Potassium tris(oxalato) ferrate (III)

Formula: K3[Fe(C2O4)3]·3H2O

Potassium salt of a complex acid, formed by Fe(III) and oxalate ions. As all ferrioxalates, has bright green color and is slightly light sensitve.

Potassium tris-oxalato ferrate

See also:


  • Crystal system: Monoclynic.
  • Crystal shape: Flat elongated hexagons, sticks.
  • Color: Deep green.
  • Stability on air: Slowly deterirated by dry air. Also, as all ferrioxalate compounds, they are slightly light sensitive.


This is one of the standard compounds chemistry students prepare in inorganic chemistry course, there are numerous preparation procedures for it inthe internet.

I perpared it by dissolving iron (III) hydroxide in excess of oxalic acid and thenneutralizing excess acid by potassium carbonate. Overal reaction equation was:

  1. Prepare iron (III) hydroxide Fe(OH)3. Oxide Fe2O3, and intermediates between them are good too, though they are less reactive. The easiest way is reaction between FeCl3 and NaOH or soda NaHCO3.

  2. Dissolve Fe(OH)3 in hot oxalic acid solution, taking 3 moles of acid per 1 mole of Fe(OH)3. This would produce dark greenish-brown solution of iron (III) oxalate

  3. Neutralize excess of acid with KOH or soda. Brown solution would turn bright green - color of the complex ion [Fe(C2O4)3]3+. Filter it and crystallize the compound from it.

Total equation is:

2Fe(OH)3 + 6H2C2O4 + 3K2CO3 → 2K3[Fe(C2O4)3] + 6H2O + 3CO2


The compound grows easily, using either evaporation or cooling method; my samples were grown using evaporation approach. Crystal shape seems to be strongly affected by impurities, present i nthe solution: excess acid, potassium oxalate and others. Depending on impurities compoistion, shape could change from flat hexagons to thin sticks or prismatic crystals.


The compound is not especially dangerous, but significant amounts of oxalates could cause poisoning.