Cerium may be determined as cerium(IV) iodate, Ce(IO3)4, which is ignited to and weighed as the oxide, CeO2. Thorium (also titanium and zirconium) must, however, be first removed. The method is then applicable in the presence of relatively large quantities of lanthanides. Titrimetric methods are generally preferred.
The solution should not exceed 50 mL in volume, all metallic elements should be present as nitrates, and the cerium content should not exceed 0.10g. Treat the solution with half its volume of concentrated nitric acid, and add 0.5 g potassium bromate (to oxidise the cerium). When the latter has dissolved, add ten to fifteen times the theoretical quantity of potassium iodate in nitric acid solution (see Note) slowly and with constant stirring, and allow the precipitated cerium(IV) iodate to settle. When cold, filter the precipitate through a fine filter paper (e.g. Whatman No. 42 or 542), allow to drain, rinse once, and then wash back into the beaker in which precipitation took place by means of a solution containing 0.8 g potassium iodate and 5 mL concentrated nitric acid in 100 mL. Mix thoroughly, collect the precipitate on the same paper, drain, wash back into the beaker with hot water, boil, and treat at once with concentrated nitric acid dropwise until the precipitate just dissolves (20-25 mL
of acid are required per 0.1 g of cerium). Add 0.25 g potassium bromate and as much potassium iodate-nitric acid solution as before. When cold, collect the cerium(IV) iodate upon the same filter paper, wash once with the washing solution, return to the beaker, stir with the washing solution, filter again, and wash three times with the same solution. Place the filter paper and precipitate in the same beaker, add 5-8 g oxalic acid and 50 mL water, and heat to boiling. After all the iodine has been expelled, set aside for several hours, filter, wash with cold water, dry, and ignite (at 500-600 °C) to constant weight in a platinum crucible. Weight as CeO2.
Note. This is prepared by dissolving 5 g of potassium iodate in 167 mL of concentrated nitric acid, and diluting to 500 mL.