Substituted diamides of dipicolinic acid as extractants and ionophores for rare earth metals

The variety of newly synthesized diamides of dipicolinic acid (DPA) bearing methyl- and oxymethyl substituents in various positions of phenyl ring was investigated as potential ligands for liquid–liquid extraction of rare earth metal ions and as membrane active compounds in plasticized polymeric membranes of potentiometric sensors. It is found that scandium is extracted with 2,6-pyridinedicarboxylic acid diamides similarly to heavy lanthanides, while yttrium is extracted together with light group lanthanides. DPA shows high selectivity for the separation of cadmium/zinc, lead/nickel or copper/nickel pairs. The four out of six studied ligands demonstrate significant potentiometric sensitivity towards scandium, yttrium and lanthanides in 10−7–10−3 mol/L concentration range in nitric acid solutions. The trends in sensitivity values vary depending on a ligand structure and the highest values are observed for Sc3+ and heavy lanthanides. According to the evaluated selectivity Sm3+ is the most preferred ion, while La3+ is the most discriminated. The differences in sensor behavior in rare earth metal solutions assume that such devices can be applied in sensor arrays for potentiometric analysis of lanthanide mixtures.