View source for Rubidium

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{{infobox rubidium}}
'''Rubidium''' is a [[chemical element]]; it has [[Symbol (chemistry)|symbol]] '''Rb''' and [[atomic number]] 37. It is a very soft, whitish-grey solid in the [[alkali metal]] group, similar to [[potassium]] and [[caesium]].<ref>{{ Ullmann |doi=10.1002/14356007.a23_473.pub2 |title=Rubidium and Rubidium Compounds |year=2010 |last1=Lenk |first1=Winfried |last2=Prinz |first2=Horst |last3=Steinmetz |first3=Anja |isbn=978-3527306732 }}</ref> Rubidium is the first [[alkali metal]] in the group to have a density higher than [[Properties of water|water]]. On Earth, natural rubidium comprises two [[isotope]]s: 72% is a stable isotope {{sup|85}}Rb, and 28% is slightly [[radioactive]] {{sup|87}}Rb, with a [[half-life]] of 48.8&nbsp;billion years – more than three times as long as the estimated [[age of the universe]].

German chemists [[Robert Bunsen]] and [[Gustav Kirchhoff]] discovered rubidium in 1861 by the newly developed technique, [[Atomic emission spectroscopy#Flame emission spectroscopy|flame spectroscopy]]. The name comes from the [[Latin]] word {{lang|la|rubidus}}, meaning deep red, the color of its emission spectrum. Rubidium's compounds have various chemical and electronic applications. Rubidium metal is easily vaporized and has a convenient spectral absorption range, making it a frequent target for [[laser]] manipulation of [[atom]]s.<ref>{{cite web |title=Rubidium (Rb) |website=American Elements (americanelements.com) |url=https://www.americanelements.com/rb.htm |access-date=2024-03-27 |lang=en-US}}</ref> Rubidium is not a known nutrient for any [[organism|living organisms]]. However, rubidium [[ion]]s have similar properties and the same charge as potassium ions, and are actively taken up and treated by [[animal cell]]s in similar ways.

==Characteristics==
=== Physical properties ===
[[File:RbH.JPG|thumb|left|Partially molten rubidium metal in an ampoule]]
Rubidium is a very soft, [[ductility|ductile]], silvery-white metal.<ref name=Ohly>{{cite book |last=Ohly |first=Julius |date=1910 |chapter=Rubidium |title=Analysis, detection and commercial value of the rare metals |publisher=Mining Science Pub. Co. |chapter-url=https://books.google.com/books?id=dGUuAQAAIAAJ |via=Google books }}</ref> It has a melting point of {{convert|39.3|°C|°F}} and a boiling point of {{convert|688|°C|°F}}.<ref name="www.rsc.org">{{cite web |title=Rubidium |department=Element information, properties and uses |series=Periodic Table |url=https://www.rsc.org/periodic-table/element/37/rubidium |access-date=2024-09-09 |website=www.rsc.org}}</ref> It forms [[amalgam (chemistry)|amalgams]] with [[mercury (element)|mercury]] and [[alloy]]s with [[gold]], [[iron]], [[caesium]], [[sodium]], and [[potassium]], but not [[lithium]] (despite rubidium and lithium being in the same periodic group).<ref name=HollemanAF>{{cite book |first1 = Arnold F. |last1 = Holleman |last2 = Wiberg |first2 = Egon |last3 =Wiberg |first3 = Nils |date = 1985 |chapter = Vergleichende Übersicht über die Gruppe der Alkalimetalle |trans-chapter=Brief overview of the Alkalai metal group |title = Lehrbuch der Anorganischen Chemie |lang = de |trans-title=Textbook of Inorganic Chemistry |edition = 91–100 |publisher = Walter de Gruyter |isbn = 978-3-11-007511-3 |pages = 953–955 }}</ref> Rubidium and potassium show a very similar purple color in the [[flame test]], and distinguishing the two elements requires more sophisticated analysis, such as spectroscopy.<ref>{{cite journal |last1=Ahrens |first1=L.H. |last2=Pinson |first2=W.H. |last3=Kearns |first3=Makgaret M. |date=1952-01-01 |title=Association of rubidium and potassium and their abundance in common igneous rocks and meteorites |journal=[[Geochimica et Cosmochimica Acta]] |volume=2 |issue=4 |pages=229–242 |issn=0016-7037 |url=https://dx.doi.org/10.1016/0016-7037%2852%2990017-3|bibcode=1952GeCoA...2..229A |doi=10.1016/0016-7037(52)90017-3 }}</ref>

