Cyclic Voltammetry: Ferro/Ferricyanide, fact and fiction
Potassium ferricyanide is a favourite choice in the laboratory to introduce students to cyclic voltammetry. This is largely due to a
describes an alternative to the use of ferri/ferrocyanide for educational cyclic voltammetry experiments in aqeuous solution.. While iridium salts are very expensive only tiny quantities are required for cyclic voltammetry.
The other paper talks about some of the problems with ferricyanide. Infrared Spectroelectrochemical Analysis of Adsorbed Hexacyanoferrate Species Formed during Potential Cycling in the Ferrocyanide/ Ferricyanide Redox Couple, Christine M. Pharr and Peter R. Griffith, Analytical Chemistry, 69, 4673-4679, 1997.
The ferro/ferrcyanide couple is has a standard reduction potential of E0 = ~436 mV however this is dependent on solution pH and
E1/2 = . While it is true that this value would be obtained as the
While is is true that a one electrode reversible reversible reaction will exhibit a a peak separation of 0.59 V at 25 deg C you should consider this
more of an 'ideal' at which to aim, rather than something you ever get to see in reality (unless you are very careful in setting up the experiment).
For example see
http://www2.chemistry.msu.edu/courses/cem419/cem372cyclicvoltammetry.pdf
which says "it is very difficult to achieve a 59/n mV splitting at most solid electrodes. If you can achieve 70 mV, you will be doing well."
As they say, even 70 mV is doing well and values of 100 mV or more are not uncommon. One major cause of this is 'uncompensated resistance'(due to 'iR drop') in the sample. See
Uncompensated Resistance. 1. The Effect of Cell Geometry Jan C. Myland and Keith B. Oldham, Anal. Chem. 2000, 72, 3972-3980 DOI:10.1021/ac0001535
Uncompensated Resistance. 2. The Effect of Reference Electrode Nonideality Keith B. Oldham and Nicholas P. C. Stevens, Anal. Chem. 2000, 72, 3981-3988. DOI:10.1021/ac000154x
To reduce this iR drop (and uncompensated resistance) to a minimum you can do the following:
1. Keep the reference electrode close to the working electrode (use of a 'Luggin probe' style electrode is recommended for the most exacting experiments).
2. Do not obscure the working electrode by placing a large reference electrode directly in front of it, blocking line of sight to the auxiliary electrode.
3. Use a high ionic strength background electrolyte if possible (eg 1 M KNO3 in place of 0.1 M KNO3)
4. As a last resort you can use the iR Compensation feature on your potentiostat, but in reality all this does is boost the actual applied potential while reporting a smaller potential so that you seem to be getting a correct answer. It's not really quite as bad as a 'fudge factor'. Adjusting electrode positions, and solution ionic strength can sometimes reduce the peak-to-peak separation dramatically. In contrast cyclic voltammetry in resistive organic solvents is much more likely to exhibit large peak-to-peak separations even with careful placement of electrodes. But in most cases think it is better to know how far you are from 'ideal' behaviour rather than disguising the problem using the potentiostat iR compensation feature.
The problems of a non-ideal response for ferricyanide can be made worse by
the quality/nature of the electrode surface. A scrupulously clean platinum
electrode is usually recommended. Glassy carbon and graphitic screen printed
electrodes can be more unpredictable as to whether they give a classic CV
shape. This is because ferri/ferrocyanide absorbs/desorbs on the surface of
the electrode so even small changes in the surface chemistry can have big
impacts on the observed CV. Ferri/ferrocyanide is notorious for this, and hexachloroiridate has been proposed as a replacement:
Cyclic Voltammetry of Hexachloroiridate(IV): An Alternative to the Electrochemical Study of the Ferricyanide Ion, Steven Petrovic, Chemical Educator 5, 231-235, 2000. DOI:10.1007/s00897000416a or Research Gate
Other materials that are used for undergaduate experiments that are more reliable than ferro/ferricyanide are:
Water soluble ferrocene derivatives
Hexammineruthenium II/III salts
In organic solvents the ferrocene/ferrocimium ion is the nearly universal choice to demonstrate reversible cyclic voltammetry.
