2.3 g of potassium oxalate monohydrate and 5.5 g of oxalic acid dihydrate are dissolved in 80 mL of water. A slow addition of 1.9 g of potassium dichromate is started with vigorous stirring, the solution is then evaporated down to 1/10th of the volume with a hot plate. The product crystallizes out in a trihydrated form.
7 H2C2O4 · 2H2O + 2 K2CO3 + K2Cr2O7 · H2O = 2 K3[Cr(C2O4)3] + 6 CO2 + 4 H2O
K3[Cr(C2O4)3] + 3 H2O = K3[Cr(C2O4)3] · 3H2O
The pure product