=== Chemical properties ===
[[File:Rb&Cs crystals.jpg|left|thumb|Rubidium crystals (silvery) compared to [[caesium]] crystals (golden)]]
Rubidium is the second most [[Electronegativity|electropositive]] of the stable alkali metals and has a very low first [[ionization energy]] of only 403&nbsp;kJ/mol.<ref name="www.rsc.org" /> It has an electron configuration of [Kr]5s<sup>1</sup> and is photosensitive.<ref name="Hart-1973" />{{rp|382}} Due to its strong electropositive nature, rubidium reacts explosively with water<ref>{{Cite book |last1=Cotton |first1=F. Albert |title=Advanced inorganic chemistry: a comprehensive text |last2=Wilkinson |first2=Geoffrey |date=1972 |publisher=Interscience Publishers |isbn=978-0-471-17560-5 |edition=3d ed., completely rev |location=New York |page=190}}</ref> to produce rubidium hydroxide and hydrogen gas.<ref name="Hart-1973" />{{rp|383}} As with all the alkali metals, the reaction is usually vigorous enough to ignite metal or the [[hydrogen]] gas produced by the reaction, potentially causing an explosion.<ref>{{Cite web |last=Stanford University |title=Information on Alkali Metals – Stanford Environmental Health & Safety |url=https://ehs.stanford.edu/reference/information-alkali-metals |access-date=2024-09-12 |language=en-US}}</ref> Rubidium, being denser than potassium, sinks in water, reacting violently; caesium explodes on contact with water.<ref>{{Cite web |title=Reactions of the Group 1 elements with water |author=Jim Clark|url=https://www.chemguide.co.uk/inorganic/group1/reacth2o.html |access-date=2024-09-12 |website=www.chemguide.co.uk}}</ref> However, the reaction rates of all alkali metals depend upon surface area of metal in contact with water, with small metal droplets giving explosive rates.<ref>Maustellar, J. W, F Tepper, and S. J. (Sheridan Joseph) Rodgers. "Alkali Metal Handling and Systems Operating Techniques" Prepared under the Direction of the American Nuclear Society for the United States Atomic Energy Commission. New York: Gordon and Breach, 1968.</ref> Rubidium has also been reported to ignite spontaneously in air.<ref name="Ohly" />

===Compounds===
{{category see also|Rubidium compounds}}
[[File:Rb9O2 cluster.png|thumb|left|upright=0.5|{{chem|Rb|9|O|2}} cluster|alt= The ball-and-stick diagram shows two regular octahedra which are connected to each other by one face. All nine vertices of the structure are purple spheres representing rubidium, and at the centre of each octahedron is a small red sphere representing oxygen.]]
[[Rubidium chloride]] (RbCl) is probably the most used rubidium compound: among several other chlorides, it is used to induce living cells to take up [[DNA]]; it is also used as a biomarker, because in nature, it is found only in small quantities in living organisms and when present, replaces potassium. Other common rubidium compounds are the corrosive [[rubidium hydroxide]] (RbOH), the starting material for most rubidium-based chemical processes; [[rubidium carbonate]] (Rb<sub>2</sub>CO<sub>3</sub>), used in some optical glasses, and rubidium copper sulfate, Rb<sub>2</sub>SO<sub>4</sub>·CuSO<sub>4</sub>·6H<sub>2</sub>O. [[Rubidium silver iodide]] (RbAg<sub>4</sub>I<sub>5</sub>) has the highest [[room temperature]] [[electrical conductivity|conductivity]] of any known [[ionic crystal]], a property exploited in thin film [[battery (electricity)|batteries]] and other applications.<ref>{{Cite book |chapter-url = https://books.google.com/books?id=pVw98i6gtwMC&pg=PA176 |title = Solid state chemistry: an introduction |chapter = RbAg<sub>4</sub>I<sub>5</sub> |first = Lesley |last = Smart |author2 = Moore, Elaine |publisher = CRC Press |date = 1995 |isbn = 978-0-7487-4068-0 |pages = [https://archive.org/details/solidstatechemis00smar_0/page/176 176–177] |url-access = registration |url = https://archive.org/details/solidstatechemis00smar_0/page/176 }}</ref><ref>{{Cite journal |title = Relationship of structure and ionic mobility in solid MAg<sub>4</sub>I<sub>5</sub> |first = J. N. |last = Bradley |author2=Greene, P. D. |journal = Trans. Faraday Soc. |date = 1967 |volume = 63 |pages = 2516 |doi = 10.1039/TF9676302516}}</ref>

Rubidium forms a number of [[Rubidium oxide|oxides]] when exposed to air, including rubidium monoxide (Rb<sub>2</sub>O), Rb<sub>6</sub>O, and Rb<sub>9</sub>O<sub>2</sub>; rubidium in excess oxygen gives the [[superoxide]] [[Rubidium superoxide|RbO<sub>2</sub>]]. Rubidium forms salts with halogens, producing [[rubidium fluoride]], [[rubidium chloride]], [[rubidium bromide]], and [[rubidium iodide]].<ref name=G&W>{{Greenwood&Earnshaw2nd}}</ref>

===Isotopes===
{{Main|Isotopes of rubidium}}
Rubidium in the Earth's crust is composed of two isotopes: the stable <sup>85</sup>Rb (72.2%) and the [[radioactive]] <sup>87</sup>Rb (27.8%).<ref name="Audi">{{NUBASE 2003}}</ref> Natural rubidium is radioactive, with specific activity of about 670 [[Becquerel|Bq]]/g, enough to significantly expose a [[photographic film]] in 110 days.<ref>{{cite journal | last1 = Strong | first1 = W. W. | title = On the Possible Radioactivity of Erbium, Potassium and Rubidium | journal = Physical Review | series = Series I | volume = 29 | issue = 2 | pages = 170–173 | date = 1909 | doi = 10.1103/PhysRevSeriesI.29.170 |bibcode = 1909PhRvI..29..170S | url = https://zenodo.org/record/1545957 }}</ref><ref>{{cite book | url = https://books.google.com/books?id=6khCAQAAIAAJ | pages = 4–25 | title = CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data | isbn = 978-0-8493-0476-7 | author1 = Lide, David R | author2 = Frederikse, H. P. R | date = June 1995| publisher = CRC-Press }}</ref> <!--CRC rubber bible gives 30 to 60 days but I could not find a source in science literature.-->Thirty additional rubidium isotopes have been synthesized with half-lives of less than 3&nbsp;months; most are highly radioactive and have few uses.<ref>{{cite web |url=http://www.nucleonica.net/unc.aspx |title=Universal Nuclide Chart |publisher=nucleonica |url-access=registration|accessdate=2017-01-03 |archive-date=2017-02-19 |archive-url=https://web.archive.org/web/20170219043412/http://www.nucleonica.net/unc.aspx |url-status=live }}</ref>

