Editing Caesium (section)

==History==
[[File:Kirchhoff Bunsen Roscoe.jpg|thumb|[[Gustav Kirchhoff]] (left) and [[Robert Bunsen]] (centre) discovered caesium with their newly invented spectroscope.| alt=Three middle-aged men, with the one in the middle sitting down. All wear long jackets, and the shorter man on the left has a beard.]]
In 1860, [[Robert Bunsen]] and [[Gustav Kirchhoff]] discovered caesium in the [[mineral water]] from [[Bad Dürkheim|Dürkheim]], Germany. Because of the bright blue lines in the [[emission spectrum]], they derived the name from the [[Latin]] word {{lang|la|caesius}}, meaning {{gloss|bluish grey}}.<ref group=note>Bunsen quotes [[Aulus Gellius|Aulus Gellius Noctes Atticae]] II, 26 by [[Nigidius Figulus]]: ''Nostris autem veteribus caesia dicts est quae Graecis, ut Nigidus ait, de colore coeli quasi coelia.''</ref><ref>[[Oxford English Dictionary]], 2nd Edition</ref><ref name="BuKi1861"/><ref name="Weeks">{{cite journal |title=The discovery of the elements. XIII. Some spectroscopic discoveries |pages=1413–1434 |last=Weeks |first=Mary Elvira |author-link=Mary Elvira Weeks |doi=10.1021/ed009p1413 |journal=[[Journal of Chemical Education]] |volume=9 |issue=8 |date=1932 |bibcode=1932JChEd...9.1413W}}</ref> Caesium was the first element to be discovered with a [[spectroscopy|spectroscope]], which had been invented by Bunsen and Kirchhoff only a year previously.<ref name="KanerACS"/>

To obtain a pure sample of caesium, {{convert|44,000|litre}} of mineral water had to be evaporated to yield {{convert|240|kg}} of concentrated salt solution. The [[alkaline earth metal]]s were precipitated either as sulfates or [[oxalate]]s, leaving the alkali metal in the solution. After conversion to the [[nitrate]]s and extraction with [[ethanol]], a sodium-free mixture was obtained. From this mixture, the lithium was precipitated by [[ammonium carbonate]]. Potassium, rubidium, and caesium form insoluble salts with [[chloroplatinic acid]], but these salts show a slight difference in solubility in hot water, and the less-soluble caesium and rubidium hexachloroplatinate ({{chem2|(Cs,Rb)2PtCl6}}) were obtained by [[fractional crystallization (chemistry)|fractional crystallization]]. After reduction of the hexachloroplatinate with [[hydrogen]], caesium and rubidium were separated by the difference in solubility of their carbonates in alcohol. The process yielded {{convert|9.2|g}} of [[rubidium chloride]] and {{convert|7.3|g}} of caesium chloride from the initial 44,000&nbsp;litres of mineral water.<ref name="BuKi1861">{{cite journal |title=Chemische Analyse durch Spectralbeobachtungen |pages=337–381 |first1=G. |last1=Kirchhoff |first2=R. |last2=Bunsen |author-link1=Gustav Kirchhoff |author-link2=Robert Bunsen |doi=10.1002/andp.18611890702 |journal=[[Annalen der Physik |Annalen der Physik und Chemie]] |volume=189 |issue=7 |date=1861 |bibcode=1861AnP...189..337K |url=http://archiv.ub.uni-heidelberg.de/volltextserver/15657/1/spektral.pdf |archive-url=https://web.archive.org/web/20160302113524/http://archiv.ub.uni-heidelberg.de/volltextserver/15657/1/spektral.pdf |archive-date=2016-03-02 |url-status=live |hdl=2027/hvd.32044080591324}}</ref>

