Caesium - History

History

In 1860, Robert Bunsen and Gustav Kirchhoff discovered caesium in the mineral water from Dürkheim, Germany. Due to the bright blue lines in its emission spectrum, they chose a name derived from the Latin word caesius, meaning sky-blue. Caesium was the first element to be discovered spectroscopically, only one year after the invention of the spectroscope by Bunsen and Kirchhoff.

To obtain a pure sample of caesium, 44,000 litres (9,700 imp gal; 12,000 US gal) of mineral water had to be evaporated to yield 240 kilograms (530 lb) of concentrated salt solution. The alkaline earth metals were precipitated either as sulfates or oxalates, leaving the alkali metal in the solution. After conversion to the nitrates and extraction with ethanol, a sodium-free mixture was obtained. From this mixture, the lithium was precipitated by ammonium carbonate. Potassium, rubidium and caesium form insoluble salts with chloroplatinic acid, but these salts show a slight difference in solubility in hot water. Therefore, the less-soluble caesium and rubidium hexachloroplatinate ((Cs,Rb)₂PtCl₆) could be obtained by fractional crystallization. After reduction of the hexachloroplatinate with hydrogen, caesium and rubidium could be separated by the difference in solubility of their carbonates in alcohol. The process yielded 9.2 grams (0.32 oz) of rubidium chloride and 7.3 grams (0.26 oz) of caesium chloride from the initial 44,000 liters of mineral water.

The two scientists used the caesium chloride thus obtained to estimate the atomic weight of the new element at 123.35 (compared to the currently accepted one of 132.9). They tried to generate elemental caesium by electrolysis of molten caesium 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 a subchloride (Cs₂Cl). In reality, the product was probably a colloidal mixture of the metal and caesium chloride. The electrolysis of the aqueous solution of chloride with a mercury anode produced a caesium amalgam which readily decomposed under the aqueous conditions. The pure metal was eventually isolated by the German chemist Carl Setterberg while working on his doctorate with Kekulé and Bunsen. In 1882, he produced caesium metal by electrolyzing caesium cyanide, and thus avoiding the problems with the chloride.

Historically, the most important use for caesium has been in research and development, primarily in chemical and electrical fields. Very few applications existed for caesium until the 1920s, when it came to be used in radio vacuum tubes. It had two functions; as a getter, it removed excess oxygen after manufacture, and as a coating on the heated cathode, it increased its electrical conductivity. Caesium did not become recognized as a high-performance industrial metal until the 1950s. Applications of nonradioactive caesium included photoelectric cells, photomultiplier tubes, optical components of infrared spectrophotometers, catalysts for several organic reactions, crystals for scintillation counters, and in magnetohydrodynamic power generators.

Since 1967, the International System of Measurements has based its unit of time, the second, on the properties of caesium. The International System of Units (SI) defines the second as 9,192,631,770 cycles of the radiation, which corresponds to the transition between two hyperfine energy levels of the ground state of the caesium-133 atom. The 13th General Conference on Weights and Measures of 1967 defined a second as: "the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of caesium-133 atoms in their ground state undisturbed by external fields".