Abstract This paper analyzed the water-solubility of ferric sulfate, composition of dispersion system of ferric hydroxide colloid prepared by ferric chloride solution, effect of sulfate on stability of ferric hydroxide colloid. It showed that the reason for ferric sulfate was not appropriate for preparing ferric hydroxide colloid was sulfate radical was more easily absorbed by the interface than chloride ion, so the number of negative charge in the solvent layer increased, and the number of negative charges in the diffusion layer decreased, then diffusion layer became thin, finally induced coagulation of ferric hydroxide colloidal particles as a result of mutual collision.
