German chemistsRobert Bunsen andGustav Kirchhoff discovered rubidium in 1861 by the newly developed technique,flame spectroscopy. The name comes from theLatin wordrubidus, 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 forlaser manipulation ofatoms.[10] Rubidium is not a known nutrient for anyliving organisms. However, rubidiumions have similar properties and the same charge as potassium ions, and are actively taken up and treated byanimal cells in similar ways.
Rubidium is a very soft,ductile, silvery-white metal.[11] It has a melting point of 39.3 °C (102.7 °F) and a boiling point of 688 °C (1,270 °F).[12] It formsamalgams withmercury andalloys withgold,iron,caesium,sodium, andpotassium, but notlithium (despite rubidium and lithium being in the same periodic group).[13] Rubidium and potassium show a very similar purple color in theflame test, and distinguishing the two elements requires more sophisticated analysis, such as spectroscopy.[14]
Rubidium is the second mostelectropositive of the stable alkali metals and has a very low firstionization energy of only 403 kJ/mol.[12] It has an electron configuration of [Kr]5s1 and is photosensitive.[15]: 382 Due to its strong electropositive nature, rubidium reacts explosively with water[16] to produce rubidium hydroxide and hydrogen gas.[15]: 383 As with all the alkali metals, the reaction is usually vigorous enough to ignite metal or thehydrogen gas produced by the reaction, potentially causing an explosion.[17] Rubidium, being denser than potassium, sinks in water, reacting violently; caesium explodes on contact with water.[18] 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.[19] Rubidium has also been reported to ignite spontaneously in air.[11]
Rubidium chloride (RbCl) is probably the most used rubidium compound: among several other chlorides, it is used to induce living cells to take upDNA; 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 corrosiverubidium hydroxide (RbOH), the starting material for most rubidium-based chemical processes;rubidium carbonate (Rb2CO3), used in some optical glasses, and rubidium copper sulfate, Rb2SO4·CuSO4·6H2O.Rubidium silver iodide (RbAg4I5) has the highestroom temperatureconductivity of any knownionic crystal, a property exploited in thin filmbatteries and other applications.[20][21]
Although rubidium ismonoisotopic, rubidium in the Earth's crust is composed of two isotopes: the stable85Rb (72.2%) and theradioactive87Rb (27.8%).[23] Natural rubidium is radioactive, with specific activity of about 670Bq/g, enough to significantly expose aphotographic film in 110 days.[24][25] Thirty additional rubidium isotopes have been synthesized with half-lives of less than 3 months; most are highly radioactive and have few uses.[26]
Rubidium-87 has ahalf-life of48.8×109 years, which is more than three times theage of the universe of(13.799±0.021)×109 years,[27] making it aprimordial nuclide. It readily substitutes forpotassium inminerals, and is therefore fairly widespread. Rb has been used extensively indating rocks;87Rbbeta decays to stable87Sr. Duringfractional crystallization, Sr tends to concentrate inplagioclase, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residualmagma may increase over time, and the progressingdifferentiation results in rocks with elevated Rb/Sr ratios. The highest ratios (10 or more) occur inpegmatites. 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 the87Sr/86Sr ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered (seerubidium–strontium dating).[28][29]
Rubidium-82, one of the element's non-natural isotopes, is produced byelectron-capture decay ofstrontium-82 with a half-life of 25.36 days. With a half-life of 76 seconds, rubidium-82 decays by positron emission to stablekrypton-82.[23]
Seawater 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.[33] Rubidium is the 18th most abundant element in seawater.[15]: 371
Because of its largeionic radius, rubidium is one of the "incompatible elements".[34] 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 zonepegmatite ore bodies formed by this enrichment process. Because rubidium substitutes forpotassium in the crystallization of magma, the enrichment is far less effective than that of caesium. Zone pegmatite ore bodies containing mineable quantities of caesium aspollucite or the lithium mineralslepidolite are also a source for rubidium as a by-product.[30]
Two notable sources of rubidium are the rich deposits ofpollucite atBernic Lake,Manitoba, Canada, and therubicline((Rb,K)AlSi3O8) found as impurities in pollucite on the Italian island ofElba, with a rubidium content of 17.5%.[35] Both of those deposits are also sources of caesium.[36]
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 4tonnes per year.[30] Several methods are available for separating potassium, rubidium, and caesium. Thefractional crystallization of a rubidium and caesium alum(Cs,Rb)Al(SO4)2·12H2O yields after 30 subsequent steps pure rubidium alum. Two other methods are reported, the chlorostannate process and the ferrocyanide process.[30][37]
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.[38] Today the largest producers of caesium produce rubidium as a by-product from pollucite.[30]