Rubidium-87 has a [[half-life]] of {{val|48.8|e=9}}&nbsp;years, which is more than three times the [[age of the universe]] of {{val|13.799|0.021|e=9}}&nbsp;years,<ref name="Planck 2015">{{cite journal
 |author=Planck Collaboration
 |year=2016
 |title=Planck 2015 results. XIII. Cosmological parameters (See Table 4 on page 31 of pfd).
 |arxiv=1502.01589
|doi=10.1051/0004-6361/201525830
 |bibcode=2016A&A...594A..13P
 |volume=594
 |journal=Astronomy & Astrophysics
 |page=A13
 |s2cid=119262962
 }}</ref> making it a [[primordial nuclide]]. It readily substitutes for [[potassium]] in [[mineral]]s, and is therefore fairly widespread. Rb has been used extensively in [[rock dating|dating rocks]]; <sup>87</sup>Rb [[beta decay]]s to stable <sup>87</sup>Sr. During [[Fractional crystallization (geology)|fractional crystallization]], Sr tends to concentrate in [[plagioclase]], leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual [[magma]] may increase over time, and the progressing [[Igneous differentiation|differentiation]] results in rocks with elevated Rb/Sr ratios. The highest ratios (10 or more) occur in [[pegmatite]]s. If the initial amount of Sr is known or can be extrapolated, then the age can be determined by measurement of the Rb and Sr concentrations and of the <sup>87</sup>Sr/<sup>86</sup>Sr ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered (see [[rubidium–strontium dating]]<!-- the hyphen is correct, it uses Rb/Rb and Sr/Sr ratios -->).<ref>{{Cite book |chapter-url = https://books.google.com/books?id=k90iAnFereYC&pg=PA162 |chapter = Rubidium-Strontium Dating |title = Isotopes in the Earth Sciences |first1 = H.-G. |last1 = Attendorn |first2 = Robert |last2 = Bowen |publisher = Springer |date = 1988 |isbn = 978-0-412-53710-3| pages = 162–165}}</ref><ref>{{Cite book |chapter-url =https://books.google.com/books?id=cYWNAZbPhMYC&pg=PA383 |title = Essentials of geochemistry |first1 =John Victor |last1 =Walther |publisher =Jones & Bartlett Learning|orig-year=1988 | date = 2009 |isbn =978-0-7637-5922-3| chapter =Rubidium-Strontium Systematics| pages = 383–385}}</ref>

[[Rubidium-82]], one of the element's non-natural isotopes, is produced by [[electron capture|electron-capture]] decay of [[strontium-82]] with a half-life of 25.36&nbsp;days. With a half-life of 76 seconds, rubidium-82 decays by positron emission to stable [[krypton-82]].<ref name="Audi" />

==Occurrence==
Rubidium is not abundant, being one of 56 elements that combined make up 0.05% of the Earth's crust; at roughly the 23rd <!-- verified in USGS ref -->[[Abundance of elements in Earth's crust|most abundant element in the Earth's crust]] it is more abundant than [[zinc]] or [[copper]].<ref name="USGS">{{cite web |url = http://pubs.usgs.gov/of/2003/of03-045/of03-045.pdf |publisher = United States Geological Survey |access-date = 2010-12-04 |title = Mineral Commodity Profile: Rubidium |first1 = William C. |last1 = Butterman |first2 = William E. |last2 = Brooks |first3 = Robert G. Jr. |last3 = Reese |date=2003}}</ref>{{rp|4}} It occurs naturally in the minerals [[leucite]], [[pollucite]], [[carnallite]], and [[zinnwaldite]], which contain as much as 1% rubidium [[oxide]]. [[Lepidolite]] contains between 0.3% and 3.5% rubidium, and is the commercial source of the element.<ref>{{Cite journal |title =Trace element chemistry of lithium-rich micas from rare-element granitic pegmatites |volume = 55
| issue = 13 |date = 1995 |doi = 10.1007/BF01162588 |pages = 203–215 |journal = Mineralogy and Petrology |first = M. A. |last = Wise |bibcode = 1995MinPe..55..203W |s2cid = 140585007
}}</ref> Some [[potassium]] minerals and [[potassium chloride]]s also contain the element in commercially significant quantities.<ref>{{cite book |last=Norton |first=J. J. |date=1973 |chapter=Lithium, cesium, and rubidium—The rare alkali metals |editor=Brobst, D. A. |editor2=Pratt, W. P. |title=United States mineral resources |publisher=U.S. Geological Survey Professional |volume=Paper 820 |pages=365–378 |chapter-url=https://pubs.er.usgs.gov/usgspubs/pp/pp820 |access-date=2010-09-26 |archive-date=2010-07-21 |archive-url=https://web.archive.org/web/20100721060544/http://pubs.er.usgs.gov/usgspubs/pp/pp820 |url-status=dead }}</ref>

