Homogeneous precipitation of Cr3+–M2+ (M=Ni, Zn, Co, Cu) oxalate by oxidation of the polyethylene glycol–cation complex

Cr3+–M2+ (M = Ni, Zn, Co, Cu) oxalate complexes were obtained by heat treatment of a polyethylene glycol (PEG) solution of a metal nitrate hydrate at 353 K for 6 h. The relation between the cation mole fraction [M2+/(Cr3+ + M2+)] in the PEG solution and in the precipitate obtained depended on the molecular weight of the PEG. The structure and the cation arrangement in the oxalate precipitate also depended on the molecular weight of the PEG. Two different models for the process of precipitate formation were examined in which it is assumed that Cr3+ and M2+ coordinate to PEG independently of and cooperatively with each other, respectively. The calculated relation between the M2+ mole fraction of the PEG solution and that of the precipitate was compared with the relation obtained from the experimental results. The results obtained from the second model agreed quite well with the experimental results. We therefore conclude that Cr3+ and M2+ coordinate cooperatively to PEGs.