Iron chemistry is complex and depends greatly upon the pH. I guess oxygen has been removed here? So, what else is present in the eleytrolyte beside CO2? At low pH, I could imagine you get a redox-potential of Fe(2+)/Fe(0). At pH 4-5 and above, it will become complicated and instable due to several insoluble Fe species at the Fe surface. Also complexing agents like halogenides may play a role.

Thank you Gerd-Uwe for your input. I meant actually that if we run many OCP experiments in which the pH is varied to be correlated to the OCP values, What kind of electrochemical information, qualitatively, can we deduce?

Hi Faysal,

the potential is controlled by the dominating redox couple present at the electrode surface. If several redox couples are present, they also can react with each other over time.

A technique called stripping potentiometry makes use of this. Open circuit potential of a mercury film electrode is recorded versus time. Oxidisers like Hg(II) or dissolved oxygen are oxidising deposited metals like zinc, cadmium, lead. The potential therefore shifts from one value to the next depending on how much metal had been deposited during the accumulation step before. Stripping time is the analytical signal here. To study this, you might look for literature such as by Daniel Jagner from Sweden. E.g. Anal. Chem. 51 (1979) 342. Here, dissolved oxygen is the oxidiser.

Best wishes,

Gerd

Let me add that the "dominating" species or redox couple seems to be the one that is producing the highest EMF, such as Zn, if Zn, Cd, Pb, Cu are present in a mercury film electrode in presence or absence of oxygen. If Zn is consumed, then Cd controlls the potential. Hg(II) or oxygen can be replaced by several other oxidisers (Au(III), Cr(VI), Mn(VII)). The Hg film has been replaced by Bi films, see Karel Vytras et al., Electroanalysis 14 (2002) 1359 for (zero current) potentiometric stripping analysis with bismuth films.

Hi Faysal,

To my knowledge, there is very little, if any, electrochemical information about your system to be deduced from the value of the Open Circuit Potential. I'm sure there are exceptions, but in most cases, the measurement of the OCP is the first step in performing an experiment such as Polarization Resistance or Potentiodynamic Polarization.

Regards,

Pete