[[Seawater]] contains an average of 125&nbsp;μg/L of rubidium compared to the much higher value for potassium of 408&nbsp;mg/L and the much lower value of 0.3&nbsp;μg/L for caesium.<ref>{{cite journal |last1 = Bolter |first1 = E. |last2 = Turekian |first2 = K. |last3 = Schutz |first3 = D. |title = The distribution of rubidium, cesium and barium in the oceans |journal = Geochimica et Cosmochimica Acta |volume = 28 |issue = 9 |pages = 1459 |date = 1964 |doi = 10.1016/0016-7037(64)90161-9 |bibcode = 1964GeCoA..28.1459B }}</ref> Rubidium is the 18th most abundant element in seawater.<ref name="Hart-1973">{{Cite book |last1=Hart |first1=William A. |url=https://doi.org/10.1016/C2013-0-05695-2 |title=The Chemistry of Lithium, Sodium, Potassium, Rubidium, Cesium and Francium |last2=Beumel Jr. |first2=O.F . |last3=Whaley |first3=Thomas P. |date=1973 |publisher=Pergamon |isbn=978-0-08-018799-0|pages= |doi=10.1016/c2013-0-05695-2}}</ref>{{rp|371}}

Because of its large [[ionic radius]], rubidium is one of the "[[incompatible element]]s".<ref>{{cite book |url = https://books.google.com/books?id=385nPZOXmYAC&pg=PA224 |page = 224 |title = Cosmochemistry |isbn = 978-0-521-87862-3 |author1 = McSween Jr., Harry Y |author2 = Huss, Gary R |date = 2010|publisher = Cambridge University Press }}</ref> During [[Fractional crystallization (geology)|magma crystallization]], rubidium is concentrated together with its heavier analogue caesium in the liquid phase and crystallizes last. Therefore, the largest deposits of rubidium and caesium are zone [[pegmatite]] ore bodies formed by this enrichment process. Because rubidium substitutes for [[potassium]] in the crystallization of magma, the enrichment is far less effective than that of caesium. Zone pegmatite ore bodies containing mineable quantities of caesium as [[pollucite]] or the lithium minerals [[lepidolite]] are also a source for rubidium as a by-product.<ref name="USGS" />

Two notable sources of rubidium are the rich deposits of [[pollucite]] at [[Bernic Lake]], [[Manitoba]], Canada, and the [[rubicline]] {{chem|((Rb,K)AlSi<sub>3</sub>O<sub>8</sub>)}} found as impurities in pollucite on the Italian island of [[Elba]], with a rubidium content of 17.5%.<ref>{{Cite journal | title = Rubicline, a new feldspar from San Piero in Campo, Elba, Italy |journal = American Mineralogist |volume = 83 |issue = 11–12 Part 1 |pages = 1335–1339 |last1 = Teertstra |first1 = David K. |first2 = Petr |last2 = Cerny |first3 = Frank C. |last3 = Hawthorne |first4 = Julie |last4 = Pier |first5 = Lu-Min |last5 = Wang |first6 = Rodney C. |last6 =Ewing |date = 1998 |author-link2 = Petr Cerny|bibcode = 1998AmMin..83.1335T |doi = 10.2138/am-1998-11-1223 }}</ref> Both of those deposits are also sources of caesium.<ref>{{Cite encyclopedia |last=Enghag |first=Per |title=Rubidium and Cesium |url=https://onlinelibrary.wiley.com/doi/book/10.1002/9783527612338 |encyclopedia=Encyclopedia of the Elements|chapter = Rubidium and Caesium |publisher=Wiley |year=2004 |pages=301–313 |isbn=978-3-527-30666-4 |edition=1 |language=en |doi=10.1002/9783527612338.ch13}}</ref>

==Production==
[[File:Die Flammenfärbung des Rubidium.jpg|thumb|upright=0.65|Flame test for rubidium]]

Although rubidium is more abundant in Earth's crust than caesium, the limited applications and the lack of a mineral rich in rubidium limits the production of rubidium compounds to 2 to 4 [[tonne]]s per year.<ref name="USGS" /> Several methods are available for separating potassium, rubidium, and caesium. The [[fractional crystallization (chemistry)|fractional crystallization]] of a rubidium and caesium alum {{chem|(Cs,Rb)Al(SO<sub>4</sub>)<sub>2</sub>·12H<sub>2</sub>O}} yields after 30 subsequent steps pure rubidium alum. Two other methods are reported, the chlorostannate process and the ferrocyanide process.<ref name="USGS" /><ref>{{cite book |url = https://books.google.com/books?id=1ikjAQAAIAAJ&q=ferrocyanide+rubidium |publisher = United States. Bureau of Mines |title = bulletin 585 |date = 1995}}</ref>

For several years in the 1950s and 1960s, a by-product of potassium production called Alkarb was a main source for rubidium. Alkarb contained 21% rubidium, with the rest being potassium and a small amount of caesium.<ref>{{cite journal |title = Cesium and Rubidium Hit Market |journal = Chemical & Engineering News |volume = 37 |issue = 22 |pages = 50–56 |date = 1959 |doi = 10.1021/cen-v037n022.p050}}</ref> Today the largest producers of caesium produce rubidium as a by-product from pollucite.<ref name="USGS" />

==History==
[[File:Kirchhoff Bunsen Roscoe.jpg|thumb|left|upright|[[Gustav Kirchhoff]] (left) and [[Robert Bunsen]] (center) discovered rubidium by spectroscopy. ''([[Henry Enfield Roscoe]] is on the right.)''| 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.]]
Rubidium was discovered in 1861 by [[Robert Bunsen]] and [[Gustav Kirchhoff]], in Heidelberg, Germany, in the mineral [[lepidolite]] through [[flame spectroscopy]]. Because of the bright red lines in its [[emission spectrum]], they chose a name derived from the [[Latin]] word {{lang|la|rubidus}}, meaning "deep red".<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|hdl = 2027/hvd.32044080591324 |url = http://archiv.ub.uni-heidelberg.de/volltextserver/15657/1/spektral.pdf }}</ref><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>

