What Is Zinc Hydroxide?
Zinc hydroxide, Zn(OH)2, is an amphoteric white powder, that reacts with both acids and alkalis but is insoluble in water. It transitions to zinc oxide upon heating and can form five structural variations, with the ε-form being orthorhombic.
Uses of Zinc Hydroxide
It’s employed in rubber compounding, surgical bandage absorbents, zinc oxide manufacturing, electrochemical synthesis, and the production of zinc salts. Zinc hydroxide derivatives find applications in rubber vulcanization, dyes, pharmaceuticals, cosmetics, fluxing agents, and agricultural chemicals.
Properties of Zinc Hydroxide
1. Amphoteric Hydroxide
Zinc hydroxide is an amphoteric hydroxide, that is, it’s soluble in both acidic and strong basic aqueous solutions. For example, when hydrochloric acid is added, it becomes zinc chloride, which is soluble in water. When sodium hydroxide is added in excess, zinc (II) tetrahydroxide acid ion [Zn(H2O)4]2+ is formed and dissolves.
- Reaction With Hydrochloric Acid
Zn(OH)2 + 2HCl → ZnCl2 + 2H2O - Reaction With Large Amount of Sodium Hydroxide
Zn(OH)2 + 2NaOH → 2Na+ + [Zn(H2O)4]2-
This property can be used as a test to detect zinc ions. However, it is not exclusive, as compounds of aluminum and lead also show similar reactions.
Reaction With Ammonia
When an excess of aqueous ammonia is added to zinc hydroxide, four ammonia molecules coordinate to Zn2+ to form the colorless tetraammonium zinc (II) ion [Zn(NH3)4]2+, which dissolves. This is not due to its amphoteric hydroxide property, but rather to the fact that Zn2+ tends to form complex ions with NH3 molecules.
- Reaction With Ammonia
Zn(OH)2 + 4NH3 → [Zn(NH3)4]2+ + 2OH–
When an excess of ammonia water is added to Zn(OH)2, an exchange reaction of H2O and NH3 for Zn2++ occurs and [Zn(NH3)4]2+ begins to form in an aqueous solution. As a result, [Zn(H2O)4]2+ decreases and Zn(OH)2 dissolves to compensate. Among the amphoteric metal ions (Al, Zn, Sn, Pb), only Zn2+ forms complex ions with ammonia.
Structure of Zinc Hydroxide
It has a tetrahedral coordination of OH groups around zinc. The α-phase is hexagonal, similar to cadmium hydroxide. The complex ion [Zn(H2O)4]2+ centered on Zn2+ also shows a tetrahedral structure.
Other Information on Zinc Hydroxide
Production Methods
Production involves adding sodium hydroxide to zinc sulfate to obtain a white precipitate or dissolving zinc oxide in hot concentrated sodium hydroxide. These processes yield zinc hydroxide, which can then precipitate as ε-phase crystals under specific conditions.
- From Zinc Sulfate
ZnSO4 + 2NaOH → Zn(OH)2 + Na2ZnSO4 - From Zinc Oxide
ZnO + 2OH– + H2O → [Zn(H2O)4]2-
[Zn(OH)4]2- → Zn(OH)2 + 2OH–
Safety Information
Zinc hydroxide can cause severe irritation upon contact with skin or eyes and poses significant environmental hazards, necessitating careful handling.