Potassium Alum

Formula: KAl(SO₄)₂·12H₂O Iconic compound for crystal growing.

Properties

• Crystal system: isometric (cubic)
• Crystal shape: octahedrons
• Stability on air: stable

Preparation

This compound usually available in chemical shops, but can be prepared too. Since it has relatively low solubility, it will readily crystallize from any solution, containing enough of K⁺, Al³⁺, SO₄^2- ions.

Some possible ways are:

• Dissolve Al in sulfuric acid, producing Al₂(SO₄)₃. Reaction is very slow due to protective film on aluminum. Then add K₂SO₄ and crystallize.
• Dissolve Al in KOH (warning: reaction very intensive, can start boiling), getting potassium aluminate:
  Al + KOH + 3H₂O → K[Al(OH)₄] + 3H₂(g).
  Then add sulfuric acid:
  K[Al(OH)₄] + 2H₂SO₄ → KAl(SO₄)₂ + 2H₂O

Growing

I used the simplest, traditional growing method: slow evaporation.'

Prepare a glass of saturated solution, make a small seed crystal and suspend the seed on a thin thread (very fine synthetic fiber, almost invisible).

Common problems in growing alum crystals:

• Crust formation: crystalline crust grows slowly on the sides of the glass, depleting the solution and slowing down crystal growth. Solved by moving crystal to a clean glass (crust can be re-dissolved and used again).
• Mold: some molds and bacteria can thrive in concentrated alum solution, dimming the crystal and reducing evaporation. I solved it by adding a few drops of iodine tincture.

Safety

This compound is safe.

More photos

References

• My blog