Palladium(II) complexes of Aminopolycarboxylic Ligands in Aqueous Solution

The binding capacity of five aminopolycarboxylic ligands (APCs) [nitrilotriacetate (NTA), ethylenediamine-N,N,N′,N′-tetraacetate (EDTA), (S,S)-ethylenediamine-N,N′-disuccinic acid (S,S-EDDS), diethylenetriamine-N,N,N′,N″,N″-pentaacetate (DTPA), and triethylenetetraamine-N,N,N′,N″,N‴,N‴-hexaacetate (TTHA)] toward the palladium(II) ion was studied by potentiometric titrations (ISE-H + electrode) in NaNO 3 and in NaClO 4/NaI (at different molar ratios) solutions and by spectrophotometric titrations (only in NaClO 4), at I = 0.1 mol·kg -1 and at T = 298.15 K. The high stability of Pd 2+-complexones species inhibits the formation of sparingly soluble hydroxo species until pH 11. In the pH range investigated and at the Pd 2+/APC ratios used in the experiments (1/1-1/4), all of the complexones form with palladium(II) only protonated or unprotonated mononuclear species and the monohydroxo species Pd(APC)(OH). As consequence of the ligand structure and of the number of amino and carboxylic groups involved in the coordination, the stability of palladium(II)-APC species follows the trend: TTHA > DTPA > EDTA > S,S-EDDS > NTA; for example, the log K PdL is 37.00, 36.31, 23.60, 23.07, and 17.82 for L = TTHA, DTPA, EDTA, S,S-EDDS, and NTA, respectively. A critical evaluation of literature evidenced the big discrepancies in stability data and also in the speciation models proposed by different authors. A comparison of the sequestering capacity of complexones toward palladium(II) and other divalent metal ions was done by using stability data from this work and literature ones.