Rubidium is a minor component in [[lepidolite]]. Kirchhoff and Bunsen processed 150&nbsp;kg of a lepidolite containing only 0.24% rubidium monoxide (Rb<sub>2</sub>O). Both potassium and rubidium form insoluble salts with [[chloroplatinic acid]], but those salts show a slight difference in solubility in hot water. Therefore, the less soluble rubidium [[hexachloroplatinate]] (Rb<sub>2</sub>PtCl<sub>6</sub>) could be obtained by [[fractional crystallization (chemistry)|fractional crystallization]]. After reduction of the hexachloroplatinate with [[hydrogen]], the process yielded 0.51&nbsp;grams of [[rubidium chloride]] (RbCl) for further studies. Bunsen and Kirchhoff began their first large-scale isolation of caesium and rubidium compounds with {{Convert|44000|litre|USgal}} of mineral water, which yielded 7.3&nbsp;grams of [[caesium chloride]] and 9.2&nbsp;grams of [[rubidium chloride]].<ref name="BuKi1861" /><ref name="Weeks" /> Rubidium was the second element, shortly after caesium, to be discovered by spectroscopy, just one year after the invention of the [[spectroscope]] by Bunsen and Kirchhoff.<ref name="autogenerated1">{{cite web |url=http://pubs.acs.org/cen/80th/print/rubidium.html |title=C&EN: It's Elemental: The Periodic Table – Cesium |publisher=American Chemical Society |access-date=2010-02-25 |first=Stephen K. |last = Ritter |date = 2003}}</ref>

The two scientists used the rubidium chloride to estimate that the [[atomic weight]] of the new element was 85.36 (the currently accepted value is 85.47).<ref name="BuKi1861" /> They tried to generate elemental rubidium by electrolysis of molten rubidium 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". They presumed that it was a [[Non-stoichiometric compound|subchloride]] ({{chem|Rb|2|Cl}}); however, the product was probably a [[colloid]]al mixture of the metal and rubidium 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=2010-09-26}}</ref> In a second attempt to produce metallic rubidium, Bunsen was able to reduce rubidium by heating charred rubidium [[tartrate]]. Although the distilled rubidium was [[pyrophoric]], they were able to determine the density and the melting point. The quality of this research in the 1860s can be appraised by the fact that their determined density differs by less than 0.1&nbsp;g/cm<sup>3</sup><!--1.52--> and the melting point by less than 1&nbsp;°C <!--38.5&nbsp;°C--> from the presently accepted values.<ref>{{cite journal |last1=Bunsen |first1=R. |title=Ueber die Darstellung und die Eigenschaften des Rubidiums |journal = Annalen der Chemie und Pharmacie |volume = 125 |issue = 3 |pages = 367–368 |date = 1863 |doi = 10.1002/jlac.18631250314|url=https://zenodo.org/record/1427191 }}</ref>

The slight radioactivity of rubidium was discovered in 1908, but that was before the theory of isotopes was established in 1910, and the low level of activity (half-life greater than 10<sup>10</sup>&nbsp;years) made interpretation complicated. The now proven decay of <sup>87</sup>Rb to stable <sup>87</sup>Sr through [[beta decay]] was still under discussion in the late 1940s.<ref>{{cite journal |doi = 10.1080/14786441008520248 | journal = Philosophical Magazine |series=Series 7| volume = 43 | issue = 345 | date = 1952 | first = G. M. | last = Lewis |pages = 1070–1074 | title =The natural radioactivity of rubidium}}</ref><ref>{{cite journal | last1= Campbell| first1 = N. R.| last2= Wood | first2= A. | date = 1908 | volume = 14 | page = 15 | title=The Radioactivity of Rubidium |journal=Proceedings of the Cambridge Philosophical Society| url=https://archive.org/stream/proceedingsofcam15190810camb/proceedingsofcam15190810camb_djvu.txt}}</ref>

Rubidium had minimal industrial value before the 1920s.<ref name="USGS"/> Since then, the most important use of rubidium is research and development, primarily in chemical and electronic applications. In 1995, rubidium-87 was used to produce a [[Bose–Einstein condensate]],<ref>{{cite press release |title = The 2001 Nobel Prize in Physics |year = 2001 |website = [[Nobel Institute]] nobelprize.org |url = http://nobelprize.org/nobel_prizes/physics/laureates/2001/press.html |access-date = 2010-02-01}}</ref> for which the discoverers, [[Eric Allin Cornell]], [[Carl Edwin Wieman]] and [[Wolfgang Ketterle]], won the 2001 [[Nobel Prize in Physics]].<ref>{{cite journal |last = Levi |first = Barbara Goss |author-link=Barbara Goss Levi |year=2001 |title = Cornell, Ketterle, and Wieman share Nobel Prize for Bose-Einstein condensates |journal = [[Physics Today]] |volume = 54 |issue = 12 |pages = 14–16|bibcode = 2001PhT....54l..14L  |doi = 10.1063/1.1445529|doi-access = free }}</ref>

