HfOCl2 + 3Na2C2O4 + H2C2O4 → Na4[Hf(C2O4)4] + 2NaCl + H2O
Hafnium(IV) dioxide-chloride react with sodium oxalate and oxalic acid to produce tetraoxalatohafnate(IV) sodium, sodium chloride and water. Sodium oxalate - solid. Oxalic acid - concentrated solution.