The separation of thorium, lanthanum and europium ions has been improved by acidic extractant bis(2-ethylhexyl)phosphoric acid (DEHPA)
in cyclohexane using 2-nitrobenzo-18-crown-6 (NB18C6) as a selective masking agent. The presence of NB18C6 shifts the extraction curve
of lanthanide ions versus pH towards higher pH region. NB18C6 was efficient masking agent towards lanthanide ions and a better separation
was obtained as compared to 18C6. The stability constants of the lanthanide-crown ether complexes in aqueous phase and the thermodynamic
parameters associated with the extraction process were evaluated.