A novel methodology for evaluation of formation constants of complexes: example of lanthanide–Arsenazo III complexes

A novel method is proposed for the computation of formation constants of complexes. It makes use essentially of the data from the first stages of the titration and is based on the Taylor expansion of the concentration of the ligand as a function of the titrant concentration. The applicability of the methodology is examplified by the computation of the complexation constants of lanthanides (Ln) by 3,6-bis[(2-arsonophenyl)azo]-4,5-dihydroxy-2,7-naphtalenedisulphonic acid disodium salt (Arsenazo III). Arsenazo III is known to form stable complexes with a large variety of ions. The spectrophotometric study of these complexes in the visible domain is facilitated by the presence of chromophore groups in the ligand. In the present work, spectral deconvolution was used to improve the accuracy in the determination of formation constants of complexes between Arsenazo III and lanthanides. It turns out that all the 11 lanthanides studied here form the same type of complexes with this ligand. For each of them, the three formation constants correspond, respectively, to Ln to Ar ratios 1:1, 1:2 and 2:2. Their approximate values are in the ranges: K11=105; K12=1010; K22=1015. Accurate values are given in the text. The results are compared with those obtained with the classical iteration method which makes use of the whole titration curve. The two techniques give satisfactorily convergent results. The new methodology might be useful in the treatment of the competition of two ligands for the complexation of the same lanthanide, where the first stages of the titration of only one of the complexants is experimentally available.