Praseodymium

Praseodymium (or /ˌpreɪsioʊˈdɪmiəm/) is a chemical element that has the symbol Pr and atomic number 59.

Characteristics

Praseodymium is a soft silvery 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. For this reason, praseodymium should be stored under a light mineral oil or sealed in glass. In its compounds, praseodymium occurs in oxidation states +3 and/or +4. Praseodymium(IV) is a strong oxidant, instantly oxidizing water to elemental dioxygen, 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.

Applications

Uses of praseodymium:

• As an alloying agent with magnesium to create high-strength metals that are used in aircraft engines.
• 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.
• Praseodymium is used to color cubic zirconia yellow-green, to simulate peridot.
• Praseodymium is a component of didymium glass, which is used to make certain types of welder's and glass blower's goggles.
• Praseodymium mixed with silicate crystal has been used to slow a light pulse down to a few hundred meters per second.
• 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.
• Doping praseodymium in fluoride glass allows it to be used as a single mode fiber amplifier.
• Praseodymium oxide in solid solution with ceria, or with ceria-zirconia, have been used as oxidation catalysts.

History

The name praseodymium comes from the Greek prasios, meaning green, and didymos, 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 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.

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. RJ Callow presents a purification scheme using double ammonium nitrate crystallization, whereby the rare earths in monazite (under steady-state conditions, using appropriate recycling of mixed fractions) provided 10% of the rare earth content as a fraction containing 40% praseodymium. In the late 1950's, the Lindsay Chemical Division of the American Potash and Chemical Corporation, at the time the largest producer of rare earths in the world, offered praseodymium salts, purified in this manner, in 30% and 45% grades. The cheapest of these were the double ammonium nitrates, straight from the purification scheme: 30%: $6.30/lb. ($3.85/lb. in 50-lb. quantities), or 45%: $8.20/lb. ($4.95/lb. in 50-lb. quantities) (Reference: Lindsay, Price List, dated October 1, 1958). The one-pound prices for the corresponding oxides were 22.50 and 29.90 for the two purities. This product line soon vanished from the price lists, and was replaced by praseodymium as purified by ion-exchange. By 1959, 99% praseodymium oxide was priced at $40/lb. and the 99.9% grade was priced at $50 per pound, or alternatively at 20 or 25 cents per gram, respectively, in small lots.

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 bastnaesite. 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, praseodymium has been found possible to incorporate into neodymium-iron-boron magnets, thereby extending the supply of the much in demand neodymium. So LC is starting to replace LCP as a result.

Occurrence

Praseodymium is available in small quantities in Earth’s crust (9.5 ppm). It is found in the rare earth minerals monazite and bastnasite, typically comprising about 5% of the lanthanides contained therein, and can be recovered from bastnasite or monazite by an ion exchange process, or by counter-current solvent extraction.

Praseodymium also makes up about 5% of misch metal.

Compounds

Praseodymium compounds include:

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

See also Praseodymium compounds.

Isotopes

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 (t_½ 2.12 hours), ^142mPr (t_½ 14.6 minutes) and ^134mPr (t_½ 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.

Precautions

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

References

• Los Alamos National Laboratory – Praseodymium

R.J. Callow, "The Industrial Chemistry of the Lanthanons, Yttrium, Thorium and Uranium", Pergamon Press, 1967.

Price Lists, Lindsay Chemical Division, American Potash & Chemical Corporation, West Chicago, Illinois, dated October 1, 1958 or January 20, 1959.

External links

• WebElements.com – Praseodymium
• It's Elemental – The Element Praseodymium