The precipitation of alkaline‐earth metal and transition metal oxalates from aqueous solution. Induction periods and nucleation rates

The precipitation of magnesium, calcium, strontium and barium oxalates and of manganous, ferrous, cobalt, nickel and copper oxalates was studied from equivalent aqueous solutions at 22°C: the initial overall concentrations (C) generally varied from 0.001 to 0.2 M and the saturation ratios ( S mox ) varied from <10 to >3000. The induction periods before the main growth surge were measured and nucleation rates were determined from final crystal numbers and induction periods. Precipitation occurred through homogenous nucleation: the critical nuclei in supersaturated alkaline‐earth metal oxalate solutions were formed by aggregation of 6–8 M ++ Ox − ion‐pairs while the critical nuclei in supersaturated transition metal oxalate solutions were formed by aggregation of 6–8 MOx complexes (to units of 3–4 M ++ MOx 2 − ion‐pairs). Over the range studied, the nucleation rates then varied with saturation ratios according to the relation, equation image Nucleation rates at any saturation ratio decreased in the order Mg > Sr, Ba > Ca and Fe > Mn > Co, Cu > Ni; that is, generally in the order of increasing M ++ –Ox − and M ++ –MOx 2 − bond strengths and increasing surface energies of the metal oxalate crystals. Induction periods decreased with increasing‐concentration and saturation ratio; over equation image The factors t C 1 and t S 1 depended in turn on the ‘rate constants’ for nucleation and growth during the induction periods and on metal oxalate solubilities.