Evaluation of the activity coefficients of some sulfonated azo dyes and of their complexes with metal ions

Abstract

The protonation and complexation conditional constants of equilibria involving large, multicharged organic ions depend on the ionic medium in which they occur. The possibility of evaluating these effects on the basis of the Specific Interaction Theory (SIT) was tested, by determining spectrophotometrically the conditional protonation constants (in concentration) of some bisulfonated azo dyes [2-hydroxy-1-(1H-tetrazol-5-ylazo)naphthalene-3,6-disulfonic acid, 1,8-dihydroxy-2-(1H-tetrazol-5-ylazo)naphthalene-3,6-disulfonic acid and 1,8-dihydroxy-2-(p-nitrophenylazo)-naphthalene-3,6-disulfonic acid] at different sodium perchlorate concentrations. The same was done for complexation with thorium(IV), indium(III) and copper(II). It was found that the dependance of protonation and complexation conditional constants on the ionic medium can be described by evaluating the activity coefficients of the ions on the basis of SIT. The activity coefficients of the multicharged free organic ligands are best estimated by considering their charge globally, exactly like in the case of small inorganic ions. On the other hand, the activity coefficients of the complexes with a metal ion are best evaluated by considering them as bearing two separate charged sites, one corresponding to the site where the metal ion is bound, and the other to the residual charge of the ligand. In this way the specific interaction coefficients were found to be constant, as expected from SIT, and their summations were experimentally determined, together with the thermodynamic constants, by linear extrapolation.