~Atomic Weight of Zirconium.~--The value of this constant is not very accurately known. The International Committee has adopted the value 90·6, but there is some uncertainty as to the value of the decimal fraction. Berzelius in 1825 employed the analytical sulphate method, and found Zr = 88·47. The numbers of Hermann (1844), obtained by the analysis of the oxychloride, 2ZrOCl₂,9H₂O,[471] were very discordant, the mean giving the value 89·56. Marignac in 1860 analysed the potassium salt, K₂ZrF₆; this he heated with strong sulphuric acid, the residue being ignited until all the zirconium sulphate was transformed to oxide; the weighed mixture was then freed from potassium sulphate, and the residual oxide dried and weighed. From the three ratios K₂ZrF₆ : ZrO₂, K₂ZrF₆ : H₂SO₄, and K₂SO₄ : ZrO₂, he obtained the mean values 90·02, 91·55, and 90·68 respectively. Weibull in 1881-1882 determined the ratios Zr(SO₄)₂ : ZrO₂ and Zr(SeO₄)₂ : ZrO₂ by ignition of the sulphate and selenate respectively; he obtained the values 89·55 and 90·81. [471] Chauvenet (_loc. cit._) could not confirm the existence of this hydrate. Bailey carried out a series of analytical sulphate determinations in 1890, obtaining the mean value 90·656. Brauner criticises the method on the ground that the preparation of the pure neutral anhydrous sulphate is almost impossible; the sulphate heated to 400° is not yet anhydrous, so that Bailey’s result, on this ground, is probably too low. Venable in 1898 analysed the oxychloride; he claimed to have obtained the compound ZrOCl₂,3H₂O, by heating the crystallised salt at 100°-125° in hydrogen chloride, a method which Chauvenet (_loc. cit._) has found to lead to the dihydrate, ZrOCl₂,2H₂O. His value was 90·803. ~Detection and Estimation.~--The following reactions may be employed to distinguish zirconium: (1) The oxalate precipitated from neutral or faintly acid solution dissolves readily in excess of oxalic acid; the oxalates of thorium and of the rare earth elements are practically insoluble under these conditions. The fluoride also dissolves in excess of hydrofluoric acid or of alkali fluoride, behaviour characteristic of this element alone among the group. (2) By fusion with sodium carbonate in the oxidising flame, a bead is obtained, which, when dissolved in boiling hydrochloric acid, forms a solution which gives a voluminous precipitate on addition of disodium hydrogen phosphate, if zirconium is present. Iron, aluminium, titanium, thorium, and rare earths have no influence on the test.[472] [472] Biltz and Mecklenburg, _Zeitsch. angew. Chem._ 1912, ~25~, 2110. (3) A solution of a zirconium salt in hydrochloric acid gives an orange colouration with curcuma paper. Ferric and titanium salts, if present, must be reduced by means of zinc before the test is applied.