there are several possibilities for that:

1) GC polishing procedure. This is the factor, that has the most influence on the measurement. In my previous experience it turned out, that manual polishing is much more affected than mechanically assisted. If you are sure, you prepare your electrode perfectly, you can check the 2 and 3 points

2) Sometimes it is about the reference electrode. You should look, if it is stable. calibrate it before measurement to make sure that this is not the problem.

3) Clean also your counter prior to each measurement electrochemically to ensure there is enough surface area available for the reaction. Also, if the surface area of your counter is not sufficient (use mashes instead of wires) it may cause difficulties in reproducibility.

Hope, something of it will work

It would be useful to have more details about your preparation procedure and what is the irreproducibility: electrode capacity, or some wierd CV-peaks, or funny CV shape, or? I agree with Xenia, there could be different reasons of irreproducibility, and polishing is one of the most crytical preparation step.

same problem I faced long time, but at the end, I thrown that GCE out and replaced by carbon paste electrode or screen printed electrode, they are easier, simpler and surely more reproducible ... try it ..

reproductibility is diff. when u work with Fe3+ /Fe 2+ . At very acid pH the Fe2+ transform at Fe3+ . at pH near to 2.0 the formation of hydroxyde may be possible.

The Problem can be solved either decreasing the concentration of your sample or changing the step potential, what kind of software you are using for it. The other thing may be the GCE is old and then replaced that one....

Xenia's suggestions are maybe the best solution for your problem. I'd even put a more special attention on the polishing step, so you should check out the grain size of the alumina you're using and try to standardize a protocol for progressive polishing using stepwise reduction of the grain size used, from, e.g. 50 um to 1 um. In the same way than mirror telescopes are fabricated.

Apart from her suggestions, you have to make sure the solution you have prepared is also standardized and the batch used each time is freshly prepared. Bear in mind that small variations in the Fe+3/Fe2+ may lead to different electrochemical profile on your results.

Thanx everybody for your interest and valuable suggestions on the problem. I would try to incorporate the above suggestions in my procedure.

before treatment with PBS you can sonicate in an ultrasonic bath newly polished electrode with alumina for 5 minutes in 0.3 M HNO3 and then five minutes in methanol, say not recommended. I've done it for years and have never damaged an electrode and with that I have improved reproducibility as remove all silica.

I suggest getting some experience with ferri/ferrocyanide couple which should perform very well as a stable test case for your instrument and cell. Likewise I recommend that you use a concentration of 1 mM and not ten times higher or lower. A scan rate of

250 mv/s should be ideal as well for typical circumstances.

I would avoid complex buffers such as PBS, the chloride ion can poison the electrode.

The electrochemically active area changes with each polishing. If the current that you observe is the irreproducible factor, try normalizing to current density, those values will be comparable. If the potential is the issue, the reference is key. I assume you are using a Ag/AgCl (aq) reference. If this is the case, replace the solution that the electrode soaks in when not in use and check against Ferrocene or ferrocene methanol. If your counter is a platinum wire, flame clean it before use. This is as simple as holding the wire (or wire mesh, as is preferable for larger volume experiments) in the flame of a propane torch until the yellow color that results ceases, this is sodium and other crud being burned off.

Make sure also that you are working within your solvent's potential window. For water this is pretty small and once you begin breaking down solvent you can generate significant quantities of new species altering the number of waves that you observe with subsequent scans.

Definitely check your polishing too, I find that 3 min on each of three alumina polish sizes smallest being 0.1um.

Good luck!

PBS has a pH of 7.4 and contains phosphate. The 3-valent Fe will form ferrous phosphate, Fe3(PO4)2; It has a molar mass of 357.478 g/mol and is insoluble in water. The solubility product of ferrous phosphate (vivianite) has been determined at 0.1 M ionic strength and 25°C. The constant was found to be 1.07 × 10-29. In water, ferric iron forms compounds that are often insoluble, at least near neutral pH. A salt of ferric iron hydrolyzes water and produces iron(III) oxide-hydroxides.

