Effect of thallium ions on anodic dissolution of gold in alkali-cyanide electrolytes at potentials more positive than the NHE potential

The gold dissolution rate i in solutions containing 0.1 M KOH, 0.1 M KCN, a nd 2.5 x 10(-7) to 1.5 x 10(-5) M TINO3 is studied as a function of potenti al E of the electrode whose surface is renewed prior to each experiment, th e TINO3 concentration c, and the time t of the electrode contact with solut ion. At c exceeding 0.5 x 10(-5) M and t similar or equal to 0, the rate is 1.5-2 times that at c = 0. Initial portions of i vs. t curves in the absen ce and presence of TINO3 coincide only at c below 10(-6) M. Potentiostatic and potentiodynamic measurements show that, at positive E, only small cover ages of the electrode surface with thallium are obtained, which make no imp act on i at E < 0 and heavily increase it at 0 < E < 0.3 V. The discovered effects are attributed to the formation, during the adsorption of oxidized thallium forms, of dipoles comprising thallium adions and gold atoms. Presu mably, the dipoles face the gold with their negative ends and make the pote ntial of zero free charge more negative.