Ammonium Zinc Sulfate

Formula: (NH₄)₂Zn(SO₄)₂·6H₂O

Extremely shiny, extremely transparent: one of the best candidates for crystal growing.

Properties

Crystal system: monoclinic
Stability on air: stable. After many months, crystals showing no signs of dehydration.

Preparation

Prepare compound by dissolving equimolar amounts of zinc sulfate (ZnSO₄·7H₂O) and ammonium sulfate ((NH₄)₂SO₄) in water. It is less soluble than its constituents, so it will crystallize when hot satrated solutions are mixed.

Zinc sulfate can be prepared by dissolving in sulfuric acid metallic zinc:

Zn + H₂SO₄ → ZnSO₄ + H₂(g)

or, preferably, zinc oxide (ZnO), used as white pigment:

ZnO + H₂SO₄ → ZnSO₄ + H₂O

The second reaction is preferable, because it does not produce gas.

Ammonium sulfate is sometimes sold as fertilizer. It can be prepared by mixing ammonia solution with sulfuric acid (warning: reaction is very exoteric and can start boiling, irritating fumes of ammonia are produced when reaction is not yet complete):

2NH₃ + H₂SO₄ → (NH₄)₂SO₄

Growing

There is a secret in making extremely clear crystals.

After preparing the solution, add small amount of baking soda NaHCO₃ to it. I used 1g of soda per 100ml of saturated solution. Stir the solution; soon, reaction starts that produces gas (CO₂) and some white sediment (ZnCO₃).

Let the solution stay undisturbed for a day (it is needed for completing reaction and forming sediment), filter it and use for growing.

Crystals, grown from such solution, would be brilliantly clear.