Sequestering ability of aminopolycarboxylic (APCs) and aminopolyphosphonic (APPs) ligands toward palladium(II) in aqueous solution

The binding capacity of three aminopolycarboxylates [nitrilotriacetic acid (NTA), ethylene-glycol-bis(2-aminoethyl ether)-N,N,N,N-tetraacetic acid (EGTA), and diethylenetriamine-N,N,N,NN-pentaacetic acid (DTPA)] and two aminopolyphosphonates {(1-hydroxyethane-1,1-diyl)bis(phosphonic acid) (HEDP) and [[(phosphonomethyl)imino]bis[2,1-ethanediylnitrilobis(methylene)]] tetrakis-phosphonic acid (DTPP)} toward palladium(II) ion was studied by potentiometric and spectrophotometric titrations at different temperatures (283.15 ≤ T/K ≤ 318.15) and ionic strengths (0.1 ≤ I/mol·dm -3 ≤ 1.0) in NaClO4. The hydrolysis of Pd2+ and the protonation of ligands were always taken into account in the speciation models of Pd2+/L systems investigated. Equilibrium reaching experiments were performed to check and confirm the reaching of the equilibrium state. Owing to the high stability of the PdL species (K > 1020), for EGTA, HEDP, and DTPP it was determined using exchange measurements with auxiliary ligands, such as iodide (I-) and ammonia (NH3). For the other ligands the stability of the PdL species was reported in the literature. The general speciation scheme consisted of mononuclear differently protonated species with general formula PdHiL and only in the case of the NTA ligand the formation of the PdL2 species was found. The stability of the PdL species is high: as an example we have log KML = 17.82, 22.60, 36.31, 23.49, and 27.27 for NTA, EGTA, DTPA, HEDP, and DTPP, respectively. Among the ligands, DTPA shows the highest formation constants and sequestering ability, evaluated using the pL0.5 parameter, as well. The complex formation reaction is always exothermic and in general the entropic contribution to the stability is dominant. Some empirical relationships were found to model the dependence of the formation constants on the number of protons and of the sequestering ability on pH.