From the caesium chloride, the two scientists estimated the [[atomic weight]] of the new element at 123.35 (compared to the currently accepted one of 132.9).<ref name="BuKi1861"/> They tried to generate elemental caesium by electrolysis of molten caesium chloride, but instead of a metal, they obtained a blue homogeneous substance which "neither under the naked eye nor under the microscope showed the slightest trace of metallic substance"; as a result, they assigned it as a [[non-stoichiometric compound|subchloride]] ({{chem|Cs|2|Cl}}). In reality, the product was probably a [[colloid]]al mixture of the metal and caesium chloride.<ref>{{cite book |last=Zsigmondy |first=Richard |title=Colloids and the Ultra Microscope |publisher=Read books |date=2007 |isbn=978-1-4067-5938-9 |page=69 |url=https://books.google.com/books?id=Ac2mGhqjgUkC&pg=PAPA69 |access-date=11 October 2015 |archive-date=5 March 2024 |archive-url=https://web.archive.org/web/20240305132925/https://books.google.com/books?id=Ac2mGhqjgUkC&pg=PAPA69 |url-status=live }}</ref> The electrolysis of the aqueous solution of chloride with a mercury cathode produced a caesium amalgam which readily decomposed under the aqueous conditions.<ref name="BuKi1861"/> The pure metal was eventually isolated by the Swedish chemist [[Carl Setterberg]] while working on his doctorate with [[Friedrich August Kekulé von Stradonitz|Kekulé]] and Bunsen.<ref name="Weeks"/> In 1882, he produced caesium metal by electrolysing [[caesium cyanide]], avoiding the problems with the chloride.<ref name="Sett">{{cite journal |title=Ueber die Darstellung von Rubidium- und Cäsiumverbindungen und über die Gewinnung der Metalle selbst |doi=10.1002/jlac.18822110105 |date=1882 |last1=Setterberg |first1=Carl |journal=Justus Liebig's Annalen der Chemie |volume=211 |pages=100–116 |url=https://zenodo.org/record/1447367 |access-date=25 August 2019 |archive-date=27 April 2021 |archive-url=https://web.archive.org/web/20210427113824/https://zenodo.org/record/1447367 |url-status=live }}</ref>

Historically, the most important use for caesium has been in research and development, primarily in chemical and electrical fields. Very few applications existed for caesium until the 1920s, when it came into use in radio [[vacuum tube]]s, where it had two functions; as a [[getter]], it removed excess oxygen after manufacture, and as a coating on the heated [[cathode]], it increased the [[electrical conductivity]]. Caesium was not recognized as a high-performance industrial metal until the 1950s.<ref>{{cite journal |last=Strod |first=A. J. |date=1957 |title=Cesium—A new industrial metal |journal=American Ceramic Bulletin |volume=36 |issue=6 |pages=212–213}}</ref> Applications for nonradioactive caesium included [[solar cell|photoelectric cells]], [[photomultiplier]] tubes, optical components of [[infrared spectroscopy|infrared spectrophotometers]], catalysts for several organic reactions, crystals for [[scintillation counter]]s, and in [[MHD generator|magnetohydrodynamic power generators]].<ref name="USGS"/> Caesium is also used as a source of positive ions in [[secondary ion mass spectrometry]] (SIMS).

Since 1967, the [[International System of Units|International System of Measurements]] has based the primary unit of time, the second, on the properties of caesium. The International System of Units (SI) defines the second as the duration of 9,192,631,770 cycles at the [[microwave]] [[frequency]] of the [[spectral line]] corresponding to the transition between two [[hyperfine structure|hyperfine]] [[energy level]]s of the [[ground state]] of [[caesium-133]].<ref name="USNO">{{cite web |title=Cesium Atoms at Work |publisher=Time Service Department—U.S. Naval Observatory—Department of the Navy |url=http://tycho.usno.navy.mil/cesium.html |access-date=20 December 2009 |url-status=dead |archive-url=https://web.archive.org/web/20150223231150/http://tycho.usno.navy.mil/cesium.html |archive-date=23 February 2015}}</ref> The 13th [[General Conference on Weights and Measures]] of 1967 defined a second as: "the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of caesium-133 atoms in their ground state undisturbed by external fields".