Cyclic voltammetry theory assumes that the initial potential is selected where no redox reaction will occur.  If both sides of a redox couple are present simultaneously at the start, it is impossible to connect a working electrode without changing the composition of reactants near the electrode surface. The above answers are all good.  If both the R and O forms were present in the bulk of the solution at the beginning, a rotating disk voltammetry is better because at steady-state hydrodynamics one can see accurate limiting currents for each side - oxidation and reduction.

If you only have Fe+2 in solution and you scan potential from negative to positive value respect to the redox potentiale of the couple Fe+2/Fe+3, youobtain the oxidation peak.

If you scan the potential from positive to negative in the same potential range, at the initial positive potential you will oxidize the iron +2 which is near to the electrode surface to iron +3 that will be reduced when your potential scan reaches the proper negative potential.

I hope I have been clear enough.

Does it work?

Thanks, Lucia Becucci

Then how do i still carry out appropriate CV scan in the case?

Bryan, If you should have only one species present Fe+2, then the appropriate scan is to start at a sufficiently negative potential for no reaction (but not so negative as to reduce Fe+2 to Fe(0).  From there, your first scan should be in the positive direction.  This is the first case mentioned by Lucia. 

Thanks Peter A. Adcock

because no diffrence betwin scannig potential or put the potential at the end ponit, when u put the potential at the end ponit, all of your matter in boundry layer is in oxidative form or reducive form

Cyclic voltammetry theory assumes that the initial potential is selected where no redox reaction will occur.  If both sides of a redox couple are present simultaneously at the start, it is impossible to connect a working electrode without changing the composition of reactants near the electrode surface. The above answers are all good.  If both the R and O forms were present in the bulk of the solution at the beginning, a rotating disk voltammetry is better because at steady-state hydrodynamics one can see accurate limiting currents for each side - oxidation and reduction.

The redox potentials we refer to, correspond to a redox couple*, such as Fe2+/Fe3+, and not to a single species Fe2+! E0 values of such couples are available in standard tables, as also E1/2 values in some cases. The value of that E0 potential (reversible potential by definition) has a unique value. The experimental values of reversible peak potentials of such couples are supposed to lead to these values, if the half cell reaction is reversible. Since you state (or assume) the peaks are reversible you cannot or should not expect to get more than one peak value irrespective of the direction of the scan. This is the reason why your experimental CV scans showed same peaks when you started scanning from both end points of voltages.

Don't mess up with the nomenclature of the electrodes. No reactions (no counter redox reactions, even) occur at the auxiliary electrode! Auxiliary electrode is a passive onlooker; it is a spectator - not an actor, in the drama of electrochemical cell reaction process. Reactions occur only at the working electrode and the counter electrode in an electrochemical cell. They don't mess up, so you don't.

* Redox potentials correspond, strictly speaking, to half cell reactions with reference to a reference electrode potential. In other words the redox potentials are cell potentials where the counter electrode is a reference electrode.

i thought auxiliary electrode is same as counter electrode

In a two electrode system we have a working electrode and a counter electrode, no auxiliary electrode.

In a three electrode system, we have a working electrode, a counter electrode and an auxiliary electrode. At auxiliary electrode no reactions occur. It is used to vary the electrode potential of the working electrode in controlled or desired manner. 

Reaction DO occur at both the auxiliary and reference electrodes, but in both cases they are not influencing what is observed at the working electrode surface in a properly functioning potentiostat.  This is explained in books by Bard and Faulkner and by Kissinger and Heineman.  In the ideal system the current at the auxiliary electrode and at the working electrode are identical (one in an anodic direction and the other cathodic).  This is a very good assumption in properly used instruments. "Counter electrode" and "auxiliary electrode" for most people describe the same thing using different words.

An ideal voltmeter has in infinite input impedance and as such allows no current to pass through it. A potentiostat works as a voltmeter between working electrode and auxiliary electrode. To the extent current passes between these electrodes the voltmeter/potentiostat is a poor device to that extent. The potentiostat is used with a 3 electrode system. Current passes between working electrode and counter electrode. the current at the two electrodes is equal in magnitude and opposite in sign. No current passes between auxiliary electrode and working electrode.

When we use a 2 electrode system the current at the two electrodes is necessarily equal in magnitude and opposite in sign (corresponding to the oxidation and reduction processes that occur at these electrodes). 

You mean to say that no current passes through the REFERENCE electrode interface when there are 3 electrodes..

The auxiliary and counter electrodes are  different names for the same electrode. The English language can indeed be confusing. Historically, the counter electrode was large in area and also served the purposes of the reference electrode by having a very low current density compared to the very small working electrode.  These confusing terms come from the very early days of polarography, a technique extinct in the USA for roughly 30 years... In the 1920s, the counter electrode was often a "mercury pool" electrode in a chloride supporting electrolyte.  This is never the situation today, but the history is interesting..

Yes, I mean no current passes in through the REFERENCE electrode interface when there are three electrodes.

You are right in the second paragraph. It is in polarography that we have two electrode system where the working electrode is a dropping Hg and the counter electrode is a Hg pool electrode with a comparatively very large surface area so as to make the current density (CD) very low and thus the counter electrode with a constant CD maintains its potential constant. Hence it is also considered as the reference electrode. In chloride solutions it is SCE.

However, with the advent of three electrode system, one could choose the counter electrode as desired and is not constrained to use Hg pool electrode. Now, to measure the potential of the working electrode one uses a reference electrode (auxiliary electrode) such as SCE with a luggin capillary positioned near the working electrode.

In the 3-electrode system we have two linearly independent current loops. One loop involves the WE, the CE and the energy supplier. Through this loop the measured cell current circulates. The second loop involves the WE, the RE and the potential measuring device. Through this loop no current circulates. To the extent there is current in this second loop, to that extent the potential measurements would be inaccurate.

Polarography is done with 3 electrodes, the WE (Hg frop), RE and CE (whatever it is, mercury pool or Pt wire).

Indeed Polarography is the best technique if one want to quantify Fe2+ and Fe3+ both present in solution. Oxidations will have negative diffusion limiting current while reductions will have positive diffusion limiting current.

Polarography is dead.  It deserves its place in history because it EVOLVED to cyclic voltammetry, hydrodynamic voltammetry, glucose sensors for diabetes, oxygen sensors for the clinic and the environment, etc.  Mass spectrometry, chromatography, and immunoassay tools (etc.) are far superior, easier to automate and cost less vs. the value of the data and speed of collecting it..

The telegraph evolved to the telephone to the radio to the semiconductor to the fax machine to the personal computer to the internet. to the smart phone.  FANTASTIC!

:) 

CV cannot do the same things of polarography.

If you would say vibrating electrodes, I may agree, but not CV. They are aimed at different purposes and different time scale. Although I agree that polarography is almost dead, I still think that in some very particular cases it has some advantages (renewable electrode surface, which is useful if you have surface adsorptions...). IMHO.