ADSORPTION AND ELECTRODE-REACTIONS OF DISULFONATED ANTHRAQUINONES AT MERCURY-ELECTRODES

Three sulfonated compounds were examined: 2,6-anthraquinonedisulfonate, sodium salt (2,6-AQDS); 1,5-anthraquinonedisulfonate, sodium salt (1,5-AQDS); and 2-anthraquinonemonosulfonate, sodium salt (2-AQMS). Extensive studies of 2,6-AQDS were carried out. This compound undergoes reversible charge exchange with the electrode in an adsorbed state. Electrocapillary curves show that desorption occurs near -0.8 V vs Ag/AgCl, KCl (1 M). Chronocoulometry and cyclic voltammetry were used to evaluate surface coverage vs the concentration of 2,6-AQDS in 0.1 M HNO3. Over the range from 2.5 × 10-8 to 5 × 10-5 M the surface excess obeys a Langmuir isotherm with a saturation limit of 0.94 × 10-10 mol/cm2, corresponding to 180 Å2/molecule. Under all conditions, the adsorbed couple shows a standard potential more positive than that for the couple involving dissolved species; hence, the reduced form is the more strongly adsorbed. Effects of pH were examined extensively. Both couples (adsorbed and dissolved) show a negative shift in formal potential of nearly 60 mV/pH unit in the pH range below 8; hence, the reactions involve one proton per electron, as expected. The formal potentials become independent of pH at pH > 11. In solutions with 2,6-AQDS concentrations above 2 × 10-5 M an extremely sharp, reversible pair of spikes develops in the cyclic voltammetry for the adsorbed couple. The spikes are not seen for 1,5-AQDS, but other aspects of behavior for 1,5-AQDS are similar to those of 2,6-AQDS. The origin of the spikes is discussed via a model involving hydrogen-bonded aggregates. Extensive exposure of a mercury surface to high concentrations of 2,6-AQDS produces a catalytic effect on the electrode reaction involving the dissolved quinone. Further exposure causes an inhibition of the same process. These effects are attributed to the growth of extended films on the mercury. © 1990 American Chemical Society.