Patent ID: 7122581

Claim:
A process for preparing a sol, the sol comprising: an aqueous phase; particles of a phosphate of one rare earth consisting of cerium; an acid other than phosphoric acid, a cerium salt of which is soluble in water, wherein the particles of phosphate are orthophosphates, and wherein said acid is selected from the group consisting of acetic acid, formic acid, citric acid and propionic acid, the process comprising: mixing a solution of salts of cerium with phosphate ions in a PO 4 3− /cerium mole ratio of more than 1 with control of the pH of the reaction medium to a value of more than 2; then ageing the precipitate obtained if the value of the pH of the reaction medium is in the range 2 to 6; separating the precipitate from the reaction medium; re-dispersing said precipitate in water; adding cerium and said acid to the dispersion in a quantity such that the final PO 4 3− /cerium mole ratio in the dispersion is equal to 1.