Those insoluble salts are also covering your electrode surface in a non-reproducible manner. Either you avoid phosphate and use e.g. sulfuric acid to avoid hydrolysis or use hexacanoferrate as suggested. The latter is well studied in electrochemistry.

Good Luck

I very much agree with Karl.... Iron without a ligand like cyanide can be a very complex

set of possibilitis. PBS is not recommended. Chloride is not a good anion to have present in such high concentrations given that it too can be oxidized.

These suggestions are very good. One other thing to consider is the purity of the reagants your are using to make the electrolyte. Try using type 1 water (R > 18.2 MOhm) water and try making the solutions with different or newer salts. Also, if you are using a Ag/AgCl reference electrode you can isolate it in a double junction bridge to prevent appreciable diffusion of chloride ions into the electrolyte. Even small impurities can have very large effects on the electrochemistry especially in redox couples like Fe 2+/3+, which can form soluble and insoluble complexes.

Also, you can check your GCE and polishing technique by running a CV after polishing in high purity perchloric acid (0.1 M) and check the CV against the one that was included with the GCE when you purchased it. If one was not provided by the manufacturer, you can contact them to get a standard CV.

you can check whether the reference electrode is Ok or not by measuring CV under the same conditions and using same solution for Fe3+/Fe2+ but using another electrode. if this is not the problem, then ur GC electrode may be damaged

From my experience I can say that GCE is the best material electrode, and has only one problem: the cleaning. But if you will get all the time the same base line (is better to check in DPV) you will have very reproducible results.

If you are using alumina for cleaning, you should wash very well the electrode surface before your experiments but I suggest to try to avoid sonication because in time you will destroy the thin layer contact between carbon and contact body.

Is only polish, wash, and try....and repeat. In time you will get enough experience. However, the redox par Fe2+/3+ is far from a good model in electrochemistry. especial at that pH; It works well in acidic media, but increasing the pH the system starts to be not very reversible, and so on....

Hello,

If you think your surface might be the issue in your measurement, you could attempt a control with Fc carboxylic acid which should in principle be giving you a consistent well-behaved response.

Hope this helps.

you can use cyclic voltametry from -0.5 to 1.5 V vs Ag/AgCl in HNO3 0.5M and NaOH 0.5M respectivly for cleaning of GCE and then recoded CV in Fe3+/Fe2+

we re polishing the GCE surface with alumina slurry 1.0, 0.3, 0.1 , 0.05 micron sized respectively. After rinse it with distilled water and sonicated 5-10 min in 1:1 EtOH:Dist. water. And sometimes as a last step, 1 V vs Ag/AgCl in 0.1 M HCl saves our GCE. And of course we check the reproducability with 5 mM K[Fe(CN)6] and 0.1 M KCl via CV. Good luck

I use very short anodization in NaOH at 1.8V during 10 seconds. This treatment does not oxidised the surface significantly, and allows very high reproducibility in both peak potential positions as we as peak currents. This anodization is performed after regular alumina cleaning and is followed by 5 min of sonication in destilled water to remove base from the surface.

This is art. PBS is terrible as an electrolyte for many things. Ferricyanide will have much better kinetics than a simple salt of iron 2 or 3. You do not want a very high concentration.

1 mM should be fine. An electrode with leaks around the circumference is best thrown away. Polishing is to just remove a film. Polishing as in sanding would is completely unproductive. The advice given above is reasonable. Keep electrodes away from tobacco smoke or oil aerosols or similar polutants.

I was very interested by Prof Kissinger’s last comment regarding tobacco smoke and glassy carbon electrodes. Can such such chemicals be readily adsorbed to electrode surface?

Yes. Hydrophobic substances like carbon surfaces. Good surface chemistry demands care with purity of both gasses and liquids. Of course labs should have a clean atmosphere for the people working there. Impure solvents and salts can be a source of trouble.

Reproducibility is a measurement of precision rather than an accurate one. Try recording CVs with different GCE electrodes(of different supplier) and with different probe molecules/ions.