I am working on electrochemical sensing of metals,
SCE is reference electrode
Pt wire as CE and GCE modified with Au-NPs as WE.
As+3 is detected and oxidized at 167-220 mV vs. SCE but at very low concentration its oxidation potential is 33-50 mV. One reason may be at low concentrations charge transfer is easier which is rendered at high conc. that's y to overcome this issue potential is shifted. But it's a big difference indeed.
Can it be possible As+3 is oxidized to As+5 at very low concentration? Is my understanding accurate?
I am surprised that there would be such a significant shift (and the direction of the shift--Nerst eq would indicate it should be harder to oxidize lower concentrations, not easier), so there is something interesting going on.
Is there something else other than the concentration of As3+ that is different, for instance pH? If the oxidation is coupled to deprotonation, we could expect about 59 mV change per pH unit, so if the two solutions differ by about 3 pH units, that could account for the observed difference. What is your source of arsenic, is it arsenous acid? If you haven't looked it up already, I suggest looking closely at the Pourbaix diagram of arsenic.
It might also be worth looking at 4 or more concentrations between the two points you mention here to see what sort of relationship the redox potential and concentration have.
Respected Sirs Thank you very much for your comments. It will help a lot.
@ Sir Aaron Bloomfield,
All parameters were kept constant pH, deposition time, deposition potential and all but concentration of Arsenic. Arsenous acid was not used as the source.
This paper suggests that Arsenic adsorbs on Au, this adsorption increases with increasing the Arsenic concentration which could result in a slower kinetics. I think you should look more in this direction.
http://pubs.acs.org/doi/abs/10.1021/acs.jpca.5b03832
In short, it could be about an ohmic loss!
It could help (you may want to see this article below)!
Article: [Re(η 5 -C 5 H 5 )(CO) 3 ] + Family of 17-Electron Compounds: Monomer/Dimer Equilibria and Other Reactions
Daesung Chong · Derek R Laws · Ayman Nafady · William E Geiger
Journal of the American Chemical Society 130(8):2692-703 · March 2008
DOI: 10.1021/ja710513n · Source: PubMed
Dear Noorulain Babar,
First you must mention the high and the low concentrations you tested. Second is the oxidation of As+3 reversible or not ? Please check for that and use activity instead of concentration, where activity = activity coefficient x concentration. Also, what kind of supporting electrolyte you use?
Dear Fakiha Heakal,
I have detected 5 ppb Arsenic and at another extreme, it was about 40 ppb. Not reversible. Sure, I will. Thank you for your attention.
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