2011年1月20日 星期四

Praseodymium www.tool-tool.com

Praseodymium is a soft, silvery, malleable and ductile metal in the lanthanide group. It is somewhat more resistant to corrosion in air than europium, lanthanum, cerium, or neodymium, but it does develop a green oxide coating that spalls off when exposed to air, exposing more metal to oxidation — a centimeter-sized sample of Pr completely oxidizes within a year.[2] For this reason, praseodymium is usually stored under a light mineral oil or sealed in glass.

Contrary to other rare-earth metals, which show antiferromagnetic or/and ferromagnetic ordering at low temperatures, Pr is paramagnetic at any temperature above 1 K.[1]

[edit] Chemical properties

Praseodymium metal tarnishes slowly in air and burns readily at 150 °C to form praseodymium(III,IV) oxide:

12 Pr + 11 O2 → 2 Pr6O11

Praseodymium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form praseodymium hydroxide:

2 Pr (s) + 6 H2O (l) → 2 Pr(OH)3 (aq) + 3 H2 (g)

Praseodymium metal reacts with all the halogens:

2 Pr (s) + 3 F2 (g) → 2 PrF3 (s) [green]2 Pr (s) + 3 Cl2 (g) → 2 PrCl3 (s) [green]2 Pr (s) + 3 Br2 (g) → 2 PrBr3 (s) [green]2 Pr (s) + 3 I2 (g) → 2 PrI3 (s) [green]

Praseodymium dissolves readily in dilute sulfuric acid to form solutions containing green Pr(III) ions, which exist as a [Pr(OH2)9]3+ complexes:[3]

2 Pr (s) + 3 H2SO4 (aq) → 2 Pr3+(aq) + 3 SO2−

4 (aq) + 3 H2 (g)

[edit] Compounds

See also: Category:Praseodymium compounds

In its compounds, praseodymium occurs in oxidation states +2, +3 and/or +4. Praseodymium(IV) is a strong oxidant, instantly oxidizing water to elemental oxygen (O2), or hydrochloric acid to elemental chlorine. Thus, in aqueous solution, only the +3 oxidation state is encountered. Praseodymium(III) salts are yellow-green and, in solution, present a fairly simple absorption spectrum in the visible region, with a band in the yellow-orange at 589-590 nm (which coincides with the sodium emission doublet), and three bands in the blue/violet region, at 444, 468, and 482 nm, approximately. These positions vary slightly with the counter-ion. Praseodymium oxide, as obtained by the ignition of salts such as the oxalate or carbonate in air, is essentially black in color (with a hint of brown or green) and contains +3 and +4 praseodymium in a somewhat variable ratio, depending upon the conditions of formation. Its formula is conventionally rendered as Pr6O11.

Other praseodymium compounds include:

  • Fluorides: PrF2, PrF3, PrF4
  • Chlorides: PrCl3
  • Bromides: PrBr3, Pr2Br5
  • Iodides: PrI2, PrI3, Pr2I5
  • Oxides: PrO2, Pr2O3, Pr6O11
  • Sulfides: PrS, Pr2S3
  • Sulfates: Pr2(SO4)3
  • Selenides: PrSe
  • Tellurides: PrTe, Pr2Te3
  • Nitrides: PrN

[edit] Isotopes

Main article: isotopes of praseodymium

Naturally occurring praseodymium is composed of one stable isotope, 141Pr. Thirty-eight radioisotopes have been characterized with the most stable being 143Pr with a half-life of 13.57 days and 142Pr with a half-life of 19.12 hours. All of the remaining radioactive isotopes have half-lives that are less than 5.985 hours and the majority of these have half-lives that are less than 33 seconds. This element also has six meta states with the most stable being 138mPr (t½ 2.12 hours), 142mPr (t½ 14.6 minutes) and 134mPr (t½ 11 minutes).

The isotopes of praseodymium range in atomic weight from 120.955 u (121Pr) to 158.955 u (159Pr). The primary decay mode before the stable isotope, 141Pr, is electron capture and the primary mode after is beta minus decay. The primary decay products before 141Pr are element 58 (cerium) isotopes and the primary products after are element 60 (neodymium) isotopes.

