1. The polished electrode has a larger active surface for deposition of As or Cu than the one found in the same electrode after several runs without the polishing step since the material deposited on the electrode surface after the deposition step is not completely removed during the stripping step. Every one of the stripping peaks is "the real one", however the observed error for the slope of the calibration plot might be lower for the peaks obtained after your first 3-4 runs, while the sensitivity of the method will be higher for the very first one. If you are willing to sacrifice sensitivity for the sake of repeatability, then it may be better to use the peak heights after the initial 3-4 runs. Nonetheless, you should be aware that the rate of reduction of the electroactive area of your electrode is probably a function of the concentration of Cu or As in solution, as well as of the scan rate and deposition time and potential, since a different fraction of the surface of the electrode will be covered for different levels of these variables.

2. As often as needed to obtain the required levels of sensitivity and repeatability. This means that you would want to polish the working electrode after each run or after a given number of runs.

yes in every cycle the surface is modified

for repeatabilty do individual tests

do not carry away by instrument manufacturers

electrochemistry needs perfection

surface changes with time  of contact

In short, and without lengthen, this depends on the product of the electrochemical process. If the product adsorbed or not on the electrode surface?. 

I agree with the answers provided by the two reseachers above. At solid electrodes unfortunately you have the problem with the modification of the surface after each scan. At the mercury eletctrodes, this problem does not exist, once you have a new electrode before each scan. You can try to apply some pre-treatment to overcome this problem, like using an anodic potencial to oxidize the rest of the material that was accumulated at the electrode. But you have to take care, because high anodic potencials may lead to the gold oxide formation.   

Most likely it's an active surface area change of your working electrode which is causing this phenomena (a decrease)

 1- I have concerns about you using a gold wire working electrode. Are you making out of it  a disc electrode? if yes make sure that assembly has no leaks.

2-  what is your stripping voltammetry strategy (reduction potential and time, and most importantly at which anodic potential are you ending your anodic scan and for how long time you staying there before next scan. make sure that no more reduced metal is present at the surface of your electrode.

3- 1M HCl is too much acidic media for those experiment. I suggest you use mild concentrated buffer electrolyte.

dear Sujeevan Kumarb Agir,

according to definition of Str.VA only the first peak is REAL, because all second peaks is not streaping (t/c/-->0)

all the best,

bf

Hi

What do you mean by multiple runs (subsequent), if you are doing stripping voltammetry?

Do you mean, that you repeat the whole experiment from the beginning including the deposition step without re- polishing your electrode in between?

or you just repeating the stripping step?

Can you explain please in details?

In my opinion, you should consider the first run as the most "real". You have probably an adsorption phenomena that affects the following runs, or perhaps a catalytic current in the first one...

Thank you very much Prof. Cordero.  For me, sensitivity is not an issue - hence as suggested, I shall go for considering peak heights after 3-4 runs.  Do you think it is a good practice to arrive at a calibration plot for various concentrations keeping scan rate and deposition time and potential constant?  I mean, deriving a calibration plot based on peak heights observed after 3-4 runs with each experiment run at the same scan rate, deposition time and potential sweep.

 Thank you Prof. Muralidharan, I shall perform individual tests as adviced.

Dr. Elrouby, Yes! the analyte is adsorbed on to the electrode's surface.

Mr. Thiago, that is a good tip regarding pre-treatment of the electrode.  I shall try it and let you know how it went or if I need some more clarity.

Thank you  Dr. Omar.

1. I am using plain 24Ct. gold wire as working electrode.

2. There is a gap of just a few seconds between each run and the scan ends at 600mV

3. Yes, I agree 1M HCl is too much acidic but I did try performing the experiment at various lower concentrations varying from 2M to 0.1M HCl and found that only 1M HCl gave me required sensititivy and reproducibility.

Thank you for the response Prof. Fadhil M. Najib.  We are looking at a very economic and easy-to-deploy solution for performing the analysis, and we choosing a simple gold wire as working electrode was a conscious decision to keep the operational and maintenance cost low.  I use 0.05 Micron Alumina on a polishing pad for cleaning the electrode.  However, I should confess that I did not consider the option of using HMDE for the experimentation.  I shall investigate using the same.  Thanks again for your valuable time :)

Thank you Dr. Boris Filanovsky and Dr. Juan Casado.  Yes, with all the valuable feedback I have received, I now understand that the first run gives the "real" response. 

Prof. Alaa A. El-Shafei, I meant, I re-run the experiment from the beginning (right through deposition step through the stripping step) without re-polishing the electrode.  Thanks.

ok, in general for stripping voltammetry, the first run is the one to be considered.

To reproduce your work, you should clean your Au surface completely. But you have to use the same pre treatment procedure including polishing step. However, getting the same exactly response is so difficult due to the change of  your electrodes surface properties including the real active area.

May be it will be better to run  a cyclic vottammetry of Au in sulphuric acid or NaOH and determine the real surface area, then you can normalize your stripping voltammetry response. The problem is you have to do this step prior to each stripping measurement. 

I think, I choose the first run.