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Technetium

History

From the Greek word technetos, artificial. Element 43 was predicted on the basis of the periodic table, and was erroneously reported as having been discovered in 1925, at which time it was named masurium. The element was actually discovered by Perrier and Segre in Italy in 1937. It was also found in a sample of molybdenum sent by E. Lawrence that was bombarded by deuterons in the Berkeley cyclotron. Technetium was the first element to be produced artificially. Since its discovery, searches for the element in terrestrial material have been made. Finally in 1962, technetium-99 was isolated and identified in African pitchblende (a uranium rich ore) in extremely minute quantities as a spontaneous fission product of uranium-238 by B.T. Kenna and P.K. Kuroda. If it does exist, the concentration must be very small. Technetium has been found in the spectrum of S-, M-, and N-type stars, and its presence in stellar matter is leading to new theories of the production of heavy elements in the stars.

Isotopes

Twenty-two isotopes of technetium with masses ranging from 90 to 111 are reported. All the isotopes of technetium are radioactive. It is one of two elements with Z < 83 that have no stable isotopes; the other element is promethium (Z = 61). Technetium has three long lived radioactive isotopes: ⁹⁷Tc (T^1/2 = 2.6 x 10⁶ years), ⁹⁸Tc (T^1/2 = 4.2 x 10⁶ years) and ⁹⁹Tc (T^1/2 = 2.1 x 10⁵ years). ⁹⁵Tc_m ("m" stands for meta state) (T^1/2 = 61 days) is used in tracer work. However, the most useful isotope of technetium is ⁹⁹Tc_m (T^1/2 = 6.01 hours) is used in many medical radioactive isotope tests because of its half-life being short, the energy of the gamma ray it emits, and the ability of technetium to be chemically bound to many biologically active molecules. Because ⁹⁹Tc is produced as a fission product from the fission of uranium in nuclear reactors, large quantities have been produced over the years. There are kilogram quantities of technetium currently existing.

Properties

Technetium is a silvery-gray metal that tarnishes slowly in moist air. The common oxidation states of technetium are +7, +5, and +4. Under oxidizing conditions technetium (VII) will exist as the pertechnetate ion, TcO_4-. The chemistry of technetium is said to be similar to that of rhenium. Technetium dissolves in nitric acid, aqua regia, and concentrated sulfuric acid, but is not soluble in hydrochloric acid of any strength. The element is a remarkable corrosion inhibitor for steel. The metal is an excellent superconductor at 11K and below.

Handling

It is reported that mild carbon steels may be effectively protected by as little as 55 ppm of KTcO₄ in aerated distilled water at temperatures up to 250°C. This corrosion protection is limited to closed systems, since technetium is radioative and must be confined. ⁹⁸Tc has a specific activity of 6.2 x 10⁸ Bq/g. Activity of this level must not be allowed to spread. ⁹⁹Tc is a contamination hazard and should be handled in a glove box.