Rubidium is a minor component inlepidolite. Kirchhoff and Bunsen processed 150 kg of a lepidolite containing only 0.24% rubidium monoxide (Rb2O). Both potassium and rubidium form insoluble salts withchloroplatinic acid, but those salts show a slight difference in solubility in hot water. Therefore, the less soluble rubidiumhexachloroplatinate (Rb2PtCl6) could be obtained byfractional crystallization. After reduction of the hexachloroplatinate withhydrogen, the process yielded 0.51 grams ofrubidium chloride (RbCl) for further studies. Bunsen and Kirchhoff began their first large-scale isolation of caesium and rubidium compounds with 44,000 litres (12,000 US gal) of mineral water, which yielded 7.3 grams ofcaesium chloride and 9.2 grams ofrubidium chloride.[39][40] Rubidium was the second element, shortly after caesium, to be discovered by spectroscopy, just one year after the invention of thespectroscope by Bunsen and Kirchhoff.[41]
The two scientists used the rubidium chloride to estimate that theatomic weight of the new element was 85.36 (the currently accepted value is 85.47).[39] 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 asubchloride (Rb 2Cl); however, the product was probably acolloidal mixture of the metal and rubidium chloride.[42] In a second attempt to produce metallic rubidium, Bunsen was able to reduce rubidium by heating charred rubidiumtartrate. Although the distilled rubidium waspyrophoric, 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 g/cm3 and the melting point by less than 1 °C from the presently accepted values.[43]
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 1010 years) made interpretation complicated. The now proven decay of87Rb to stable87Sr throughbeta decay was still under discussion in the late 1940s.[44][45]
Rubidium compounds are sometimes used infireworks to give them a purple color.[48] Rubidium has also been considered for use in athermoelectric generator using themagnetohydrodynamic principle, whereby hot rubidium ions are passed through amagnetic field.[49] These conduct electricity and act like anarmature of a generator, thereby generating anelectric current. Rubidium, particularly vaporized87Rb, is one of the most commonly used atomic species employed forlaser cooling andBose–Einstein condensation. Its desirable features for this application include the ready availability of inexpensivediode laser light at the relevantwavelength and the moderate temperatures required to obtain substantial vapor pressures.[50][51] For cold-atom applications requiring tunable interactions,85Rb is preferred for its richFeshbach spectrum.[52]
Rubidium has been used for polarizing3He, producing volumes of magnetized3He gas, with the nuclear spins aligned rather than random. Rubidium vapor is optically pumped by a laser, and the polarized Rb polarizes3He through thehyperfine interaction.[53] Suchspin-polarized3He cells are useful for neutron polarization measurements and for producing polarized neutron beams for other purposes.[54]
The resonant element inatomic clocks utilizes thehyperfine 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. Theserubidium standards are often used withGNSS to produce a "primary frequency standard" that has greater accuracy and is less expensive than caesium standards.[55][56] Such rubidium standards are often mass-produced for thetelecommunications industry.[57]
Other potential or current uses of rubidium include a working fluid in vapor turbines, as agetter invacuum tubes, and as aphotocell component.[58] Rubidium is also used as an ingredient in special types of glass, in the production ofsuperoxide by burning inoxygen, in the study ofpotassiumion channels in biology, and as the vapor in atomicmagnetometers.[59] In particular,87Rb is used with other alkali metals in the development of spin-exchange relaxation-free(SERF) magnetometers.[59]
Rubidium-82 is used forpositron 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 ismyocardial perfusion imaging. As a result of changes in theblood–brain barrier in brain tumors, rubidium collects more in brain tumors than normal brain tissue, allowing the use of radioisotope rubidium-82 innuclear medicine to locate and image brain tumors.[60] Rubidium-82 has a very short half-life of 76 seconds, and the production from decay ofstrontium-82 must be done close to the patient.[61]
Rubidium was tested for the influence on manic depression and depression.[62][63] Dialysis patients suffering from depression show a depletion in rubidium, and therefore a supplementation may help during depression.[64] In some tests the rubidium was administered as rubidium chloride with up to 720 mg per day for 60 days.[65][66]
Rubidium reacts violently with water and can cause fires. To ensure safety and purity, this metal is usually kept under drymineral oil or sealed in glass ampoules in an inert atmosphere. Rubidium formsperoxides on exposure even to a small amount of air diffused into the oil, and storage is subject to similar precautions as the storage of metallicpotassium.[68]
Rubidium, like sodium and potassium, almost always has +1oxidation state when dissolved in water, even in biological contexts. The human body tends to treat Rb+ ions as if they were potassium ions, and therefore concentrates rubidium in the body'sintracellular fluid (i.e., inside cells).[69] The ions are not particularly toxic; a 70 kg person contains on average 0.36 g of rubidium, and an increase in this value by 50 to 100 times did not show negative effects in test persons.[70] Thebiological half-life of rubidium in humans measures 31–46 days.[62] 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.[71][72]
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