==Applications==
[[File:USNO rubidium fountain.jpg|thumb|left|upright|A rubidium fountain [[atomic clock]] at the [[United States Naval Observatory]]]]
Rubidium compounds are sometimes used in [[fireworks]] to give them a purple color.<ref>{{Cite journal |first = E.-C. |last = Koch |title = Special Materials in Pyrotechnics, Part II: Application of Caesium and Rubidium Compounds in Pyrotechnics |journal = Journal Pyrotechnics |date = 2002 |volume = 15 |pages = 9–24 |url = http://www.jpyro.com/wp/?p=179 |access-date = 2010-01-29 |archive-date = 2011-07-13 |archive-url = https://web.archive.org/web/20110713122322/http://www.jpyro.com/wp/?p=179 |url-status = dead }}</ref> Rubidium has also been considered for use in a [[thermoelectric generator]] using the [[magnetohydrodynamics|magnetohydrodynamic]] principle, whereby hot rubidium ions are passed through a [[magnetic field]].<ref>{{cite book |url = https://books.google.com/books?id=59XvAAAAMAAJ&q=%22rubidium%22+%22magnetohydrodynamic%22 |page = 193 |title = Chemical principles |isbn = 978-0-06-040808-4 |author1 = Boikess, Robert S |author2 = Edelson, Edward |date = 1981|publisher = Harper & Row }}</ref> These conduct electricity and act like an [[armature (electrical engineering)|armature]] of a generator, thereby generating an [[electric current]]. Rubidium, particularly vaporized <sup>87</sup>Rb, is one of the most commonly used atomic species employed for [[laser cooling]] and [[Bose–Einstein condensation]]. Its desirable features for this application include the ready availability of inexpensive [[diode laser]] light at the relevant [[wavelength]] and the moderate temperatures required to obtain substantial vapor pressures.<ref>{{cite journal |journal = Journal of Research of the National Institute of Standards and Technology |url = http://nvl.nist.gov/pub/nistpubs/jres/101/4/cnt101-4.htm |title = Bose-Einstein condensation (all 20 articles) |date = 1996 |volume = 101 |issue = 4 |pages = 419–618 |author = Eric Cornell |display-authors = etal |doi = 10.6028/jres.101.045 |pmid = 27805098 |pmc = 4907621 |access-date = 2015-09-14 |archive-url = https://web.archive.org/web/20111014234040/http://nvl.nist.gov/pub/nistpubs/jres/101/4/cnt101-4.htm |archive-date = 2011-10-14 |url-status = dead }}</ref><!--All the 20 article are valid because they all describe the use of rubidium for Bose-Einstein condensation --><ref>{{cite journal | doi = 10.1088/0953-4075/32/12/322 | title = Output coupling of a Bose-Einstein condensate formed in a TOP trap | date = 1999 | last1 = Martin | first1 = J. L. | last2 = McKenzie | first2 = C. R. | last3 = Thomas | first3 = N. R. | last4 = Sharpe | first4 = J. C. | last5 = Warrington | first5 = D. M. | last6 = Manson | first6 = P. J. | last7 = Sandle | first7 = W. J. | last8 = Wilson | first8 = A. C. | journal = Journal of Physics B: Atomic, Molecular and Optical Physics | volume = 32 | issue = 12 | pages = 3065|arxiv = cond-mat/9904007 |bibcode = 1999JPhB...32.3065M | s2cid = 119359668 }}</ref> For cold-atom applications requiring tunable interactions, <sup>85</sup>Rb is preferred for its rich [[Feshbach resonance|Feshbach spectrum]].<ref>{{Cite journal |last1=Chin |first1=Cheng |last2=Grimm |first2=Rudolf |last3=Julienne |first3=Paul |last4=Tiesinga |first4=Eite |date=2010-04-29 |title=Feshbach resonances in ultracold gases |journal=Reviews of Modern Physics |volume=82 |issue=2 |pages=1225–1286 |doi=10.1103/RevModPhys.82.1225 |bibcode=2010RvMP...82.1225C |arxiv=0812.1496|s2cid=118340314 }}</ref>

Rubidium has been used for polarizing [[Helium-3|<sup>3</sup>He]], producing volumes of magnetized <sup>3</sup>He gas, with the nuclear spins aligned rather than random. Rubidium vapor is optically pumped by a laser, and the polarized Rb polarizes <sup>3</sup>He through the [[hyperfine structure|hyperfine]] interaction.<ref>{{Cite journal |url=http://www.ncnr.nist.gov/equipment/he3nsf/SEOP/nistSlowNeutronconf2005.pdf |journal=Journal of Research of the National Institute of Standards and Technology |title=Polarized <sup>3</sup>He spin filters for slow neutron physics |volume=110 |issue=3 |pages=299–304 |first1=T. R. |last1=Gentile |first2=W. C. |last2=Chen |first3=G. L. |last3=Jones |first4=E. |last4=Babcock |first5=T. G. |last5=Walker |doi=10.6028/jres.110.043 |pmid=27308140 |pmc=4849589 |year=2005 |access-date=2015-08-06 |archive-date=2016-12-21 |archive-url=https://web.archive.org/web/20161221234735/https://ncnr.nist.gov/equipment/he3nsf/SEOP/nistSlowNeutronconf2005.pdf |url-status=dead }}</ref> Such [[spin polarization|spin-polarized]] <sup>3</sup>He cells are useful for neutron polarization measurements and for producing polarized neutron beams for other purposes.<ref>{{Cite web |url=http://www.ncnr.nist.gov/AnnualReport/FY2002_html/pages/neutron_spin.htm |publisher=NIST Center for Neutron Research 2002 Annual Report |title=Neutron spin filters based on polarized helium-3 |access-date=2008-01-11}}</ref>

