Hofmann voltameter 3006512 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]] [[it:Voltametro di Hofmann]] [[nl:Toestel van Hoffman]] [[sr:Хофманов волтаметар]]