Determination of lanthanum in monazite sand after selective solvent extraction separation of the picrate with dibenzo-[24]-crown-8

Abstract

The solvent extraction of La^III from picrate media was carried out with a dichloromethane solution containing large cavity size crown ethers (> 4Å), namely dibenzo-[24]-crown-8 (DB24C8), dicyclohexano-[24]-crown-8 (DCH24C8) and dibenzo-[30]-crown-10 (DB30C10). It was found that La^III was poorly extracted when DB30C10 was used, whereas both DB24C8 and DCH24C8 were good extractants. However, DB24C8 was preferred over DCH24C8 owing to its superior selectivity over the other lanthanides. The effects of variables such as solvent, counter ion concentration, pH, ligand concentration, equilibration time, stripping agent and selectivity over other elements were investigated. Under the optimized conditions, La^III was quantitatively extracted from 0.0002 mol l^–1 picric acid at pH 9 with 0.02 mol l^–1 DB24C8 in dichloromethane. The extracted metal ion was stripped from the organic phase with 0.5 mol l^–1 perchloric acid, and its concentration determined spectrophotometrically at 651.5 nm as its complex with Arsenazo III (pH, 3.2). A significant selectivity for La^III over other lanthanides was observed. Replicate analysis of a monazite sand sample for lanthanum yielded results comparable to those obtained by atomic absorption spectrometry. The composition of the extracted species, viz., La^III: DB24C8: picrate, was 1:1:3.