Rubidium ( /rʉˈbɪdiəm/ roo-BID-ee-əm) is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. The atomic weight is 85.4678. Elemental rubidium is very soft and highly reactive, with properties similar to other elements in group 1, such as very rapid oxidation in air. 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.
Two German chemists, Robert Bunsen and Gustav Kirchhoff, discovered rubidium in 1861 by the newly developed method of flame spectroscopy. 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 ions: it is actively taken up by plants and by living animal cells.
- 1 Characteristics
- 1.1 Compounds
- 1.2 Isotopes
- 1.3 Occurrence
- 2 Production
- 3 History
- 4 Uses and applications
- 5 Precautions and biological effects
- 6 References
- 7 Further reading
- 8 External links
Rubidium is the second most electropositive of the non-radioactive alkali elements and melts at a temperature of 39.3 °C (102.7 °F). Like other group 1 elements, this metal reacts violently with water. As with potassium (which is slightly less reactive) and caesium (which is slightly more reactive), this reaction is usually vigorous enough to ignite the hydrogen gas it liberates. 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 and its ions give a reddish-violet color to a flame. It was named after two strong emission lines in the dark red area of the spectrum.
As a symmetrical effect of rubidium metal's high reactivity toward oxidation and tendency to subsequent formation of the rubidium cation Rb+, this cation, once formed, is very stable, and is normally unreactive toward further oxidative or reductive chemical reactions.
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.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.
Main article: Isotopes of rubidium
There are 26 isotopes of rubidium known. Naturally occurring rubidium is composed of just two isotopes: stable 85Rb (72.2%) and the radioactive87Rb (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 87Sr 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 inpegmatites. 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 the87Sr/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.
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. 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. Some potassium minerals andpotassium chlorides also contain the element in commercially significant amounts.
Sea water contains an average of 125 µg/l of rubidium compared to the much higher value for potassium of 408 mg/l and the much lower value of 0.3 µg/l for caesium
Due to its large ionic radius, rubidium is one of the "incompatible elements." Duringmagma 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 does substitute for potassium in the crystallization of magma the enrichment is far less effective than in the case of caesium. Zone pegmatite ore bodies containing mine-able quantities of caesium as pollucite or the lithium minerals lepidolite are also a source for rubidium as a by product.
One notable source is 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 crystals of the mineral rubicline ((Rb,K)AlSi3O8) with a rubidium content of 17.5 %.
Although rubidium is more abundant in Earth's crust than caesium the limited applications and the lack of a mineral rich in rubidium limits its production to 2 to 4 tonnes per year. The are several methods to separate potassium, rubidium and caesium. The fractional crystallization of a rubidium and caesium alum (Cs,Rb)Al(SO4)2·12H2O yields after 30 subsequent steps pure rubidium alum. Reports of two other methods are given in the literature the chlorostannate process and the ferrocyanide process. For several years in the 1950s and 1960s a by-product of the potassium production called ALKARB was a main source for rubidium. Alkarb contained 21 % rubidium while the rest was potassium and a small fraction of caesium.
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.
Rubidium was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff in the mineral lepidolite through the use of a spectroscope. Due to the bright red lines in its emission spectrum, they chose a name derived from the Latin word rubidus, dark red.
Gustav Kirchhoff (left) and Robert Bunsen (center) discovered rubidium spectroscopically.
Rubidium is present as a minor component in lepidolite. Kirchhof and Bunsen processed 150 kg of a lepidolite containing only 0.24 % rubidium oxide. Potassium, rubidium form insoluble salts with chloroplatinic acid, but these salts show a slight difference in solubility in hot water. Therefore, the less-soluble rubidium hexachloroplatinate ((Rb)2PtCl6) could be obtained by fractional crystallization. After reduction of the hexachloroplatinate with hydrogen, rubidium could be separated by the difference in solubility of their carbonates in alcohol. This process yielded 0.51 grams of rubidium chloride for further studies. The first large scale isolation of caesium and rubidium compounds, performed from 44,000 liters of mineral water by Bunsen and Kirchhoff, yielded, besides 7.3 grams of caesium chloride, also 9.2 grams of rubidium chloride. Rubidium was the second element, shortly after caesium, to be discovered spectroscopically, only one year after the invention of the spectroscope by Bunsen and Kirchhoff.
The two scientists used the rubidium chloride thus obtained to estimate the atomic weight of the new element at 85.36 (compared to the currently accepted one of 85.47). They tried to generate elemental rubidium by electrolysis of molten rubidium chloride, but instead of a metal, they obtained a blue homogenous 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 asubchloride (Rb2Cl). In reality, the product was probably a colloidal mixture of the metal and rubidium chloride. In a second experiment to produce metallic rubidium Bunsen was able to reduce rubidium oxide with carbon. The distilled rubidium was pyrophoric and the density differed less than 0.1 g/cm3 and the melting point by less than 1 °C from the now established values.
 Uses and applications
A rubidium fountain atomic clock at the United States Naval Observatory
Rubidium had minimal industrial use before the 1920s. 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, for which the discoverers won the 2001 Nobel Prize in Physics.
Rubidium compounds are sometimes used in fireworks to give them a purple color. 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. Spin-polarized 3He cells are becoming popular for neutron polarization measurements and for producing polarized neutron beams for other purposes.
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 and 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 and a photocell component. The resonant element in atomic clocks utilizes the hyperfine structure of rubidium's energy levels. 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 channels in biology and as the vapor to make atomic magnetometers. In particular, 87Rb is currently being used, with other alkali metals, in the development of spin-exchange relaxation-free (SERF) magnetometers.
Rubidium-82 is used for positron emission tomography. Rubidium is very similar to potassium and therefore tissue with high potassium content will also accumulate the radioactive rubidium. One of the main uses are Myocardial perfusion imaging. The very short half-life of 76 seconds makes it necessary to produce the rubidium-82 from decay of strontium-82 close to the patient. As a result of changes in the blood brain barrier in brain tumors, rubidium collects more in brain tumors than normal brain tissue, allowing to use the radioisotopes rubidium-82 in nuclear medicine to locate and image brain tumors.
 Precautions and biological effects
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 even to air diffusing into oil, and is thus subject to some of the same peroxide precautions as storage of metallic potassium.
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, a 70 kg person contains on average 0.36 g of rubidium and an increase by 50 to 100 times this value did not showed negative effects in test persons. The half life in humans was measured to be between 31 and 46 days. Although a partial substitution of potassium by rubidium is possible, rats with a 100% substituted diet died after a few weeks. Rubidium was tested for the influence on manic depression.
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