Determination of Lanthanides By Conductometry Using Some Sequestering Agents

Abstract

The present work envisages establishing conductometry as an analytical technique for the determination of lanthanides by employing the use of different polyaminocarboxylic acids (PACA) as sequestering agents. The technique offers the advantage of eliminating the need of separation or pre-concentration of the sample under study. Five different polyaminocarboxylic acids viz., Ethylenediaminetetracetic acid (EDTA); trans-1,2- Diaminocyclohexane-N,N,N′,N′-tetraacetic Acid (DCTA); Ethylene glycol-bis(2- aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA); N-(2-hydroxyethyl) ethylenediamine- N,N΄,N΄-triacetic acid (EDTA-OH) and 1,3-Diaminopropane-N,N,N′,N′-tetraacetic acid (TMDTA) have been used as sequestering agents for this work. Effect of different parameters has been studied to establish appropriate conditions for the conductometric study of lanthanide‒PACA complexes which enabled precise lanthanide determinations. For this purpose, firstly, the appropriate cell constant was established by carrying out a number of conductometric titrations at four different cell constant values using two-pole and five ring conductivity cells. The latter was found suitable for the study due to its linearity over a wide concentration range and a quick and stable response. Secondly, the detection limit of the five ring conductivity cell has been determined as 10-5 M by direct as well as conductometric titration methods. Thirdly, the effect of pH on the reaction system has been studied over four different pH ranges. Lastly, the effect of temperature on the reaction system was also studied by carrying out conductometric titrations at four different temperatures. Following the standardization of these parameters all the polyaminocarboxylic acids have been tested for use as potential ligands for the determination of lanthanides in solution containing single lanthanide ion or a mixture of two lanthanide ions. The use of polyaminocarboxylic acid (PACA), α-hydroxyisobutyric acid (HIBA) as a co- ligand has been established for simultaneous determinations of lanthanides in their binary mixtures. After circumstantiating their potential, the selectivity of ligand and co-ligand in the presence of different interfering ions was adjudged to establish the competence of conductometry as an analytical tool for lanthanide determination. The interfering ions chosen for the study included metal ions like barium, cobalt, nickel, thorium, etc. that are v present along with lanthanides in various matrices and metal ions like calcium that bear chemical properties similar to those of lanthanides. The established method did not suffer interference from any of the species under study. The results of all the titrations have been used to furnish the efficacy of ligands for lanthanide determination by conductometry. The variation in exactitude of different ligands used has been explained on the basis of stabilities of resulting complexes. The adequacy of the performance parameters of the proposed technique has been validated by conducting lanthanide determination in different real time samples. Tap water, river water, clinical waste water, Welsbach gas mantle and MRI contrast - Gadopentetate dimeglumine samples have been tested for the presence of lanthanides. The results obtained were found to be coherent with those obtained by spectrophotometry. Spectroscopic studies have been conducted to inspect the effect of different polyaminocarboxylic acids on Arsenazo III‒lanthanide (III) complexes. Different spectrophotometric titrations have been performed in order to predicate the behavior of polyaminocarboxylic acids as competing and substituting ligands with respect to Arsenazo III for lanthanide bonding. The complexation behavior observed for different polyaminocarboxylic acids with lanthanides by spectrophotometric technique has been found to be congruent to that observed by conductometric technique. Keywords: Conductometric titration, lanthanide, polyaminocarboxylic acid, Ethylenediaminetertracetic acid, trans-1,2-Diaminocyclohexane-N,N,N′,N′-tetraacetic Acid, Ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid, N-(2-hydroxyethyl) ethylenediamine-N,N΄,N΄-triacetic acid, 1,3-Diaminopropane-N,N,N′,N′-tetraacetic acid, co- ligand, α-hydroxyisobutyricacid, interfering ions, spectrophotometric titrations, Arsenazo III.