It is a bit difficult to answer without more detailed information. What is the pH? The classical problem is that the pH is not correctly set or is shifting. That will make peaks broader, shifted or disappear. And the buffer should have enough buffering capacity to hinder a pH shift during the scan. Have the literature voltammograms been measured with this working electrode? Many species adsorb or interact differently with different working electrodes. Many electrodes should also be cleaned before (can be done by polishing and/or electrochemical cleaning) to enable good adsorption. I dont know your system, but it might have a too large background current to enable the detection of distinct peaks at these concentrations. Have you tried another working electrode? Are you sure that ferricyanide should adsorb at the working electrode? Are you stirring the electrode? There is a diffusion problem usually, which is solved by stirring and/or stripping voltammetry. And last but not least, how sensitive is ferricyanide to degradation / reaction? Might it have been degraded by oxygen/light/etc.? Have you purged argon or nitrogen to remove dissolved oxygen from the electrolyte? Are you sure that the buffer does not react with ferricyanide? The voltammogram looks normal otherwise, so the reference and counter electrodes could be fine (but you can always check them by standard tests - check manuals, dummy cells, or any system that you know potential/current responses of).

Hi Yolanda, thank you for your response. The pH of the BR buffer is 7.2, though I have not checked it with the addition of the potassium ferricyanide (will get on that). The working electrode in question is a pad printed carbon electrode; a carbon graphite paste from Gwent group LTD was printed onto a 250 micron sheet of cellulose acetate, about 8 deposits were printed. The electrodes were not polished but were sonicated in ultra pure water for 5 minutes before use. Given as these electrodes are meant to be disposable, I was under the impression that polishing may not be necessary? or am I way off?

The system I'm using is relatively small, it has been used previously by past students of the university with (what I imagine) relative success. I'm trying to trouble shoot issues with my supervisor, but they are very busy. 

I've tried it with several other printed electrodes and though there may be a slight variation amongst them, however the voltammograms all have similar presentation. We've ordered a new GCE to assess the circuit and to do a preliminary to this preliminary in order to compare the printed electrodes but that will have to wait.

The ferricyanide was prepared on the day of experimentation, the room I work in is relatively dark and the powdered stock is stored in a brown bottle in a dark cupboard so I can't imagine that's the issue. Also, the samples were purged with nitrogen before assessment. 

I've done a bit more looking around at the literature and found some that present CV staircases slightly similar to my own, which is a bit of a relief. 

Thanks again for your response; I'll find a way to get it working.

Hi,

Few points to check

1.Check your potential window, its to0 high for the ferri/ferro system. It should reside in between 0 V (may be -0.1) and 0.5 V(may be 0.6 V) vs Ag/AgCl.

2. Voltammogram should not start at high -ve potential like -0.7 V, as you can see that you have two peaks but because of high background current (dotted line) you are unable to justify them as peaks. For safer side, you could start at 0 V.

3. You didn't mention the scan rate but it is also an important consideration in CV.

Hope these points will help you to get a perfect voltammogram. :)

Hi Sujoy, thanks for your response. The graph here is a bit old, since posting it I've reduced the window to -0.7 - 1.2 V and started at 0V ( I will now go even smaller); I was trying to remove this untidy line from the graph and figured starting at a lower V would remove it, though it did give me a higher background current! I've tried a few scan rates, I believe this particular one was most probably 100mV/s.

Since posting this I've had a few better results but it's still not great, I'll just keep working at it!

Sonication in ultrapure water for 5 min will probably not be enough, and your shift in the hydrogen evolution peak (at -0.7 V or lower) also suggests that the electrode surface is not clean. Try to clean it electrochemically by cycling it in the buffer without the ferricyanide. You should get cycles that overlap in the end and where the hydrogen/oxygen peaks are not shifting anymore. Some of my colleagues had similar problems before and tried different cleaning methods: check "Lowe, T. A.; Hedberg, J.; Lundin, M.; Wold, S.; Odnevall Wallinder, I., Chemical Speciation Measurements of Silver Ions in Alkaline Carbonate Electrolytes Using Differential Pulse Stripping Voltammetry on Glassy Carbon Compared With Ion Selective Electrode Measurements. Int. J. Electrochem. Sci 2013, 8, 3851-3865.".