Potassium tris(oxalato) aluminate

Formula: K₃[Al(C₂O₂)₃]·3H₂O

Also known as potassium alumooxalate or potassium oxalatoaluminate. Similar compounds:

• Sodium analog that have different structure: sodium tris(oxalato) aluminate.
• Iron analog, with the same structure and nice green color: potassium tris(oxalato) ferrate(III).

Properties

• Crystal system: monoclinic
• Crystal shape: elongated prisms, sticks
• Color: colorless
• Stability on air: not stable, crystals start cracking after several days on air, eventually turning white. More stable than sodium salt though.

Preparation

The simplest procedure is: dissolve metallic aluminium in aqueous KOH solution, taking 3 mols of KOH for each mol of Al. Then add 3 mols of oxalic acid. Initially, addition of oxalic acid causes white Al(OH)₃ to precipitate, but after heating an stirring, the precipitate dissolves in acid, forming alumooxalate complex. Then filter the solution and let it crystallize. Total equation of the reaction is:

2 Al + 6 KOH + 6 H₂C₂O₄·2H₂O = 2 K₃[Al(C₂O₄)₃] + 3 H₂ + 18 H₂O

Growing

I used slow evaporation method, as always. Solubility of the salt at room temperature is not very high, below 20g/100ml. It increases with temperature, so slow cooling crystallization method could work well.

Safety

All oxalates are moderately toxic. The biggest safety risk, however, is the preparation step. Reaction of metallic Al with KOH solution is very exothermic and tend to go runaway when done on a larger scale, don't drop all the aluminium at once. Beware of the extremely irritating alkaline aerosol that is produced in this reaction.

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