In full disclosure I'm fairly new to electrochemistry so I might just be doing something wrong, but that's not to say I don't know how to replicate published experiments or follow a protocol. With that said, I've recently undertaken some work experimenting with Potassium Hexacyanoferrate (III) with a focus on Cyclic voltammetry redox peaks- Much of what I've seen experimentally has made sense and reflects whats been well defined in copious amounts of literature (ie. Increasing/decreasing concentration increases/decreases respectively the peak current observed). What hasn't made sense and the reason for my lack of sleep and inevitable inquiry here is that I occasionally get a gradual increase in signal over time... But why!?
I do CV scans in the potential range of -0.1V to 0.5V and at a scan rate of 50mV/s. The working electrode is a gold electrode, the reference is calomel and the counter is platinum while the solution this experiment is routinely done in is typically a 10uM potassium hexacyanoferrate (III) in pH 7.4 PBS solution.
The oxidation peak can increase by as much as 20% within 2 hours for seemingly no reason (as can the reduction peak). Evaporation is not an issue.
I eagerly look forward to any thoughts in regards to this!
Thanks y'all,
-Vlad
10uM potassium hexacyanoferrate (III) seems to be too low concentration. Try at least 1 mM, use at least 100 mM KNO3 as an additional electrolyte to your buffer.
1. Follow Yurii's recommendations.
2. For the signal increase you may think to diffusion from the bulk electrolyte in the diffusion layer (a thin layer near the working electrode) during the repeated scans. The scan duration is 24 s for one cycle. The concentration of Potassium Hexacyanoferrate (III) is depleted during the cathodic scan and the proposed diffusion is probable, thus increasing the oxidation (and also reduction) current peak. The current increase is observable if you maintain the repeated scans for some hours. For confirmation compare the situations, if you take a break for some hours.
Follow Yurii’s recommendation but I would also add the same concentration of hexacyanoferrate(II). See this application note for a foolproof method.
http://www.ecochemie.nl/download/Applicationnotes/Autolab_Application_Note_EC01.pdf
I would also use a potassium carbonate supporting electrolyte rather than sodium hydroxide.
Hi Vlad,
All the recommendations seem fine! The concentration seems to be low for preliminary studies. I will go to 1 mM (not higher). Adding more electrolyte (we don't know the concentration) is also fine.
Looking at the "thin layer near the electrode" is also important. In the website address of John, there is rotation of the electrode, this is the reason to see a plateau, but you talk about a peak, so you don't have stirring. When I repeat CVs I stir in between measurements, to renew the thin layer near the electrode. Maybe this is the reason.
The other two options are: i) adsorption of the analyte on the electrode (It is not common but adsorption phenomena of the ferri/ferrocyanide system on Pt have been reported, then is better to increase the concentration to favor diffusion) and ii) increase of the area of the electrode, that seems not very probable (usually the opposite happens).
Summarising: increase concentration, increase electrolyte and stir 5s in between scans.
Best regards
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