Adsorptive square wave voltammetry of metal complexes. Effect of ligand concentration - Part II. Experimental applications

Theoretical models developed in Part I of this work for the square wave voltammetric response of inert metal complexes are applied to Cd(II)- and Cu(II)-8-OH-quinoline species adsorbed on mercury at pH 6.7. A wide range of experimental conditions were considered, taking into account the effect of ligand concentration. Cyclic voltammetry experiments were also performed for the purpose of comparison. Fitting between experimental and theoretical curves was excellent, assuming that the predominant species in bulk solution were CdOx(+) and Cu(Ox), and that the electrochemical reaction mechanism involved ligand desorption after the reduction of both complexes. From the simulation procedure the following parameters were estimated: electrochemical rate constants k(s),(CdOx+) = (7 +/- 1) s(-1) and k(s),(Cu(Ox))(2) = (1.5 +/- 0.1) s(-1); charge transfer coefficients alpha (CdOx+) = 0.51 +/- 0.01 ant alpha (Cu(Ox)2) = 0.48 +/- 0.01; adsorption constants for the electroactive species K-ad,K-CdOx = (4.5 +/- 0.5) x 10(-3) cm and K-ad,K-CuOx = (8 +/- 2) x 10(-3) cm. Surface concentrations Gamma (M) were also evaluated, observing that, for the ligand to complex ratios analysed, Gamma (CdOx) increased steadily with oxine concentration whereas Gamma (CuOx) remained constant. Local ligand concentrations close to the electrode were also evaluated. (C) 2001 Elsevier Science B.V. All rights reserved.