Hello,
I using the ferri- ferrocyanide redox couple, and my CV plots show very good reversibility in terms of peak current ratios (ipa/ipc=1). However I am seeing peak separation values of 80 mV. What are some of the factors that can cause the peak separation, but not result in deviations in peak current ratio?
I am using a planar 3 electrode system with WE/RE/CE all gold.
Thanks,
Hunter
A 59 mV peak-to-peak separation is essentially too good to be true! This is a theoretical value, and assumes an infinitely small step potential; in reality, the potentiostat applies a series of steps so different potentiostats can have different peak-to-peak separations. The peak separation (in volts) is only theoretically equal to 59 mV ideal at exactly 25*C.
Thiago mentioned the uncompensated resistance (iR) drop. A Luggin capillary is typically required to minimise this effect. Often people simply apply an arbitrary resistance compensation to their models or even via the potentiostat's software, to 'correct' the peak separation to get their expected 59 mV, but I would not recommend this.
Hedieh mentioned quasi-reversibility, and indeed the ferri/ferrocyanide redox couple is only a quasi-ideal outer sphere redox couple. It's kinetics are sensitive to the oxygen content on the electrode surface (some gold oxide = better). Finally, it is extremely sensitive to the alkali metal content in solution (more = better), and the nature of the alkali metal (Cs best, Li worst, from memory). If you are using something like a large excess of KNO3 as supporting electrolyte in water it should not be significant, but if you do not have a large excess of alkali metal present then the rate of electron transfer could be slow enough to become quasi-reversible (on the timescale of your scans).
Hello,
Generally, (E2-E1) is greater than 59 mV for quasi-reversible systems and increases with increasing sweep scan rate. So, maybe it is better to conduct a diagnostic test and evaluate the Cyclic voltammetry diagram for different scan rates.
It is not so easy to reach a peak separation of 59 mV. The distance between WE and RE should be very small. Beyond this, the sotution resistence needs to be very small too. To reach the value of 59 mV of peak separation, you will need to reapeat ideal conditions.
A 59 mV peak-to-peak separation is essentially too good to be true! This is a theoretical value, and assumes an infinitely small step potential; in reality, the potentiostat applies a series of steps so different potentiostats can have different peak-to-peak separations. The peak separation (in volts) is only theoretically equal to 59 mV ideal at exactly 25*C.
Thiago mentioned the uncompensated resistance (iR) drop. A Luggin capillary is typically required to minimise this effect. Often people simply apply an arbitrary resistance compensation to their models or even via the potentiostat's software, to 'correct' the peak separation to get their expected 59 mV, but I would not recommend this.
Hedieh mentioned quasi-reversibility, and indeed the ferri/ferrocyanide redox couple is only a quasi-ideal outer sphere redox couple. It's kinetics are sensitive to the oxygen content on the electrode surface (some gold oxide = better). Finally, it is extremely sensitive to the alkali metal content in solution (more = better), and the nature of the alkali metal (Cs best, Li worst, from memory). If you are using something like a large excess of KNO3 as supporting electrolyte in water it should not be significant, but if you do not have a large excess of alkali metal present then the rate of electron transfer could be slow enough to become quasi-reversible (on the timescale of your scans).
Since, all electrodes are gold, maybe there is an issue[1] of a (slow) gold surface modification, in alkali metal solutions (as supporting electrolyte).
1. Ionic-metallic transition in gold-based liquid alkali metal alloys http://iopscience.iop.org/article/10.1088/0305-4608/11/5/009/pdf
Have a look at
http://demonstrations.wolfram.com/CyclicVoltammetryForARedoxReactionWithDiffusion/
Enjoy !
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