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Abstract

It has been found that phosphoric acid decreases the first excitation maximum of Ce(III) at 256 nm, increases the second excitation maximum at 297 nm and shifts the fluorescence maximum from 350 to 346 nm. Under optimum conditions, with λ_exc = 297 nm and λ_em = 346 nm, Ce(III) can be determined fluorimetrically with a detection limit of 1.2 ng ml^-1 in 12M-H₃PO₄ medium. No interference was observed from a 20-200 fold excess of HCl, H₂SO₄, Na, K, NH₄⁺, Al and the rare earth elements. HNO₃ interferes and Ce(IV) and Fe(III) interfere strongly. It follows from the stereofluorograms of Ce and Tb that the spectra of the two elements are practically independent. The detection limit for Tb(III) in 0.02-2.5M-H₂SO₄ medium for λ_exc = 222 nm and λ_em = 494 nm is 33 ng ml^-1. No interference was observed from a 5-20 fold excess of Al³⁺ and the other rare earth elements. The determination is slightly less sensitive in H₃PO₄ or HCl medium. The relative standard deviation of the measurement for 10 ng ml^-1 Ce(III) or 50 ng ml^-1 Tb(III) is about 3%.