Praseodymium
Praseodymium | ||||||||||||||||||||||||||||
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Pronunciation | /ˌpreɪziːəˈdɪmiəm/[1] | |||||||||||||||||||||||||||
Appearance | grayish white | |||||||||||||||||||||||||||
Standard atomic weight Ar°(Pr) | ||||||||||||||||||||||||||||
Praseodymium in the periodic table | ||||||||||||||||||||||||||||
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kJ/mol | ||||||||||||||||||||||||||||
Heat of vaporization | 331 kJ/mol | |||||||||||||||||||||||||||
Molar heat capacity | 27.20 J/(mol·K) | |||||||||||||||||||||||||||
Vapor pressure
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Atomic properties | ||||||||||||||||||||||||||||
Discovery | Carl Auer von Welsbach (1885) | |||||||||||||||||||||||||||
Isotopes of praseodymium | ||||||||||||||||||||||||||||
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Praseodymium is a
Praseodymium always occurs naturally together with the other rare-earth metals. It is the sixth-most abundant rare-earth element and fourth-most abundant lanthanide, making up 9.1
Like most rare-earth elements, praseodymium most readily forms the +3 oxidation state, which is the only stable state in aqueous solution, although the +4 oxidation state is known in some solid compounds and, uniquely among the lanthanides, the +5 oxidation state is attainable in matrix-isolation conditions. The 0, +1, and +2 oxidation states are rarely found. Aqueous praseodymium ions are yellowish-green, and similarly, praseodymium results in various shades of yellow-green when incorporated into glasses. Many of praseodymium's industrial uses involve its ability to filter yellow light from light sources.
Physical properties
Praseodymium is the third member of the
Neutral praseodymium's 59 electrons are arranged in the configuration [Xe]4f36s2. Like most other lanthanides, praseodymium usually uses only three electrons as valence electrons, as the remaining 4f electrons are too strongly bound to engage in bonding: this is because the 4f orbitals penetrate the most through the inert xenon core of electrons to the nucleus, followed by 5d and 6s, and this penetration increases with higher ionic charge. Even so, praseodymium can in some compounds lose a fourth valence electron because it is early in the lanthanide series, where the nuclear charge is still low enough and the 4f subshell energy high enough to allow the removal of further valence electrons.[13]
Similarly to the other early lanthanides, praseodymium has a
Praseodymium, like all of the lanthanides, is
Chemical properties
Praseodymium metal tarnishes slowly in air, forming a
- 12 Pr + 11 O2 → 2 Pr6O11
This may be reduced to praseodymium(III) oxide (Pr2O3) with hydrogen gas.[17] Praseodymium(IV) oxide, PrO2, is the most oxidised product of the combustion of praseodymium and can be obtained by either reaction of praseodymium metal with pure oxygen at 400 °C and 282 bar[17] or by disproportionation of Pr6O11 in boiling acetic acid.[18][19] The reactivity of praseodymium conforms to periodic trends, as it is one of the first and thus one of the largest lanthanides.[13] At 1000 °C, many praseodymium oxides with composition PrO2−x exist as disordered, nonstoichiometric phases with 0 < x < 0.25, but at 400–700 °C the oxide defects are instead ordered, creating phases of the general formula PrnO2n−2 with n = 4, 7, 9, 10, 11, 12, and ∞. These phases PrOy are sometimes labelled α and β′ (nonstoichiometric), β (y = 1.833), δ (1.818), ε (1.8), ζ (1.778), ι (1.714), θ, and σ.[20]
Praseodymium is an electropositive element and reacts slowly with cold water and quite quickly with hot water to form praseodymium(III) hydroxide:[16]
- 2 Pr (s) + 6 H2O (l) → 2 Pr(OH)3 (aq) + 3 H2 (g)
Praseodymium metal reacts with all the stable halogens to form trihalides:[16]
- 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)
The
Praseodymium dissolves readily in dilute sulfuric acid to form solutions containing the chartreuse Pr3+ ions, which exist as [Pr(H2O)9]3+ complexes:[16][22]
- 2 Pr (s) + 3 H2SO4 (aq) → 2 Pr3+ (aq) + 3 SO2−
