Rubidium - History

History

Rubidium was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff, in Heidelberg, Germany, in the mineral lepidolite through the use of a spectroscope. Because of the bright red lines in its emission spectrum, they chose a name derived from the Latin word rubidus, meaning "dark red".

Rubidium is present as a minor component in lepidolite. Kirchhoff and Bunsen processed 150 kg of a lepidolite containing only 0.24% rubidium oxide (Rb₂O). Both potassium and 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 (Rb₂PtCl₆) could be obtained by fractional crystallization. After reduction of the hexachloroplatinate with hydrogen. 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 as 85.36 (the currently accepted value is 85.47). 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 assigned it as a subchloride (Rb₂Cl); however, the product was probably a colloidal mixture of the metal and rubidium chloride. 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, it was possible to determine the density and the melting point of rubidium. The quality of the research done in the 1860s can be appraised by the fact that their determined density differs less than 0.1 g/cm3 and the melting point by less than 1 °C from the presently accepted values.

The slight radioactivity of rubidium was discovered in 1908 but before the theory of isotopes was established in the 1910s and the low activity due to the long half-life of above 1010 years made interpretation complicated. The now proven decay of 87Rb to stable 87Sr through beta decay was still under discussion in the late 1940s.

Rubidium had minimal industrial value before the 1920s. Since then, the most important use of rubidium has been in research and development, primarily in chemical and electronic applications. In 1995, rubidium-87 was used to produce a Bose–Einstein condensate, for which the discoverers, Eric Allin Cornell, Carl Edwin Wieman and Wolfgang Ketterle, won the 2001 Nobel Prize in Physics.