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
Monazite
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.
引用出處：
http://en.wikipedia.org/wiki/Praseodymium
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