Praseodymium: the essentials

Praseodymium is soft, silvery, malleable, and ductile. It was prepared in relatively pure form in 1931. 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" away when exposed to air. The metal should be stored under an inert atmosphere or under mineral oil or petroleum.

The rare-earth oxides, including Pr2O3, are among the most refractory substances known. It is a component of misch metal, used for lighter flints, and of the glass in welders' goggles.

Table: basic information about and classifications of praseodymium.

Praseodymium: historical information

Praseodymium was discovered by Carl F. Auer von Welsbach at 1885 in Austria. Origin of name: from the Greek words "prasios didymos" meaning "green twin".

In 1885, Carl Auer von Welsbach separated an "earth" called didymia obtained from the mineral samarskite into two earths, praseodymia and neodymia, which gave salts of different colours. The separation required the repeated fractionation of ammonium didymium nitrate.

Praseodymium: physical properties

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Praseodymium: orbital properties

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Isolation: praseodymium metal is available commercially so it is not normally necessary to make it in the laboratory, which is just as well as it is difficult to isolate as the pure metal. This is largely because of the way it is found in nature. The lanthanoids are found in nature in a number of minerals. The most important are xenotime, monazite, and bastnaesite. The first two are orthophosphate minerals LnPO4 (Ln deonotes a mixture of all the lanthanoids except promethium which is vanishingly rare) and the third is a fluoride carbonate LnCO3F. Lanthanoids with even atomic numbers are more common. The most comon lanthanoids in these minerals are, in order, cerium, lanthanum, neodymium, and praseodymium. Monazite also contains thorium and ytrrium which makes handling difficult since thorium and its decomposition products are radioactive.

For many purposes it is not particularly necessary to separate the metals, but if separation into individual metals is required, the process is complex. Initially, the metals are extracted as salts from the ores by extraction with sulphuric acid (H2SO4), hydrochloric acid (HCl), and sodium hydroxide (NaOH). Modern purification techniques for these lanthanoid salt mixtures are ingenious and involve selective complexation techniques, solvent extractions, and ion exchange chromatography.

Pure praseodymium is available through the reduction of PrF3 with calcium metal.

2PrF3 + 3Ca → 2Pr + 3CaF2

This would work for the other calcium halides as well but the product CaF2 is easier to handle under the reaction conditions (heat to 50°C above the melting point of the element in an argon atmosphere). Excess calcium is removed from the reaction mixture under vacuum.

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