[edit] History

The name praseodymium comes from the Greek prasios (πράσιος), meaning green, and didymos (δίδυμος), twin. Praseodymium is frequently misspelled as praseodynium.

In 1841, Mosander extracted the rare earth didymium from lanthana. In 1874, Per Teodor Cleve concluded that didymium was in fact two elements, and in 1879, Lecoq de Boisbaudran isolated a new earth, samarium, from didymium obtained from the mineral samarskite. In 1885, the Austrian chemist baron Carl Auer von Welsbach separated didymium into two elements, praseodymium and neodymium, which gave salts of different colors.

Leo Moser (son of Ludwig Moser, founder of the Moser Glassworks in what is now Karlovy Vary, Bohemia, in the Czech Republic, not to be confused with Leo Moser, a mathematician) investigated the use of praseodymium in glass coloration in the late 1920s. The result was a yellow-green glass given the name "Prasemit". However, a similar color could be achieved with colorants costing only a minute fraction of what praseodymium cost in the late 1920s, such that the color was not popular, few pieces were made, and examples are now extremely rare. Moser also blended praseodymium with neodymium to produce "Heliolite" glass ("Heliolit" in German), which was more widely accepted. The first enduring commercial use of purified praseodymium, which continues today, is in the form of a yellow-orange stain for ceramics, "Praseodymium Yellow", which is a solid-solution of praseodymium in the zirconium silicate (zircon) lattice. This stain has no hint of green in it. By contrast, at sufficiently high loadings, praseodymium glass is distinctly green, rather than pure yellow.[4]

Using classical separation methods, praseodymium was always difficult to purify. Much less abundant than the lanthanum and neodymium from which it was being separated (cerium having long since been removed by redox chemistry), praseodymium ended up being dispersed among a large number of fractions, and the resulting yields of purified material were low. Praseodymium has historically been a rare earth whose supply has exceeded demand. This has occasionally led to its being offered more cheaply than the far more abundant neodymium. Unwanted as such, much praseodymium has been marketed as a mixture with lanthanum and cerium, or "LCP" for the first letters of each of the constituents, for use in replacing the traditional lanthanide mixtures that were inexpensively made from monazite or bastnäsite. LCP is what remains of such mixtures, after the desirable neodymium, and all the heavier, rarer and more valuable lanthanides have been removed, by solvent extraction. However, as technology progresses, it has been found that praseodymium can be incorporated into neodymium-iron-boron magnets, thereby extending the supply of the much in demand neodymium[citation needed]. So LC is starting to replace LCP as a result.

[edit] Occurrence


Praseodymium is available in small quantities in Earth’s crust (9.5 ppm). It is found in the rare earth minerals monazite and bastnäsite, typically comprising about 5% of the lanthanides contained therein, and can be recovered from these minerals by an ion exchange process, or by counter-current solvent extraction. Misch metal, used in making cigarette lighters, contains about 5% praseodymium metal.[5]

[edit] Applications

Uses of praseodymium:

  • As an alloying agent with magnesium to create high-strength metals that are used in aircraft engines.[6][7]
  • Praseodymium forms the core of carbon arc lights which are used in the motion picture industry for studio lighting and projector lights.
  • Praseodymium compounds give glasses and enamels a yellow color.[8]
  • Praseodymium is used to color cubic zirconia yellow-green, to simulate the mineral peridot.
  • Praseodymium is a component of didymium glass, which is used to make certain types of welder's and glass blower's goggles.[8]
  • Silicate glass doped with praseodymium ions has been used to slow a light pulse down to a few hundred meters per second.[9]
  • Praseodymium alloyed with nickel (PrNi5) has such a strong magnetocaloric effect that it has allowed scientists to approach within one thousandth of a degree of absolute zero.[10]
  • Doping praseodymium in fluoride glass allows it to be used as a single mode fiber optical amplifier.[11]
  • Praseodymium oxide in solid solution with ceria, or with ceria-zirconia, have been used as oxidation catalysts.[12]
  • Modern ferrocerium firesteel products, commonly referred to as "flint", used in lighters, torch strikers, "flint and steel" fire starters, etc., contain around 4% praseodymium.[8]

[edit] Precautions

Like all rare earths, praseodymium is of low to moderate toxicity. Praseodymium has no known biological role.



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