Its a Kinetic parameter.

As Joseph noted, it can be a kinetic parameter. Additionally, if you are starting your scan in the water reduction region, you could be changing the local pH, and hence altering both thermodynamic and kinetic parameters. 

Joseph and Leigh: I know that the peak potential could be a kinetic parameter if we speak about under potential deposition (UPD). And of course, fixing the active material (gold in my case) and the analyte (lead), it can change position depending on the kinetics but I didn't think it could change of 200mV due to this. Moreover I can't explain this change of kinetics with the starting potential of SW....

Leigh: I thought about the local pH too. But starting from -1V I produce hydrogen that means increase the local pH. So the peak should go and the left, isn't? In my case, increasing the local pH the peak moves of 200mV in the positive way....

Hi Christian,

thank you for your replay. 

I agree with you about the local pH during the accumulation step but I have this problem if I change the window of the stripping step (-0.6/1V or -1/1 V). Do you think that just some seconds (scan rate: 40mV/s) from -1 to -0.6V could give this effect on the local pH? I think that the amount of OH- generated during the stripping step, in particular during the scan from -1 to -0.6V, should be negligible compared to the amount produced in the accumulation step (3mins at -0.4V). 

Anyway my electrolyte composition is: DI water, 10mM NaCl, 1mM HNO3, pH 3.5

About the sign of the shift, I think it too, is really odd!

thank you,

Bernardo