When ferrous sulphate reacts with oxalic acid then Iron (II) oxalate is obtained, which on reaction with H2O2 and potassium oxalate gives potassium trioxalato ferrate(III).

FeSO4 + H2C2O4 ...> FeC2O4 + H2SO4

6FeC2O4 + H2O2 + 6K2C2O4 ...> 4 K3[Fe(C2O4)3] + 2Fe(OH)2

In the resulting mixture Fe(OH)2 is present. To convert this Fe(OH)2 oxalic acid is added to the resulting mixture.

2Fe(OH)2 + 3 H2C2O4 --> 2 K3[Fe(C2O4)3] + 6 H2O