Thursday, January 17, 2013

Happy Birthday to U

1989 marks the 200th anniversary of the establishment of uranium as an element, and its naming. In 1989 Martin Heinrich Klaproth (1743-1817) a German apothecary turned chemist postulated the existence of a new element in pitchblende, previously considered an ore of iron and zinc. In fact, so sure was he that he even named it uranium, after the newly discovered planet Uranus (in 1781, by Herschel). M. Klaproth was a self-taught chemist who went on to become Professor of Chemistry at the University of Berlin in 1810. In quantitative analytical chemistry he was a precise worker, introducing corrections for impurities of reagents and apparatus faults. He used flint and agate mortars for their inertness. Klaproth begun by dissolving pitchblende in nitric acid, to which he added potash, getting a yellow precipitate. This precipitate was soluble in excess potash. When the precipitate was dried to constant weight, it was found to be small greenish yellow crystals in hexagonal prisms. Convinced that these were of the compound of a new element, he tried to isolate the metal. When Klaproth obtained a black, lustrous powder on the bottom of his Crucible, he thought he had the metal. He was mistaken. We now know this to be the oxide.

So, although uranium managed to find its way into the textbooks of chemistry, the metal was not yet available. Six years after Klaproth's death, J. Arfvedson, a Swedish pupil of Berzelius tried to reduce a dark green oxide of uranium with hydrogen. The reaction yielded a brown powder, which he took to be the metal. This was because Arfvedson had started with U3O8, which he believed was the lower oxide of uranium. He obtained UO2, which he therefore thought was the metal itself.

It was left to Eugène-Melchior Péligot in 1841 to isolate the metal with the new reducing methods of the time. He heated anhydrous uranium chloride with potassium in a platinum crucible, and obtained a black metallic powder with properties noticeably different from the black powder got by Klaproth.

Although the first ingot of the metal was prepared by A. Noissan in May 1896, (who melted it in a high temperature electric furnace) uranium still did not still become as important as it is in the present century. True, its properties were studied and its atomic mass measured; but the atomic mass was wrongly determined to be 120, which gave Dmitri Mendeleev trouble in placing it in the periodic table correctly. He originally placed it in the third group of his table as the heavy analogue of aluminum. But since the properties were different from what they were expected to be, Mendeleev proposed a revaluation, and finally placed it in Group VI under tungsten, which made it the heaviest and last of the naturally occurring elements of the periodic table.

Before the importance of the metal was realized in the modern sense, its salts were used even in cheap ceramics and glassware to give colors ranging from pale yellow to green. During the First World War and shortly after (1914-1926) it was used as a carbide former or hardening agent in tool steels, since tungsten and molybdenum were scarce. Several tons of ferro-uranium containing 30% uranium were produced for this purpose.

Uranium is a radioactive metal. This was discovered by Antoine Henri Becquerel in 1896 while doing an experiment to find out whether phosphorescent materials emit X-rays. When a photographic plate got exposed through black paper on a cloudy day through the agency of the crystals he was using for the experiment, Becquerel concluded that some unknown rays from the crystals had caused the exposure. This prompted him to analyze the crystals and find a trace of uranium to which he attributed the phenomenon. Thus it was uranium that led to the discovery of radioactivity.

In 1938, the Germans Otto Hahn and Fritz Strassman produced the first artificial nuclear fission reaction using uranium. Four years later, Enrico Fermi and his co-workers at the University of Chicago made the first nuclear pile containing uranium, which supported a controlled chain reaction. This of course led to the nuclear reactor and on the bleak side, the atomic bomb.

Uranium is obtained from its major ore, pitchblende or uraninite, which although rare, is the richest source. This black mineral with a pitchy luster has oxides of uranium and also traces of thorium, yttrium, lead, radium, and helium. It is found in Canada, Zaire, Czechoslovakia, and France. Uranium is also concentrated from ores containing gold and silver, which are the primary products. Another ore is carnotite and very poor sources are some shales and phosphate rocks. In fact, these are so poor that only because phosphate is extracted from them that it is practicable to extract uranium.

Uranium­235 supports a chain reaction easily, so it has to be concentrated to be used, (its occurrence is 0.71% naturally) up to 90% for nuclear weapons. Basically three methods are used:

  1.       The Gaseous Diffusion Method: Here, gaseous compounds of uranium (UF6) are passed through porous barriers where the U-235 fraction being lighter passes through more easily. If this process is repeated, relatively high proportions of U-235 can be obtained over the much more abundant U-238.
  2.       The Centrifugal Method: Uses rapidly rotating cylinders to separate the two gaseous fractions by centrifugation.
  3.       The Spectroscopic Method: This employs two lasers lasing at such frequencies, that while one excites the U-235 atom, the other ionizes it. The U-255 is then separated electrostatically.

After enrichment, the uranium fuel is fabricated, that is converted to UO2 and packed into cylindrical pellets which are placed in hollow stainless steel rods for use as reactor fuel. Although U-238 is not easily fissionable, it is not without its uses: fast neutrons in breeder reactors convert it to U-239 which beta-decays to neptunium and further to plutonium-239. Pu-239 is also a much-used nuclear fuel.

Thus, in these 200 years, uranium has seen a slow, then rapid rise in the uses to which it is put. Also, the work of many scientists has gone into giving us the picture that we now see: miraculously potent in its energy production, yet threateningly dangerous in radiation and nuclear warheads.