[325] Compt. rend. 1900, 131, 343; ibid. 1901, 132, 1484.
[326] Ibid. 1905, 140, 583, etc.
[327] Zeitsch. anorg. Chem. 1905, 54, 374.
The gadolinia obtained by ignition of the salts of volatile acids should be perfectly white; presence of terbia causes it to assume a yellow colour.[328] The salts are colourless, and their solutions show no absorption in the visible region, though Urbain[329] has shown that there are four strong bands in the ultraviolet.
[328] Eberhard (loc. cit.) has shown that even in the perfectly white oxide, traces of terbia can be distinguished by spectroscopic examination.
[329] Compt. rend. 1905, 140, 1233.
The hydroxide, Gd(OH)₃, is a gelatinous precipitate with strongly basic properties, rapidly absorbing carbon dioxide from the air. The oxide, Gd₂O₃, also absorbs carbonic anhydride from the air, and is easily soluble in acids, even after strong ignition. The element is therefore strongly electropositive. Its position among the yttrium elements, however, is justified by the properties of the platinocyanide, 2Gd(CN)₃,3Pt(CN)₂,18H₂O, which forms long, pointed red crystals, with a green metallic lustre, belonging to the rhombic system, and isomorphous with the corresponding yttrium and erbium salts; the cerium elements, on the other hand, give yellow platinocyanides, with a blue metallic lustre, which crystallise in the monoclinic system.
The nitrate, Gd(NO₃)₃,6H₂O, separates from aqueous solutions at the ordinary temperatures in large crystals belonging to the anorthic system, and is isomorphous with the corresponding compounds of praseodymium and neodymium.[330] From solutions in strong nitric acid, a pentahydrate is obtained, which melts at 92°; the hexahydrate melts at 91°. The sulphate separates from aqueous solution as the octohydrate, Gd₂(SO₄)₃,8H₂O, isomorphous with the corresponding salts of both groups. The anhydrous sulphate is much less soluble in water at 0° than the corresponding compounds of the cerium elements. The selenate forms hydrates with 10 and 8 molecules of water of crystallisation respectively; these are isomorphous with the corresponding selenates of yttrium and the erbium metals.
[330] Lang and Haitinger, Annalen, 1907, 351, 450.