Potential-dependent depassivation

Figure 2.15 In the presence of rslOgmol L-1 CP, the blocking PEG-CI film is metastable as revealed by sharp potential-dependent depassivation. At the critical potential the inhibiting layer is disrupted and metal deposition accelerates to the transport-controlled limit.

The simple / uGai model of the electrochemical cell provides a challenging control situation. The presence of dif-fusional faradaic current reduces the reactance of the working electrode interface by adding a parallel noncapaci-tive current path across However, some electrode processes can transiently increase the reactance of the interface, thus decreasing the control loop stability. For example, potential-dependent adsorption or desorption of ions at the interface or passivation/depassivation phenomena can destabilize an otherwise... 

Figure 8. a transient currents on the Fe working (upstream) electrode, b detection currents on the glassy carbon collector (downstream) electrode IM Na2S04 acidified to pH 3. c potential dependence of the charge for dissolution of ferrous cations and for passive film formation at various pH. Depassivated surface area 0.08 cm. d potential dependence of the charge ratio QJQ, (integral emission efficiency) at various PH. 

In an insonated system, it is possible to optimize the ultrasound power and deposition potential in order to target a specific heavy metal. Fig. 3(a,b) shows the variation in ultrasound power and deposition potential for copper and lead from a dilute mucous solution. It is evident that the optimum deposition of copper is effected at 200 W cm ultrasound power and potential of —1.6 V. For lead, this is found to be 100 W cm ultrasound power and a potential of —1.7 V. We conclude that stability of the deposit on the surface of the electrode is dependent on the interaction between the deposit and the electrode surface and the cavitational activity, which also acts to depassivate the electrode in the presence of glycoproteins from the mucus. 

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