[344] Trans. Chem. Soc. 1889, 55, 258.
The element gives the white sesquioxide, Tb₂O₃, and colourless salts.[345] The peroxide, of which the composition corresponds approximately to the formula Tb₄O₇, is obtained as a brownish-black powder by ignition of suitable salts. Its presence, even in small quantities, gives so deep a colouration to the other earths that some kind of salt formation seems probable. It is insoluble in cold acids; it dissolves in hot nitric acid with evolution of oxygen, forming a solution from which the nitrate, Tb(NO₃)₃,6H₂O, melting at 89·3°, separates on cooling. In hot hydrochloric acid, the peroxide dissolves with evolution of chlorine, forming solutions from which the chloride, TbCl₃,6H₂O, can be isolated with difficulty; this salt is extremely deliquescent, and easily forms supersaturated solutions. The sulphate, Tb₂(SO₄)₃,8H₂O, can be precipitated from a sulphuric acid solution of the oxide by addition of considerable quantities of alcohol; it is isomorphous with the other sulphate octohydrates, and is completely dehydrated at 360°.
[345] The terbium compounds here described have been prepared by Urbain (loc. cit.) from carefully purified material; other compounds have been described by Potratz (Chem. News, 1905, 92, 3), but her material contained a large proportion of gadolinium.
Atomic Weight.
—The value adopted by the International Committee is 159·2, which was obtained by Urbain in 1905 (loc. cit.) from the ratio Tb₂(SO₄)₃,8H₂O : Tb₂(SO₄)₃. This is the only determination on which reliance can be placed, as the material of the earlier workers was seldom even approximately pure.
Detection.
—Solutions of terbium salts show only one band in the visible spectrum, at 487·7 in the blue. This band was observed by Lecoq de Boisbaudran in a specimen of terbia containing dysprosia, and assumed by him to belong to a new element, Zδ (loc. cit.) In the ultraviolet nine absorption bands have been observed (Urbain, loc. cit.)
The spark spectrum shows the lines observed by Demarçay in 1900, and attributed by him to the new element Γ. Lecoq de Boisbaudran’s element Zβ showed a green fluorescence with the reversed spark, a phenomenon which Urbain has found to be exhibited by pure terbium compounds.
The arc spectrum of Urbain’s pure terbia was examined by Eberhard[346]—see also Exner and Haschek, and Eder and Valenta.[347] The element may be detected in minerals and earth mixtures by the following lines:
| 3523·82 | 3704·01 |
| 3676·52 | 4005·62 |
| 3703·05 | 4278·71 |