glowing ember, indicating oxygen is detected in the collecting tube.
</gallery>
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 [[electroplating|electroplates]] the cathode with an amount of metal from the anode directly proportional to the total current ([[Faraday's law of electrolysis]]). The modern name is "electrochemical [[coulometer]]".
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 [[stoichiometry|stoichiometric]] principles, as the two-to-one ratio of the volumes of [[hydrogen]] and [[oxygen]] gas produced by the apparatus illustrates the chemical formula of water, <math>H_{2}O\,</math>.
==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'''. [http://books.google.com/books/pdf/Introduction_to_Modern_Chemistry.pdf?id=KT8JAAAAIAAJ&output=pdf&sig=Vooloj2sSW4b5XKRsWBcoqAYpZ4]
{{electrolysis}}
[[Category:Electrolysis]]
[[Category:Electrochemistry]]
[[Category:Hydrogen technologies]]
[[Category:Hydrogen production]]
{{analytical-chemistry-stub}}
[[de:Hofmannscher Wasserzersetzungsapparat]]
[[fr:Voltamètre d'Hoffmann]]
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[[nl:Toestel van Hoffman]]
[[sr:Хофманов волтаметар]]