[252] Sitzungsber. kaiserl. Akad. Wiss. Wien, 1910, 119, IIa, 39.
Praseodymium, Pr = 140·6
This element occurs only in small quantities in the commoner rare earth minerals, and its separation in the pure state is in consequence a matter of very great difficulty. The salts and their solutions have a characteristic green colour. The salts are derived from the sesquioxide, Pr₂O₃, but a dioxide, PrO₂, and an intermediate oxide of uncertain composition are known. The absorption spectrum has five absorption bands, one of which coincides with a band in the absorption spectrum of neodymium; this fact has been interpreted as an indication of the non-elementary nature of both metals.[253] Difference in the absorption spectra have been put forward by several workers as indicating the complex nature of praseodymium, but an exhaustive examination by Stahl[254] in 1909 showed that there is no reason to doubt that the metal is really an element.
[253] Auer von Welsbach, Sitzungsber. kaiserl. Akad. Wiss. Wien, 1903, 112, IIa, July; also Urbain, Ann. Chim. Phys. 1900, [vii], 19, 184.
[254] Le Radium, 1909, 6, 215.
The metal is prepared by electrolysis of the fused chloride; in order to attain the temperature required to fuse the element, a very thin cathode is employed; if too powerful a current be used, the dioxide is formed. The metal is purified by remelting it in crucibles of magnesia, under a layer of anhydrous barium chloride. It has a yellowish shade, and is more stable in the air than lanthanum and cerium. For physical properties, see p. 115. No alloys have been prepared.
The hydroxide is thrown down by alkalies as a gelatinous green precipitate; in the presence of hydrogen peroxide, an hydrated peroxide, which closely resembles the corresponding lanthanum compound, is thrown down.
The Oxides.—By ignition of salts of volatile acids, Auer von Welsbach[255] obtained an oxide to which he assigned the formula Pr₄O₇. More recent work[256] has shown that the composition of the oxide obtained depends upon the conditions under which the various salts are decomposed. By fusing the nitrate in presence of potassium nitrate at 400-450°C., Meyer obtained the dioxide, PrO₂; at higher temperatures this decomposes, giving the intermediate oxides. The formation of the dioxide is greatly influenced by the presence of other oxides,[257]—ceric oxide, acting as an oxygen carrier, favouring whilst the other oxides hinder. The pure dioxide is a brownish-black powder, which resembles manganese dioxide, but is less stable. It liberates halogens from the halogen acids, and oxidises manganese salts to permanganates, but does not completely oxidise ferrous or stannous salts, losing instead a part of its oxygen in the gaseous form. The dioxide cannot be obtained in the wet way.
[255] Monats. 1885, 6, 477.
[256] See, e.g. Meyer, Zeitsch. anorg. Chem. 1904, 41, 94.