Potassium ferric oxalate is an inorganic complex which dissociates in a suitable solvent to give simple potassium (K+) and the complex ferric oxalate, [Fe(C2O4)3]3– ions. In the complex ferricoxalate ion, ferric (Fe3+) ion is linked to three oxalate groups (C2O4)2– through six covalent bonds.

Three molecules of water of hydration are present in potassium ferric oxalate.

It is prepared by heating solid ferrous oxalate with saturated solution of potassium oxalate at 40°C by adding required amount of 3% hydrogen peroxide slowly in two lots under stirring. Finally the solution is heated to boiling. Then requisite amount of 1 M oxalic acid is added drop wise to the boiling solution till solution becomes green. It is cooled to get crystalline salt.

How To Perform Experiment

Preparation of ferrous oxalate, Fe(C2O4).2H2O

(i) Dissolve 5.5 g of Mohr’s salt (ferrous ammonium sulphate), Fe(NH4)2(SO4)2.6H2O in 17 ml of warm water in 250 ml.

(ii) Add a few drops of (4 M) sulphuric acid to the solution necessary for complete dissolution of Mohr’s salt to avoid hydrolysis of ferrous ion in water.

(iii) In another 250 ml beaker dissolve about 3g of oxalic acid in 30 ml of water to prepare 30 ml of oxalic acid solution.

(iv) Add 30 ml of oxalic acid solution to the ferrous sulphate solution.

(v) Heat the mixed solution to boiling with ion continuous stirring.

(vi) Remove the beaker from the flame and allow the precipitate of ferrous oxalate Fe(C2O4).2H2O to settle.

(vii) Decant the supernatant liquid and leave the precipitate in the beaker.

(viii) Wash the precipitate. This is done by heating the precipitate with 30 ml of distilled water to 50°C, allowing the precipitant to settle down and decanting off the supernatant liquid. Better repeat this procedure once again.

Preparation of potassium ferric oxalate K3[Fe(C2O4)3].3H2O

(i) Prepare a saturated solution of potassium oxalate solution.

(ii) Take the ferrous oxalate precipitate on a 250 ml beaker and add to it 11 ml of saturated solution of potassium oxalate.

(iii) Heat the mixture to 40°C and add 20 ml of 3% hydrogen peroxide (H2O2) slowly with stirring.

(iv) Add another 20 ml of 3% H2O2 and heat the mixture to boiling.

(v) Add 10 ml of 1 M oxalic acid (the first 5 ml at once and the last 5 ml drop wise) to the boiling solution continue heating until the colour of the solution becomes green. 

If the required colour is not obtained, repeat the whole procedure.

Crystailization of potassium ferric oxalate

(i) Filter the hot green Coloured solution into a conical flask.

(ii) Add to the filtrate about 20 ml of ethyl alcohol and heat the mixture to 70°C. Add some more alcohol drop wise until the solution just term cloudy.

(iii) Cool the solution and let the crystals of the complex settle down. 

If the crystals are obtained, suspend a short piece of thread into the liquid and gently cork the flask. Keep the flask in the cupboard. After a few days good crystals of potassium ferric oxalate will be formed.

(iv) Filter, dry (by pressing between the sheets of filter paper) and weigh the potassium ferric oxalate.