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)2 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 ks,CdOx+= (7±1) s−1 and ks,Cu(Ox)2= (1.5±0.1) s−1; charge transfer coefficients αCdOx+=0.51±0.01 and αCu(Ox)2=0.48±0.01; adsorption constants for the electroactive species Kad,CdOx=(4.5±0.5) × 10−3 cm and Kad,CuOx=(8±2)×10−3 cm. Surface concentrations ΓM were also evaluated, observing that, for the ligand to complex ratios analysed, ΓCdOx increased steadily with oxine concentration whereas ΓCuOx remained constant. Local ligand concentrations close to the electrode were also evaluated.