The resonant element in [[atomic clock]]s utilizes the [[hyperfine structure]] of rubidium's energy levels, and rubidium is useful for high-precision timing. It is used as the main component of secondary frequency references (rubidium oscillators) in cell site transmitters and other electronic transmitting, networking, and test equipment. These [[rubidium standard]]s are often used with [[GNSS]] to produce a "primary frequency standard" that has greater accuracy and is less expensive than caesium standards.<ref>{{cite book |chapter-url = https://books.google.com/books?id=jmfkJYdEANEC&pg=PA32 |page = 32 |chapter = GPS |title = Measurement, control, and communication using IEEE 1588 |isbn = 978-1-84628-250-8 |author1 = Eidson, John C |date = 2006-04-11|publisher = Springer }}</ref><ref name="Clock">{{cite book |chapter-url = https://books.google.com/books?id=ttYt5bZqX0AC&pg=PA300 |page = 300 |chapter = Rubidium and crystal oscillators |title = Data network engineering |isbn = 978-0-7923-8594-3 |author1 = King, Tim |author2 = Newson, Dave |date = 1999-07-31|publisher = Springer }}</ref> Such rubidium standards are often mass-produced for the [[telecommunications industry]].<ref>{{cite book |chapter-url = https://books.google.com/books?id=LesrjSVQMPQC&pg=PA72 |chapter = Rubidium Vapor Cell |title = Advances in electronics and electron physics |isbn = 978-0-12-014644-4 |author1 = Marton, L. |date = 1977-01-01|publisher = Academic Press }}</ref>

Other potential or current uses of rubidium include a working fluid in vapor turbines, as a [[getter]] in [[vacuum tube]]s, and as a [[photocell]] component.<ref>{{cite book |url = https://books.google.com/books?id=GEVt3kpFw64C&pg=PA274 |page = 274 |title = Introduction To Nuclear And Particle Physics |isbn = 978-81-203-3610-0 |author1 = Mittal |date = 2009|publisher = Prentice-Hall Of India Pvt. Limited }}</ref> Rubidium is also used as an ingredient in special types of glass, in the production of [[superoxide]] by burning in [[oxygen]], in the study of [[potassium]] [[ion channel]]s in biology, and as the vapor in atomic [[magnetometer]]s.<ref name="MAG">{{Cite journal |title=Parametric modulation of an atomic magnetometer |journal=Applied Physics Letters| volume=89| date=2006 |issue=13 |pages=23575531–23575533 |doi=10.1063/1.2357553 |last1=Li |first1=Zhimin |last2=Wakai |first2=Ronald T. |last3=Walker |first3=Thad G. |bibcode = 2006ApPhL..89m4105L |pmc=3431608 |pmid=22942436}}</ref> In particular, <sup>87</sup>Rb is used with other alkali metals in the development of spin-exchange relaxation-free [[SERF|(SERF) magnetometers]].<ref name="MAG" />

[[Rubidium-82]] is used for [[positron emission tomography]]. Rubidium is very similar to potassium, and tissue with high potassium content will also accumulate the radioactive rubidium. One of the main uses is [[myocardial perfusion imaging]]. As a result of changes in the [[blood–brain barrier]] in brain tumors, rubidium collects more in brain tumors than normal brain tissue, allowing the use of radioisotope rubidium-82 in [[nuclear medicine]] to locate and image brain tumors.<ref>{{cite journal |last1 = Yen |first1 = C. K. |last2 = Yano |first2 = Y. |last3 = Budinger |first3 = T. F. |last4 = Friedland |first4 = R. P. |last5 = Derenzo |first5 = S. E. |last6 = Huesman |first6 = R. H. |last7 = O'Brien |first7 = H. A. |title = Brain tumor evaluation using Rb-82 and positron emission tomography |journal = Journal of Nuclear Medicine |volume = 23 |issue = 6 |pages = 532–7 |date = 1982 |pmid = 6281406}}</ref> Rubidium-82 has a very short half-life of 76&nbsp;seconds, and the production from decay of [[strontium-82]] must be done close to the patient.<ref>{{cite book |chapter-url = https://books.google.com/books?id=FhkLE8MC71IC&pg=PA59 |page = 59 |chapter = Rubidium-82 |title = Clinical PET and PET/CT |isbn = 978-1-85233-838-1 |last1= Jadvar |first1= H. |last2 = Anthony Parker | first2 = J. |date = 2005|publisher = Springer }}</ref>

Rubidium was tested for the influence on manic depression and depression.<ref name="manic" /><ref>{{cite journal |last1 = Malekahmadi |first1 = P. |title = Rubidium in psychiatry: Research implications |journal = Pharmacology Biochemistry and Behavior |volume = 21 |pages = 49–50 |date = 1984 |doi = 10.1016/0091-3057(84)90162-X |pmid = 6522433 |last2 = Williams |first2 = John A.|s2cid = 2907703 }}</ref> Dialysis patients suffering from depression show a depletion in rubidium, and therefore a supplementation may help during depression.<ref>{{cite journal| last1 = Canavese| first1 = Caterina| last2 = Decostanzi| first2 = Ester| last3 = Branciforte| first3 = Lino| last4 = Caropreso| first4 = Antonio| last5 = Nonnato| first5 = Antonello| last6 = Sabbioni| first6 = Enrico| title = Depression in dialysis patients: Rubidium supplementation before other drugs and encouragement?| journal = Kidney International| volume = 60| issue = 3| pages = 1201–2| date = 2001| doi = 10.1046/j.1523-1755.2001.0600031201.x| pmid=11532118| doi-access = free}}</ref> In some tests the rubidium was administered as rubidium chloride with up to 720&nbsp;mg per day for 60 days.<ref name="isbn1-58890-299-4">{{cite book | last = Lake | first = James A. | title = Textbook of Integrative Mental Health Care | publisher = Thieme Medical Publishers | location = New York | date = 2006 | pages =164–165 | isbn = 978-1-58890-299-3 |url = https://books.google.com/books?id=Bt5euqMwbpYC&pg=PA165}}</ref><ref>{{cite journal |pmid=8412574 |date=1993 |last1=Torta |first1=R. |last2=Ala |first2=G. |last3=Borio |first3=R. |last4=Cicolin |first4=A. |last5=Costamagna |first5=S. |last6=Fiori |first6=L. |last7=Ravizza |first7=L. |title=Rubidium chloride in the treatment of major depression |volume=34 |issue=2 |pages=101–110 |journal=Minerva Psichiatrica}}</ref>

