Lanthanum, La = 139·0
As the most electropositive element of the rare earth group, lanthanum is the most similar in its chemical properties to the metals of the alkaline earths. The metal itself (see [p. 115]) oxidises even in dry air, and in moist air rapidly becomes coated with a white layer of hydroxide; it attacks water, and burns vigorously when heated in the air. An alloy with aluminium, of the formula LaAl₄, has been prepared by Muthmann and Beck[244]; it forms lustrous white crystals, very stable in the air and very resistant towards acids.
[244] Annalen, 1904, 331, 46.
The hydroxide is of interest from the fact that, if precipitated under suitable conditions, it has the power of taking up solid iodine to form a deep blue adsorption compound[245]; colloidal solutions of basic lanthanum acetate are also coloured blue by addition of a few drops of iodine solution. If precipitation with alkali be carried out in presence of hydrogen peroxide, an hydrated peroxide of the composition La₂O₅,nH₂O is obtained.[246] This compound partially decomposes with evolution of oxygen at ordinary temperatures; towards carbon dioxide and acids it acts as a true peroxide, with formation of hydrogen peroxide.
[245] Damour, Compt. rend. 1857, 43, 976; see also Biltz, Ber. 1904, 37, 719
[246] Melikoff and Pissarjewski, Zeitsch. anorg. Chem. 1899, 21, 70.
The oxide is colourless, and forms colourless salts with those acids in which the anion is not coloured. The oxide is distinguished from the other rare earth oxides in that it turns moistened litmus paper blue; it resembles lime, in hissing when slaked, absorbing carbon dioxide from the air, and liberating ammonia from ammonium salts. By fusion with alkali carbonates, and by digestion with concentrated alkali hydroxides, Baskerville and Catlett[247] claim to have obtained lanthanates and metalanthanates, but their work has not yet been confirmed.
[247] J. Amer. Chem. Soc. 1904, 26, 75.
The sulphate, La₂(SO₄)₃,9H₂O, is the least soluble of all the rare earth sulphates. The enneahydrate is the only form stable at ordinary temperatures,[248] though under special conditions, hydrates with 6 and with 16 molecules of water of crystallisation have been obtained. It separates in needles belonging to the hexagonal system; 100 parts of water dissolve at 0°, 3·01, and at 100°, 0·69 parts of the salt. The acetylacetone compound melts at 185°.
[248] Muthmann and Rölig, Ber. 1898, 31, 1718.