Zirconium forms only one series of compounds, in which the metal is tetravalent. Its chemical behaviour accords well with its position in the periodic classification. It is somewhat more electropositive than titanium, as shown by the fact that the hydroxide will not dissolve in alkalies, though zirconates may be obtained by the fusion methods; the oxide, however, is still a weak base, and the salts are to a large extent hydrolysed in solution. The formation of a stable oxychloride, which can be recrystallised without change in composition, shows clearly the strengthening of the electropositive character. It has still, however, in a high degree, the property of forming complex salts, which is characteristic of the less electropositive metals.
The group relations are borne out by the isomorphism of many related salts. The hydroxide and oxide show polymeric modifications, and the former has the usual tendency of compounds of this group to form colloidal solutions, a tendency which extends to the element itself. The metal resembles titanium in the eagerness with which it combines with other elements, especially with oxygen, nitrogen, and carbon, whilst the chloride closely resembles titanium tetrachloride in general properties, and in the ease with which it forms addition and condensation products with other substances.
The Metal.—All the difficulties which attend the attempts to prepare metallic titanium in the pure state have to be encountered in the preparation of metallic zirconium. The attempts which have been made have used the same methods, and obtained much the same kind of result as those employed in the case of titanium.[456] The reduction of potassium fluozirconate by metallic potassium, first employed by Berzelius, gives an amorphous product of unknown metal-content; it certainly contains a considerable percentage of oxygen. The monoxide is obtained when zirconia is reduced by magnesium (Winkler’s method). The reduction of the fluozirconates of potassium by means of sodium gives better results if the reaction is carried out in presence of sodium chloride in a sealed iron bomb; the product after careful washing contains 97-98 per cent. of the metal. Reduction with aluminium leads to the formation of alloys; Weiss and Neumann[457] have used these in the form of pencils as electrodes between which they pass the electric arc in vacuo, and so obtain an almost pure zirconium. The 97-98 per cent. amorphous product obtained by the sodium reduction also yields the practically pure metal when treated in this way (compare Titanium, [p. 223]). A very pure zirconium has been obtained by Wedekind[458] by heating the oxide with fine calcium turnings in an evacuated iron tube; the powdered product is washed, in absence of air, and heated in an evacuated porcelain tube to 800°-1000°, at which temperature the powder sinters into lumps which take a brilliant polish and contain 99·1 per cent. of the metal. Attempts to prepare a purer product from this by the method of Weiss and Neumann were unsuccessful.
[456] For a detailed account of these, see Lewis, Studien über das elementare Zirconium, Stuttgart, 1912.
[457] Zeitsch. anorg. Chem. 1909, 65, 248.
[458] Annalen, 1913, 395, 149.
The amorphous metal is a dark powder, which when washed with water on the filter paper passes through as a dark blue colloidal solution; it burns readily when heated in the air. According to Wedekind and Lewis,[459] amorphous zirconium is really the colloidal form of the metal. The fused metal is very hard (7-8, Mohs’ scale—it scratches quartz but not topaz) and very brittle; it has the density 6·4, and is of a whitish colour, with good metallic lustre on freshly broken surfaces. The atomic heat is abnormally high, being approximately 7·3; the element is paramagnetic. The melting-point was given by Wedekind and Lewis[460] as 2330°-2380°, but later work of the former author[461] gives the much lower value of 1530°, which seems more probable in view of the fact that the element cannot be employed for electric lamp filaments (see [p. 322]).
[459] Ibid. 1910, 371, 367.
[460] Weiss and Neumann, loc. cit.; also Wedekind, loc. cit.
[461] Annalen, 1913, 395, 149.