Synthesis of potassium trisoxalatoferrate(III) trihydrate

This procedure for the synthesis of potassium trisoxalatoferrate(III) trihydrate, K₃[Fe(C₂O₄)₃]·3H₂O, is presented here:

Synthesis

(procedure by 'Endimion'[1])

Equipment:

• two beakers (100, 250 cm³)
• small graduated cyllinder or a pipette with 0.1 cm³ precision
• Büchner funnel with filter paper, hooked up to a Büchner flask
• vacuum pump with rubber tubing
• thermometer (-10 °C - 150 °C or similar)
• glass rod

Chemicals:

• Mohr salt
• sulfuric acid, 2 mol dm^-3
• aqueous solution of oxalic acid dihydrate, 10%
• potassium oxalate monohydrate
• ethanol, conc.
• distilled water
• acetone
• hydrogen peroxide, 20%

Procedure:

Dissolve 2 g of Mohr's salt in 7 cm³ of hot distilled water acidified with ~0.35 cm³ of 2 M sulphuric acid.

Add 9.8 cm³ of oxalic acid solution. Heat carefully to the boiling point. Stop the heating and wait for the yellow iron(II) oxalate to settle down. Decant the water and add 7 cm³ of hot distilled water. Mix thoroughly and repeat the decanting.

Prepare a hot aqueous solution of potassium oxalate monohydrate (put 1.3 g of the salt in 4.5 cm³ of water).

Add it to the iron(II) oxalate precipitate and mix the suspension. Slowly add 4 cm³ of hydrogen peroxide drop by drop while mixing with a thermometer, ensuring the temperature doesn't exceed 40 °C.

After that, heat to the boiling point and add 2.6 cm³ of the oxalic acid solution. The precipitate should dissolve.

Add 0.7 cm³ more, drop by drop, while maintaining the temperature close to the boiling point. Filter the hot solution and let it cool down.

Start adding 5 cm³ of ethanol, drop by drop. If crystals do not form, reduce the volume by boiling in the dark or leave it in the dark for several days.

Filter off the crystals using a Büchner funnel and wash them with a 1:1 solution of ethanol and water. Repeat the washing using acetone.

Store it at a dark place (black plastic bottle).

Reactions involved:

Fe²⁺ + C₂O₄^2- → FeC₂O₄(s)

2 FeC₂O₄(s) + 6 K⁺ + 4 C₂O₄^2- + H₂O₂(l) + 2 H⁺ + H₂O → 2 K₃[Fe(C₂O₄)₃]·3H₂O(s)