Rubidium

Rubidium (pronounced /rʉˈbɪdiəm/ roo-BID-ee-əm) is a chemical element with the symbol Rb and atomic number 37. Rb is a soft, silvery-white metallic element of the alkali metal group.

Rubidium is very soft and highly reactive, with properties similar to other elements in group 1, such as very rapid oxidation in air. Its compounds have 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 atoms.

Rubidium is not known to be necessary for any living organisms. However, like caesium, rubidium ions are handled by living organisms in a manner similar to potassium: it is actively taken up by plants and by living animal cells.

Rubidium has one stable isotope,85Rb. The isotope 87Rb which composes almost 28% of naturally occurring rubidium is slightly radioactive, with a half-life of 49 billion years—more than three times longer than the estimated age of the universe.
Characteristics

Rubidium is the second most electropositive of the stable alkali elements and liquefies at a temperature of 39.3 °C (102.7 °F). Like other group 1 elements, this metal reacts violently in water. In common with potassium and caesium this reaction is usually vigorous enough to ignite the liberated hydrogen. Rubidium has also been reported to ignite spontaneously in air. Like other alkali metals, it forms amalgams with mercury and it can form alloys with gold, caesium, sodium, and potassium. The element gives a reddish-violet color to a flame. It was named after two strong emission lines in the dark red area of the spectrum.

History
Rubidium (Latin: rubidus, deepest red) was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff in the mineral lepidolite through the use of a spectroscope.[2] Processing 150 kg of lepidolite yielded only a few grams for analysis. Rubidium metal was first produced by the reaction of rubidium chloride with potassium by Bunsen.[2][3]

The first large scale isolation of caesium compounds from 44,000 liters of mineral water done by Bunsen and Kirchhoff yielded besides 7.3 grams of caesium chloride also 9.2 grams of rubidium chloride.[2]

Occurrence

Rubidium is about the twenty-third most abundant element in the Earth's crust, roughly as abundant as zinc and rather more common than copper.[4] It occurs naturally in the minerals leucite, pollucite, carnallite and zinnwaldite, which contain up to 1% of its oxide. Lepidolite contains between 0.3% and 3.5% rubidium and this is the commercial source of the element.[5] Some potassium minerals and potassium chlorides also contain the element in commercially significant amounts. One notable source is also in the extensive deposits of pollucite at Bernic Lake, Manitoba (also a source of the related element caesium). The caesium mineral pollucite found on the Italian island Elba contains small crystalls of the mineral rubicline ((Rb,K)AlSi3O8) with a rubidium content of 17.5 %.[6]

Rubidium metal can be produced by reducing rubidium chloride with calcium among other methods. In 1997 the cost of this metal in small quantities was about US$25/gram.

Isotopes
Main article: Isotopes of rubidium

There are 26 isotopes of rubidium known with naturally occurring rubidium being composed of just two isotopes; 85Rb (72.2%) and the radioactive 87Rb (27.8%). Natural rubidium is radioactive with specific activity of about 670 Bq/g, enough to expose a photographic film in approximately 30 to 60 days.

Rubidium-87 has a half-life of 4.88 × 1010 years. It readily substitutes for potassium in minerals, and is therefore fairly widespread. Rb has been used extensively in dating rocks; 87Rb decays to stable strontium-87 by emission of a negative beta particle. During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. Highest ratios (10 or higher) occur in pegmatites. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the 87Sr/86Sr ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered. See Rubidium-Strontium dating for a more detailed discussion.

Uses and applications

Rubidium had minimal industrial use before the 1930s. Since then, the most important use for rubidium historically has been in research and development, primarily in chemical and electronic applications.

In 1995 rubidium-87 was used to make a Bose-Einstein condensate[7], for which the discoverers won the 2001 Nobel Prize in Physics[8].

Rubidium is easily ionized, so it has been considered for use in ion engines for space vehicles (but caesium and xenon are more efficient for this purpose).

Rubidium compounds are sometimes used in fireworks to give them a purple color.[9]

RbAg4I5 has the highest room temperature conductivity of any known ionic crystal. This property could be useful in thin film batteries and in other applications.[10]

Rubidium has also been considered for use in a thermoelectric generator using the magnetohydrodynamic principle, where rubidium ions are formed by heat at high temperature and passed through a magnetic field. These conduct electricity and act like an armature of a generator thereby generating an electric current.

Rubidium, particularly vaporized 87Rb, 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.

Rubidium has been used for polarizing 3He (that is, producing volumes of magnetized 3He gas, with the nuclear spins aligned toward a particular direction in space, rather than randomly). Rubidium vapor is optically pumped by a laser and the polarized Rb polarizes 3He by the hyperfine interaction.[11] Spin-polarized 3He cells are becoming popular for neutron polarization measurements and for producing polarized neutron beams for other purposes.[12]

Rubidium is the primary compound used in secondary frequency references (rubidium oscillators) to maintain frequency accuracy in cell site transmitters and other electronic transmitting, networking and test equipment. Rubidium references are often used with GPS to produce a "primary frequency standard" that has greater accuracy but is less expensive than caesium standards. Rubidium references such as the LPRO series from Datum were mass-produced for the Telecom industry. The general life expectancy is 10 years or better for most designs.

