Sodium aluminium sulfate

Formula: NaAl(SO₄)₂·12H₂O

Also known as sodium alum, this double sulfate is among the most elusive alums species. Typycally, alums have comparatively low solubility and crystallize easily, but for sodium aluminium sulfate it is not the case.

Properties

Crystal system: isometric
Crystal shape: octahedral
Color: colorless
Stability on air: very unstable

Preparation

Make solution of sodium and aluminium sulfates in stoichiometric proportion (1:1 molar ratio) and crystallize it carefully. An easy way to prepare such solution is to dissolve metallic aluminium in NaOH solution, and then add stoichiometric amount of sulfuric acid:

Al + NaOH + 3H₂O = Na[Al(OH)₄] + 1.5H₂

Na[Al(OH)₄] + 2H₂SO₄ + 8H₂O = NaAl(SO₄)₂·12H₂O

Growing

Definitely, some luck is needed to obtain crystals of this material. Solubility of sodium alum is much higher then of its potassium analog, reaching 208g/100ml at 15 °C, according to the Wikipedia.

Moreover, nucleation appears to be complicated. Sometimes, evaporation of a small amount of the solution does not produce any alum crystals.

Safety

Toxicity is low, but solutions can be irritating.

More photos

Mix of various crystalline species obtained by evaporating a solution of Na₂SO₄ and Al₂(SO₄)₃. Octahedral crystals, such as the large one at the right, are alum. I don't know what are the flat rhombic crystals.

References

Wikipedia on sodium alum