Heat 100 mL of water to 90°C, dissolve 7 g of oxalic acid and add slowly 4.4 g of potassium hydroxide. Add 6 g of copper (II) oxide and stir rapidly until the solution turns clear blue. Cool the solution down to 0°C, and filter, wash the crystals with very cold water.
H2C2O4 + KOH = C2HKO4 + H2O
C2HKO4 + CuO = CuC2O4 + KOH