4 (aq) + 3 H2 (g)
Dissolving praseodymium(IV) compounds in water does not result in solutions containing the yellow Pr4+ ions;
Although praseodymium(V) in the bulk state is unknown, the existence of praseodymium in its +5 oxidation state (with the stable electron configuration of the preceding noble gas xenon) under noble-gas matrix isolation conditions was reported in 2016. The species assigned to the +5 state were identified as [PrO2]+, its O2 and Ar adducts, and PrO2(η2-O2).[26]
Organopraseodymium compounds
Organopraseodymium compounds are very similar to
Isotopes
Praseodymium has only one stable and naturally occurring isotope, 141Pr. It is thus a
History
In 1751, the Swedish mineralogist
While lanthanum turned out to be a pure element, didymium was not and turned out to be only a mixture of all the stable early lanthanides from praseodymium to
Occurrence and production
Praseodymium is not particularly rare, despite it being in the rare-earth metals, making up 9.2 mg/kg of the Earth's crust.[42] Praseodymium's classification as a rare-earth metal comes from its rarity relative to "common earths" such as lime and magnesia, the few known minerals containing it for which extraction is commercially viable, as well as the length and complexity of extraction.[43] Although not particularly rare, praseodymium is never found as a dominant rare earth in praseodymium-bearing minerals. It is always preceded by cerium and lanthanum and usually also by neodymium.[44]
The Pr3+ ion is similar in size to the early lanthanides of the cerium group (those from lanthanum up to samarium and europium) that immediately follow in the periodic table, and hence it tends to occur along with them in phosphate, silicate and carbonate minerals, such as monazite (MIIIPO4) and bastnäsite (MIIICO3F), where M refers to all the rare-earth metals except scandium and the radioactive promethium (mostly Ce, La, and Y, with somewhat less Nd and Pr).[40] Bastnäsite is usually lacking in thorium and the heavy lanthanides, and the purification of the light lanthanides from it is less involved. The ore, after being crushed and ground, is first treated with hot concentrated sulfuric acid, evolving carbon dioxide, hydrogen fluoride, and silicon tetrafluoride. The product is then dried and leached with water, leaving the early lanthanide ions, including lanthanum, in solution.[40]
The procedure for monazite, which usually contains all the rare earth, as well as thorium, is more involved. Monazite, because of its magnetic properties, can be separated by repeated electromagnetic separation. After separation, it is treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earth. The acidic filtrates are partially neutralized with
Praseodymium may then be separated from the other lanthanides via ion-exchange chromatography, or by using a solvent such as tributyl phosphate where the solubility of Ln3+ increases as the atomic number increases. If ion-exchange chromatography is used, the mixture of lanthanides is loaded into one column of cation-exchange resin and Cu2+ or Zn2+ or Fe3+ is loaded into the other. An aqueous solution of a complexing agent, known as the eluant (usually triammonium edtate), is passed through the columns, and Ln3+ is displaced from the first column and redeposited in a compact band at the top of the column before being re-displaced by NH+
4. The Gibbs free energy of formation for Ln(edta·H) complexes increases along with the lanthanides by about one quarter from Ce3+ to Lu3+, so that the Ln3+ cations descend the development column in a band and are fractionated repeatedly, eluting from heaviest to lightest. They are then precipitated as their insoluble oxalates, burned to form the oxides, and then reduced to metals.[40]
Applications
Leo Moser (not to be confused with
Like many other lanthanides, praseodymium's shielded
