Zincates

Zinc hydroxide dissolves in excess of potassium hydroxide. A precipitate falls when the solution (a) stands; (b) is treated with alcohol; (c) is treated with carbon dioxide; (d) is diluted and boiled.

The zinc hydroxide acts as an extremely weak acid, and it has been suggested that it exists in alkaline solution largely in colloidal form, though the electrolysis of these solutions indicates zinc anions. When solutions of zinc hydroxide in sodium hydroxide are dialysed, the passage of the zinc through the membrane seems to vary with the physical condition of the zinc hydroxide before solution.

A maximum value has been found for the solubility of zinc hydroxide in caustic soda when the atomic ratios were Zn:Na = l:3. This might be due partly to hydrolysis and partly to dehydration of the hydroxide by the alkali. According to Moir, the solution of zinc hydroxide in caustic alkalies is essentially an equilibrium between zincic acid and the alkali. The reactions

K₂ZnO₂+H₂O = KZnO₂+KO = H₂ZnO₂+2KO

are slow, but hydrolysis is finally very complete. Klein found that the solubility of the zinc hydroxide was proportional to the alkali present, and referred the separation of zinc hydroxide on boiling to its conversion into a more stable and less soluble form. The differences in solubility may be due to both physical and chemical causes.

According to Hildebrand and Bowers, part of the zinc dissolves colloidally on the addition of the alkali and the rest reacts as a monobasic acid. Finally, the colloidal particles disappear and the solution contains the ion HZnO₂.

Crystalline potassium zincate, K₂O.ZnO, has been obtained by covering a solution of zinc hydroxide in caustic potash with alcohol. It dissolved in cold water, and boiling water decomposed it into K₂O.ZnO. The latter compound was obtained crystalline: water appears to decompose it. Prescott also obtained crystalline K₂O.ZnO, and the corresponding sodium compound probably occurs in aqueous solutions that are not too dilute. Zinc has been very generally assumed to dissolve in aqueous sodium hydroxide with the formation of Zn(ONa)₂, and a stable sodium zincate, Na₂O.ZnO.4H₂O, has been described.

The two crystalline sodium zincates, H₂Na₄Zn₃O₆.17H₂O and HNaZnO₂.3½H₂O, were isolated by alcoholic treatment of a solution of zinc oxide in strong aqueous sodium hydroxide. The former melted at 100° C. and the latter at 300° C.

Needle-shaped crystals of Zn(OH)(ONa).3H₂O have separated from a solution of zinc hydroxide in caustic soda which was kept in the winter cold.

Some zincoxides that may have the constitution M''(O.ZnOH)₂ + xH₂O, where M'' = Ca, Sr, or Ba, have been described.

Zinc hydroxide dissolves in excess of ammonia. Up to 1880 several compounds of zinc oxide with ammonia had been reported. The solutions of zinc hydroxide or oxide in ammonia probably contain the complex Zn(NH₄)₄, or, according to Bonsdorff, the complex Znm(NH₄)₃m.

A solution described as containing cuprammonium and zinc-ammonium hydroxide was used for producing parchment from paper.