Abnormal delay in precipitation of magnesium oxalate with aged solutions

Abstract

Precipitation behavior of magnesium oxalate using oxalic acid dissolved in water/ethanol (W/E) solvents was investigated. This study examined the effects of various process parameters, including solvent composition, aging time of the starting solution, aging temperature, and alcohol species. Special attention was given to the delay time (incubation period, IP), defined as the time from solution mixing to the appearance of turbidity. A remarkably long delay exceeding 4 days was observed at W/E = 0/10 and with an aging time of 14 days. Regarding the solvent composition, an unexpected minimum point for IP was observed at W/E = 1/9. Furthermore, the addition of fresh oxalic acid to the aged solution drastically decreased the IPs. Molecular dynamics (MD) simulations qualitatively suggested that monovalent oxalic acid (HC₂O₄⁻) may experience locally hindered diffusion in pure ethanol. These findings suggest that oxalic acid, while appearing highly soluble macroscopically, might not exhibit similar solubility at the microscopic level, influencing the precipitation kinetics.