Why we need to add an internal reference (Ferrocene, for instance) when we study the electrochemical activity of an organic electroactive species using cyclic voltammetry?
Organic electroactive species are most often studied in non-aqueous media. Unlike aqueous systems, where the NHE or SHE (and other reference electrodes such as AgCl/Ag or SCE) are ideal, non-aqueous studies often require the use of non-traditional references like quasi-references such as Ag wire, or special precautions to prevent water from leaking from the reference electrode an into the non-aqueous medium.
Also, due to differences such as the geometry of the electrodes in individual set-ups, it is difficult to reproduce exact potentials as reported by various groups. The use of an internal reference such as ferrocene eliminates the need to reproduce the exact conditions of an experiment, since irrespective of the fine details of one's respective experimental set-up, the potential of the electroactive species relative to ferrocene couple should be unchanged. It also eliminates the need to try and adjust the value of your reference electrode relative to the SHE or NHE based on the choice of the solvent.
This therefore allows for comparative as well as quantitative calculations based on values reported in the literature, that were not executed under your conditions.
Of note though, is that in some cases it might be difficult to use ferrocene as an internal standard in the strictest of sense, especially if it overlaps with the redox wave of your species of interest (or if it participates in redox reactions as can occur with TM complexes). In such a case one will have measure the value of the ferrocene couple by keeping everything the same, but exclude the analyte from the solution.
Hope that was helpful.
Dear Mark,
Many thanks for your explanation, and it would be really helpful if there is any advice regarding the optimum concentration of Ferrocene. For example if I prepare a solution of my electrochemical active species at concentration of 5 mM, what is the optimum concentration of ferrocene (half of this concentration, less or more than this?).
From what I have observed, the concentration sometimes affect the values of the ferrocene couple, especially if one is using a quasi reference electrode. You might find it convenient to use the same concentration as the analyte you are using. By using the same concentration as the analyte, it might help in assigning the number of electrons involved with a particular redox wave, if (and only if) the ferrocene has a similar diffusion coefficient to your analyte. Outside of that the Delta Ep (which in non-aqueous solvents can be a lot more than 59 mV, especially on glassy carbon electrodes) for the ferrocene couple might also assists with determining the whether a particular redox couple is reversible or if there are other processes following the electron transfer.
However, if you are only doing qualitative work, the concentration of the ferrocene will not be of much importance.
Hope you found that information helpful as well.
Dear Mark,
Many thanks for your explanation, it was really helpful.
Saddam
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