Determination of Thorium and Uranyl in Nitrophosphate Solution by On-Line Matrix-Elimination Reversed-Phase Chromatography

A reversed-phase chromatographic method combined with an on-line matrix-elimination procedure is described for the determination of thorium and uranium in the nitric acid digest of phosphate rocks. Phoslphate and other anions were fiirst removed from the sample solution using a cation-exchange pre-column to bind cations in the sample and a dilute nitric acid eluent to elute the anions. In the second stage of the procedure, a short C18 column was employed with a phenylhydroxyacetic acid solution used as the washing eluent. Thorium(IV) and uranyl were trapped on the C18 column whilst the transition and lanthanide metals present were flushed to waste due to their weaker retention. In the final step thorium(IV) and uranyl were transferred onto a reversed-phase C18 column, where they were sepoarated and quantified using -hydroxyisobutyric acid eluent with visible detection at 658 nm asfter post-column reaction with Arsenazo III. The entire procedure was performed automatically using a programmable HPLC pump.