Metallic zirconium is highly resistant to acids; it is attacked only by hydrofluoric acid and by aqua regia. In the compact form it burns in the air only at very high temperatures, though when powdered it glows in the air at a red heat, forming probably a mixture of lower oxides. It is attacked by chlorine and by hydrogen chloride at a red heat, with formation of the chloride; fused potash also oxidises it, with evolution of hydrogen. When heated in a current of hydrogen at a red heat, it forms the hydride, ZrH₂,[462] as a velvet-black powder, which burns with an intense bluish flame in oxygen, forming the sesquioxide, Zr₂O₃. When heated in nitrogen or ammonia, amorphous zirconium yields nitrides, which are also obtained when any attempt is made to reduce zirconium compounds to the metal in air. The most definite is the compound Zr₂N₃,[462] which forms a bronze-coloured powder, resistant to all mineral acids except hydrofluoric acid. Chlorine and bromine transform this to the halide.
[462] Wedekind and Lewis, Annalen, 1910, 371, 367.
The hydroxide is of doubtful individuality, since on drying it loses water progressively as the temperature is raised, no definite stable compound being known; in this respect zirconium resembles titanium. When heated to 100°, its composition corresponds approximately with that required by the formula ZrO₂,H₂O, but the percentage of water varies with the history of the specimen. When precipitated by alkalies in the cold, it forms the so-called α or ortho modification, which, like the analogous titanium compound, is readily soluble in dilute acids, and glows when heated. By precipitation at the boiling point, the β form is obtained; this is less soluble in acids, and does not glow when heated. The differences between the two forms are by no means sharply marked; they are rather the limiting forms of a continuously varying series than distinct chemical individuals, and the properties of any hydroxide precipitate depend very largely on the conditions under which it is thrown down.
The hydroxide is insoluble in water, but can be obtained in colloidal solution after it has been repeatedly heated with dilute acids, which serve to break down the molecular complexes; it can be also readily obtained in colloidal solution by dialysis of the nitrate, chloride, or acetate. In these solutions it is positively charged; electrolytes precipitate it with great ease. The gel has a very high power of forming adsorption products. When thrown down from solution by soda or potash, it carries down considerable quantities of alkali, to which it clings so tenaciously that the most careful washing cannot entirely remove them. If the gel be placed in contact with an ammoniacal solution of a cupric compound, it removes the cuprammonium complex entirely from the solution, becoming itself deep blue in colour, and leaving the liquid quite clear and colourless. In colloidal solution it forms adsorption compounds with negatively charged colloids, especially metals, the gels obtained from such solutions containing both colloids.
In the presence of hydrogen peroxide, ammonia throws down an hydrated peroxide, which is also obtained[463] by electrolysis of a brine solution in which the hydroxide is suspended, oxidation being effected by the sodium hypochlorite formed. This reaction is expressed by the equation:
Zr(OH)₄ + NaOCl = Zr(OOH)(OH)₃ + NaCl
[463] Pissarjewski, Zeitsch. anorg. Chem. 1900, 25, 378.
It is an endothermic compound, and is very unstable, losing oxygen on standing; by the action of acids it gives hydrogen peroxide. It dissolves in alkalies containing hydrogen peroxide; from such solutions, alcohol precipitates salts of the formula R´₄Zr₂O₁₁,9H₂O.
Zirconium oxide, ZrO₂, occurs in nature; it can be obtained in the laboratory as a voluminous white powder by ignition of the hydroxide or a suitable salt. The physical properties are described under the mineral Baddeleyite ([p. 75]) and in [Chapter XXI] ([p. 323]), in which an account of its technical applications is given. The melting-point is probably about 2700°; at 3000° it begins to volatilise. It dissolves readily in mineral acids, unless previously ignited very strongly; all specimens dissolve easily in hydrofluoric acid, and are readily converted by concentrated sulphuric acid into the sulphate.
When fused with metallic oxides or carbonates, it gives crystalline zirconates, of which a large number have been prepared; the calcium compound, CaZrO₃, is said to be isomorphous with perovskite, CaTiO₃.