Zinc Hydroxide, Zn(OH)₂

When solutions of caustic alkalies or ammonia are added to solutions of zinc salts, Zn(OH)₂ falls as a white amorphous precipitate which dissolves in excess of the precipitant. It was obtained in rhombic prisms by allowing zinc to remain in contact with iron or copper under aqueous solutions of ammonia or caustic alkalies, or by adding zinc carbonate to an excess of potassium hydroxide solution. Becquerel obtained colourless octahedra by coiling copper strips round strips of zinc and leaving them in a solution of either silicic acid or clay in caustic alkali.

The amorphous variety is difficult to obtain pure by precipitating zinc salts with alkali, but a pure product has been prepared by treating a solution of the nitrate with carbonate-free alkali. Pure zinc hydroxide can be readily prepared by electrolysing a solution of a salt of an alkali metal (such as sodium sulphate) with a platinum cathode and a zinc anode. Zinc is dissolved by the anions, and precipitation is effected, more readily if the liquid is agitated, by the hydroxyl ions formed at the cathode. Electrolytically prepared zinc hydroxide has occurred crystalline.

The density of zinc hydroxide has been given as 3.053.

Octahedral crystals of the monohydrate Zn(OH)₂.H₂O have been obtained by allowing a saturated solution of zinc oxide in sodium hydroxide to stand for some weeks. According to de Forcrand only the crystalline variety is the anhydrous hydroxide. This becomes anhydrous zinc oxide at 125° C. in a current of dry air, which combines with water to form the crystalline ZnO.H₂O again - the heat of solution being 2.190 Cal. The amorphous hydroxide prepared by precipitation of zinc salts with alkalies corresponds, according to the conditions and temperature of desiccation, to 3ZnO.5H₂O, or 3ZnO.4H₂O, or 4ZnO.5H₂O. These hydrates still retain water when heated to 250° C. (perhaps forming 5ZnO.2H₂O), and even resist a prolonged temperature of 400° C. The anhydrous ZnO prepared at a red heat gives a number of differently hydrated compounds.

The zinc hydroxide precipitated by potassium hydroxide is said to become less soluble in the alkali on drying at 60°-70° C., and by the action of caustic potash on zinc sulphate three different varieties are said to be produced. One of these corresponds to the formula 2ZnO.H₂O, and two to ZnO.H₂O: each one is differently soluble in potassium hydroxide from the others.

Zinc hydroxide is very sparingly soluble in water, but the solubility is increased by the presence of various salts.

Its amphoteric character is very marked - both acids and alkalies readily dissolving it. Its acid character is less pronounced than that of aluminium hydroxide.