As the lanthanides are so similar, praseodymium can substitute for most other lanthanides without significant loss of function, and indeed many applications such as mischmetal and ferrocerium alloys involve variable mixes of several lanthanides, including small quantities of praseodymium. The following more modern applications involve praseodymium specifically or at least praseodymium in a small subset of the lanthanides:[51]
- In combination with neodymium, another rare-earth element, praseodymium is used to create high-power magnets notable for their strength and durability.[53] In general, most alloys of the cerium-group rare earths (lanthanum through samarium) with 3d transition metals give extremely stable magnets that are often used in small equipment, such as motors, printers, watches, headphones, loudspeakers, and magnetic storage.[51]
- Praseodymium–nickel intermetallic (PrNi5) has such a strong magnetocaloric effect that it has allowed scientists to approach within one thousandth of a degree of absolute zero.[54]
- As an alloying agent with magnesium to create high-strength metals that are used in aircraft engines; yttrium and neodymium are also viable substitutes.[55][56]
- Praseodymium is present in the rare-earth mixture whose fluoride forms the core of projector lights.[54]
- Praseodymium compounds give glasses, enamels and ceramics a yellow color.[10][51]
- Praseodymium is a component of
- Praseodymium oxide in solid solution with catalyst.[57]
Due to its role in permanent magnets used for wind turbines, it has been argued that praseodymium will be one of the main objects of geopolitical competition in a world running on renewable energy. However, this perspective has been criticized for failing to recognize that most wind turbines do not use permanent magnets and for underestimating the power of economic incentives for expanded production.[58][59]
Hazards | |
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GHS labelling: | |
Danger | |
H250 | |
P222, P231, P422[60] | |
NFPA 704 (fire diamond) |
Biological role and precautions
The early lanthanides have been found to be essential to some
Notes
- ^ The thermal expansion is highly anisotropic: the parameters (at 20 °C) for each crystal axis are αa = 1.4×10−6/K, αc = 10.8×10−6/K, and αaverage = αV/3 = 4.5×10−6/K.
References
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- ^ a b c d "Chemical reactions of Praseodymium". Webelements. Retrieved 9 July 2016.
- ^ a b Greenwood and Earnshaw, pp. 1238–9
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- ^ Greenwood and Earnshaw, p. 1424
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- ^ Weeks, Mary Elvira (1956). The discovery of the elements (6th ed.). Easton, PA: Journal of Chemical Education.
- ^ Marshall, James L.; Marshall, Virginia R. (Winter 2015). "Rediscovery of the elements: The Rare Earths – The Confusing Years" (PDF). The Hexagon: 72–77.
- ^ (Berzelius) (1839) "Nouveau métal" (New metal), Comptes rendus, 8 : 356–357. From p. 356: "L'oxide de cérium, extrait de la cérite par la procédé ordinaire, contient à peu près les deux cinquièmes de son poids de l'oxide du nouveau métal qui ne change que peu les propriétés du cérium, et qui s'y tient pour ainsi dire caché. Cette raison a engagé M. Mosander à donner au nouveau métal le nom de Lantane." (The oxide of cerium, extracted from cerite by the usual procedure, contains almost two fifths of its weight in the oxide of the new metal, which differs only slightly from the properties of cerium, and which is held in it so to speak "hidden". This reason motivated Mr. Mosander to give to the new metal the name Lantane.)
- ^ (Berzelius) (1839) "Latanium — a new metal," Philosophical Magazine, new series, 14 : 390–391.
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Bibliography
- Emsley, John (2011). Nature's Building Blocks: An A-Z Guide to the Elements. ISBN 978-0-19-960563-7.
- ISBN 978-0-08-037941-8.
Further reading
- R. J. Callow, The Industrial Chemistry of the Lanthanons, Yttrium, Thorium, and Uranium, Pergamon Press, 1967.
- Bouhani, H (2020). "Engineering the magnetocaloric properties of PrVO3 epitaxial oxide thin films by strain effects". Applied Physics Letters. 117 (7). arXiv:2008.09193. doi:10.1063/5.0021031.