Synthesis of potassium trisoxalatoferrate(III) trihydrate

K₃[Fe(C₂O₄)₃].3H₂O

(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 - sulphuric 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+ + C2O42- -> FeC2O4(s)

2FeC2O4(s) + 6K+ + 4C2O42- + H2O2(l) +2H+ + H2O -> 2K3[Fe(C2O4)3]×3H2O(s)