Other potential or current uses of rubidium include:

* A working fluid in vapor turbines.
* A getter in vacuum tubes.
* A photocell component.
* The resonant element in atomic clocks. This is due to the hyperfine structure of rubidium's energy levels.
* An ingredient in special types of glass.
* The production of superoxide by burning in oxygen.
* The study of potassium ion channels in biology.
* Rubidium is used to locate brain tumors, due to its slight radioactivity.[4]
* Rubidium vapor has been used to make atomic magnetometers. 87Rb is currently being used, with other alkali metals, in the development of spin-exchange relaxation-free (SERF) magnetometers.[13]

Compounds
See also: Category:Rubidium compounds

Rubidium chloride is probably the most-used rubidium compound; it is used in biochemistry to induce cells to take up DNA, and as a biomarker since it is readily taken up to replace potassium, and does not normally occur in living organisms. Rubidium hydroxide is the starting material for most rubidium-based chemical processes; rubidium carbonate is used in some optical glasses.

Rubidium has a number of oxides, including Rb6O and Rb9O2 which form if rubidium metal is exposed to air; the final product of reacting with oxygen is the superoxide RbO2. Rubidium forms salts with most anions. Some common rubidium compounds are rubidium chloride (RbCl), rubidium monoxide (Rb2O) and rubidium copper sulfate Rb2SO4·CuSO4·6H2O. A compound of rubidium, silver and iodine, RbAg4I5, has interesting electrical characteristics and might be useful in thin film batteries.[14]

Precautions

Rubidium reacts violently with water and can cause fires. To ensure health, safety and purity, this element must be kept under a dry mineral oil, and in practice is usually sealed in glass ampoules in an inert atmosphere. Rubidium forms peroxides on exposure to even air diffusing into oil, and is thus subject to some of the same peroxide precautions as storage of metallic potassium.

Biological effects

Rubidium, like sodium and potassium, is almost always in its +1 oxidation state when dissolved in water, and this includes all biological systems. The human body tends to treat Rb+ ions as if they were potassium ions, and therefore concentrates rubidium in the body's intracellular fluid (i.e., inside cells). The ions are not particularly toxic, and are relatively quickly removed in the sweat and urine. As a result of changes in the blood brain barrier in brain tumors, rubidium collects more in brain tumors than normal brain tissue, allowing short-lived radioisotopes of rubidium to be used in nuclear medicine to locate and image brain tumors.

References

1. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
2. ^ a b c Kirchhoff,, G.; Bunsen, R. (1861). "Chemische Analyse durch Spectralbeobachtungen". Annalen der Physik und Chemie 189 (7): 337–381. doi:10.1002/andp.18611890702.
3. ^ Weeks, Mary Elvira (1932). "The discovery of the elements. XIII. Some spectroscopic discoveries". Journal of Chemical Education 9 (8): 1413–1434. doi:10.1021/ed009p1413.
4. ^ a b Chemical Olympics. Rubidium
5. ^ Wise, M. A. (1995). "Trace element chemistry of lithium-rich micas from rare-element granitic pegmatites". Mineralogy and Petrology 55 (13): 203–215. doi:10.1007/BF01162588.
6. ^ Teertstra, David K.; Cerny, Petr; Hawthorne, Frank C.; Pier, Julie; Wang, Lu-Min; Ewing, Rodney C. (1998). "Rubicline, a new feldspar from San Piero in Campo, Elba, Italy". American Mineralogist 83 (11–12 Part 1): 1335–1339. http://ammin.geoscienceworld.org/cgi/content/abstract/83/11-12_Part_1/1335.
7. ^ "Press Release: The 2001 Nobel Prize in Physics". http://nobelprize.org/nobel_prizes/physics/laureates/2001/press.html. Retrieved 2010-02-01.
8. ^ Levi, Barbara Goss (2001). "Cornell, Ketterle, and Wieman Share Nobel Prize for Bose-Einstein Condensates". Search & Discovery. Physics Today online. http://www.physicstoday.org/pt/vol-54/iss-12/p14.html. Retrieved 2008-01-26.
9. ^ Koch, E.-C. (2002). "Special Materials in Pyrotechnics, Part II: Application of Caesium and Rubidium Compounds in Pyrotechnics". Journal Pyrotechnics 15: 9–24. (Abstract).
10. ^ Bradley, J. N.; Greene, P. D. (1967). "Relationship of structure and ionic mobility in solid MAg4I5". Trans. Faraday Soc. 63: 2516. doi:10.1039/TF9676302516.
11. ^ Gentile, T.R. et al.. "Polarized 3He spin filters for slow neutron physics". Journal of Research of the National Institute of Standards and Technology 100: 299. http://nvl.nist.gov/pub/nistpubs/jres/110/3/j110-3gen.pdf.
12. ^ "Neutron spin filters based on polarized helium-3". NIST Center for Neutron Research 2002 Annual Report. http://www.ncnr.nist.gov/AnnualReport/FY2002_html/pages/neutron_spin.htm. Retrieved 2008-01-11.
13. ^ Li, Zhimin et al. (2006). "Parametric modulation of an atomic magnetometer". Applied Physics Letters 89: 134105. doi:10.1063/1.2357553.
14. ^ Smart, Lesley; Moore, Elaine (1995). "RbAg4I5". Solid state chemistry: an introduction. CRC Press. pp. 176–177. ISBN 9780748740680. http://books.google.com/books?id=pVw98i6gtwMC&pg=PA176.