[372] Loc. cit.
[373] Ber. 1887, 20, 2134.
[374] Ber. 1908, 41, 308; also Hofmann, ibid. 1910, 43, 2631.
The element forms a rose-coloured oxide, and rose-coloured salts, which give to the compounds of the mixed erbia earths their characteristic colour. The oxide gives a very definite and characteristic reflection spectrum, but the salts do not possess this property;[375] the reflection spectrum remains unchanged in the presence of foreign oxides, provided no combination occurs. From the atomic weight determinations, it seems clear that the salts described by Cleve and his pupils[376] were not pure erbium compounds; a few salts only appear to have been recently obtained in the pure state for the [atomic weight determination] (q.v.).
[375] See Kruss and Bugge, Ber. 1908, 41, 3783.
[376] See Compt. rend. 1880, 91, 381.
The sulphate separates from aqueous solutions at ordinary temperatures as the octohydrate, Er₂(SO₄)₃,8H₂O, which forms rose-coloured monoclinic crystals isomorphous with the corresponding sulphates of the whole group. The anhydrous sulphate is formed by long heating at 400°, more quickly at 475°, and can be heated to 630° without decomposition. At 845° a basic salt, Er₂O₃,SO₃, is formed, which begins to decompose at 950°; at 1055° the transformation to the oxide is complete. The ammonium and potassium double sulphates are easily soluble in cold water.
The oxalate is thrown down in rosettes of bright rosy plates, which according to Hofmann[377] have the formula Er₂(C₂O₄)₃,10H₂O, even when dried in the air. Cleve believed the salt to be thrown down as the enneahydrate. When kept in vacuo over phosphoric anhydride, the decahydrate passes into the trihydrate, which when heated decomposes, passing into the oxide at a temperature of 575°. The nitrate, Er(NO₃)₃,5H₂O, separates from aqueous solution as the pentahydrate, in large stable red crystals. The platinocyanide, Er₂[Pt(CN)₄]₃,21H₂O, has the characteristic red colour with green fluorescence. The formate, Er(HCOO)₃—Cleve, loc. cit.—is a red powder, obtained by dissolving the oxide in formic acid; it crystallises from water as the dihydrate.
[377] Loc. cit.