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==Precautions and biological effects==

Rubidium reacts violently with water and can cause fires. To ensure safety and purity, this metal is usually kept under dry [[mineral oil]] or sealed in glass ampoules in an inert atmosphere. Rubidium forms [[peroxide]]s on exposure even to a small amount of air diffused into the oil, and storage is subject to similar precautions as the storage of metallic [[potassium]].<ref>{{cite book |chapter-url = https://books.google.com/books?id=vKBqqiCTB7MC&pg=PA215 |page = 215 |chapter = Rubidium |title = Chemical risk analysis: a practical handbook |isbn = 978-1-903996-65-2 |author1 = Martel, Bernard |author2 = Cassidy, Keith |date = 2004-07-01|publisher = Butterworth-Heinemann }}</ref>

Rubidium, like sodium and potassium, almost always has +1 [[oxidation state]] when dissolved in water, even in biological contexts. The human body tends to treat Rb<sup>+</sup> ions as if they were potassium ions, and therefore concentrates rubidium in the body's [[intracellular fluid]] (i.e., inside cells).<ref>{{cite journal |last1 = Relman |first1 = A. S. |title =The Physiological Behavior of Rubidium and Cesium in Relation to That of Potassium |journal = The Yale Journal of Biology and Medicine |volume = 29 |issue = 3 |pages = 248–62 |date = 1956| pmid = 13409924|pmc = 2603856}}</ref> The ions are not particularly toxic; a 70&nbsp;kg person contains on average 0.36&nbsp;g of rubidium, and an increase in this value by 50 to 100 times did not show negative effects in test persons.<ref>{{cite journal |last1 = Fieve |first1 = Ronald R. |last2 = Meltzer |first2 = Herbert L. |last3 = Taylor |first3 = Reginald M. |title = Rubidium chloride ingestion by volunteer subjects: Initial experience |journal = Psychopharmacologia |volume = 20 |issue = 4 |pages = 307–14 |date = 1971 |pmid = 5561654 |doi = 10.1007/BF00403562|s2cid = 33738527 }}</ref> The [[biological half-life]] of rubidium in humans measures 31–46&nbsp;days.<ref name="manic">{{cite journal |last1 = Paschalis |first1 = C. |last2 = Jenner| first2 = F. A. |last3 = Lee |first3 = C. R. |title = Effects of rubidium chloride on the course of manic-depressive illness |journal = J R Soc Med |volume = 71 |issue = 9 |pages = 343–352 |date = 1978 |pmid = 349155|pmc = 1436619|doi = 10.1177/014107687807100507 }}</ref> Although a partial substitution of potassium by rubidium is possible, when more than 50% of the potassium in the muscle tissue of rats was replaced with rubidium, the rats died.<ref>{{cite journal | last1 = Meltzer | first1 = H. L. | title = A pharmacokinetic analysis of long-term administration of rubidium chloride | url = http://jcp.sagepub.com/content/31/2/179 | journal = Journal of Clinical Pharmacology | volume = 31 | issue = 2 | pages = 179–84 | date = 1991 | pmid = 2010564 | doi = 10.1002/j.1552-4604.1991.tb03704.x | s2cid = 2574742 | url-status = dead | archive-url = https://archive.today/20120709223213/http://jcp.sagepub.com/content/31/2/179 | archive-date = 2012-07-09 }}</ref><ref>{{cite journal| author=Follis, Richard H. Jr. |date=1943|title=Histological effects in rats resulting from adding rubidium or cesium to a diet deficient in potassium|url=https://journals.physiology.org/doi/abs/10.1152/ajplegacy.1943.138.2.246|journal=AJP: Legacy Content|volume=138|issue=2|pages=246–250|doi=10.1152/ajplegacy.1943.138.2.246}}</ref>

==References==
{{Reflist|30em}}

==Further reading==
* Meites, Louis (1963). ''Handbook of Analytical Chemistry'' (New York: McGraw-Hill Book Company, 1963)
* {{Cite web| author=Steck, Daniel A.| title=Rubidium-87 D Line Data| url=http://george.ph.utexas.edu/~dsteck/alkalidata/rubidium87numbers.pdf| publisher=Los Alamos National Laboratory (technical report LA-UR-03-8638)| access-date=2008-02-09| archive-date=2013-11-02| archive-url=https://web.archive.org/web/20131102072437/http://george.ph.utexas.edu/~dsteck/alkalidata/rubidium87numbers.pdf| url-status=dead}}

==External links==
*{{cite EB1911 |wstitle=Rubidium|volume=23 |pages=809 |short=x}}
* [http://www.periodicvideos.com/videos/037.htm Rubidium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)

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[[Category:Rubidium| ]]
[[Category:Chemical elements]]
[[Category:Alkali metals]]
[[Category:Reducing agents]]
[[Category:Chemical elements with body-centered cubic structure]]
[[Category:Pyrophoric materials]]