Zinc hydroxide

Zinc hydroxide Zn(OH)2 is an inorganic chemical compound. It also occurs naturally as 3 rare minerals: wülfingite (orthorhombic), ashoverite and sweetite (both tetragonal).

Like the hydroxides of other metals, such as lead, aluminium, beryllium, tin and chromium, Zinc hydroxide (and Zinc oxide), is amphoteric. Thus it will dissolve readily in a dilute solution of a strong acid, such as HCl, and also in a solution of an alkali such as sodium hydroxide.

Preparation
It can be prepared by first dissolving zinc oxide in concentrated aqueous solution of sodium hydroxide. The resulting solution is strongly diluted.
 * Zn2+ + 2 OH− → Zn(OH)2.

The initial colorless solution contains the zincate ion:
 * Zn(OH)2 + 2 OH− → Zn(OH)42−.

Zinc hydroxide will dissolve because the ion is normally surrounded by water ligands; when excess sodium hydroxide is added to the solution the hydroxide ions will reduce the complex to a −2 charge and make it soluble. When excess ammonia is added, it sets up an equilibrium which provides hydroxide ions; the formation of hydroxide ions causes a similar reaction as sodium hydroxide and creates a +2 charged complex with a co-ordination number of 4 with the ammonia ligands - this makes the complex soluble so that it dissolves.

Unlike the hydroxides of aluminium and lead, zinc hydroxide also dissolves in excess aqueous ammonia to form a colorless, water-soluble ammine complex.

Applications
One major use is as an absorbent in surgical dressings. It is also used to find zinc salts by mixing sodium hydroxide with the suspect salt.