A potentiometric titration technique has been used to determine the stability constants for the various complexes of Ni(II) with adenine (A) as primary ligand and selected amino acids (L) as secondary ligands. Ternary complexes of amino acids are formed in a stepwise mechanism, whereby (A) binds to Ni(II), followed by interaction with ligand (L), whereas thiol-containing ligands form ternary complexes through a simultaneous mechanism. The formation constants of the complexes were determined at 25 degrees C and ionic strength 0.1 M NaNO3. The relative stabilities of the ternary complexes are compared with those of the corresponding binary complexes in terms of Delta logK values. The concentration distribution of the complexes are evaluated. (C) 2010 Elsevier B.V. All rights reserved.