Dear Riaan,

Regarding SWV, it is very intuitive the section SWV in "Electrochemical methods:Fundamentals and applications", Bard and Faulkner, 2001, p. 293-298. There are two figures that are extremely intuitive for this discussion (Fig. 7.3.14 and 7.3.15), that were extracted from J. Osteryoung and J.J. O'Dea, Electroanal.Chem., 14, 209 (1986). Here you can se that even when you scan in the negative (or the positive) direction you have anodic and cathodic currents (in a reversible system). You sample the current at the end of the forward pulse, that is in the direction of the staircase scan (if) and at the end of the reverse pulse, which is in the opposite direction (ir). A difference current is calculated if-ir, and plotted: your square wave voltammogram.

If you put all the forward current samples together you get a voltammogram (let's say "forward") and if you put all the reverse current samples you get another scan (let's say "reverse"). The difference (Bard and Faulkner talk about difference) is the net current (higher because since one is positive and the other negative, you obtain the addition). It is true that most of the software allow to visualize all the three, "forward", "reverse" and this with the net current.

This is why is considered a diagnostic technique as CV, because it keeps all the information!

I encourage you to check these figures. They are very intuitive!

Best,

Teresa

Dear Riaan Laubscher,

In SWV, the current is shown as the difference between two current samples. One is measured at the end of forward potential pulse and the other is at the end of reverse pulse. So, whenever you perform a SWV, you should get two peaks for forward and reverse pulses. But the difference of them will display a single peak.

Since the potential was swept from well positive to well negative potential, it was the reduction of redox species dominated over oxidation. Since the peak potential separation is very high between cathodic and anodic peaks, it is highly unlikely to get anodic peak for reverse potential step at the experimental negative potential sweep shown in SWV. It is because at this negative potential, Fe(CN)63- species could only reduce to Fe(CN)64-. Hence, the difference (SWV peak) would be between two reduction current sampled for forward and reverse pulses.

Regards,

Tamanna

Dear Riaan Laubscher,

I agree with Tamanna Islam's answer. The plot usually shows the difference current and thus only a single peak. But the data for both reverse and forward scan are there. If you are using a CH instrument then you can see both plots simply by using the graph options.

I have attached here the image of an experimental SWV plot of K3[Fe(CN)6]. It contains the difference plot, forward plot and reverse plot. You will see that it shows reduction and oxidation peaks for forward and reverse scan, respectively. You will see that the reduction and oxidation peak separation is only 20 mV and the oxidation peak is about 75% of the reduction peak. This is due to the fact that the scan is towards reduction potential.

Sincerely,

Md. Mahedi Hasan

I don’t agree with the previous two answers: a SWV record is not a difference of two peaks or a difference between two plots (forward/reverse). The theory of SWV is well explained here:

https://www.ameteksi.com/-/media/ameteksi/download_links/documentations/library/princetonappliedresearch/application_note_s-7.pdf?la=en

For your case, at the starting potential (0.5V), Fe is present near the working electrode as Fe(III). As the potential decreases, a reduction current is recorded for each pulse: a higher current for the forward pulse (because this pulse anticipates a more reductive potential) and a lower current for the reverse pulse (because the pulse returns to a less reductive potential). The difference between the two cathodic/reduction currents, in the same SW cycle, is used for the SWV plot (di vs E). Similar explanation for an anodic scan (the plot relates to anodic/oxidation current).

Think to a SWV plot to be something similar to a differential plot (di/dE vs E) of the original record (i vs E from CV).

1.If I sweeping the SWV from well positive of the oxidation potential to well negative of the reduction potential (as identified in the CV), why am I only seeing one peak current?

Seeing only one peak in a cathodic SWV scan, if the cathodic part of the CV has only one reduction peak, is absolutely normal. Try to differentiate the cathodic part of the CV plot and compare the results).

2. Is the peak in SWV due to the reduction or oxidation of the ferricyanide, or due to both? (If I had to take a guess I would say both since the SWV peak is more or less halfway between the reduction and oxidation peak potentials as identified in CV - but it is just a guess.

In the cathodic scan you record only reduction currents. Don’t confuse peak potentials with the reduction potential (avoid to say oxidation potential). The reduction potential is at E1/2, as it can be seen in a CV. A peak signal in SWV close to E1/2 as seen in CV is normal, because SWV is similar to a differential plot of the CV plot.

3. How would the scan direction influence the test if I started at -0.5 V and ended at 0.5 V?

You may expect to have a current peak in SWV at the same E1/2 seen in CV. You may expect to have similar high currents, in anodic and cathodic scans, only if you use a 1:1 Fe(III)/Fe(II) mixture as a test sample.

Dear Dr. Cornel Radu,

I think there is some misinterpretation here. If you read the para under page 2 of your attachment you will find these lines, " The current is sampled twice during each square wave cycle, one at the end of the forward pulse, and again at the end of the reverse pulse. The technique discriminates against charging current by delaying the current measurement to the end of the pulse. The difference current between the two measurements is plotted vs. the potential staircase." We meant this difference.

Also please see the figure 2 of your attachment.

Also you will always get 2 peaks while taking SWVs. This I say based on my experimental findings and both of them may correspond to reduction. (see the attached figure)

Here we used an unpolished GCE and got the peak at around 0.196 Vs. So, for both forward and reverse scan we got only reduction currents. Because the amplitude was only 0.025 Vs.

If there's any wrong information here please let me know. SWV is a very interesting technique and I would very much like to learn more about it.

Sincerely,

Md. Mahedi Hasan

The common SWV plot is " the difference current between the two measurements (forward pulse - reverse pulse) vs. the potential staircase". Both are here for a cathodic scan, thus both record reduction currents. There is no "forward and reverse scans" in SWV, but "forward and reverse pulses".

CH software allows to collect all forward pulse measurements in one plot and all reverse pulse measurements in another plot. This may suggest that a SWV plot is a difference between two plots. I don't know why CH has adopted this practice, the common SWV plot (a differential, not a difference) can be obtained directly (as di vs E). I don't say that this practice is wrong, I say it is not necessary and may be confusing.

Dear Dr. Cornel Radu,

I think you are right about the word 'pulse'. It should be used in case of SWV in order to avoid confusion.

As an expert, your kind advice would be helpful for CHI programmers and chemists (who use CH instrument like me) to adopt more scientific methods.

Sincerely,

Tamanna

Dear Riaan,

Regarding SWV, it is very intuitive the section SWV in "Electrochemical methods:Fundamentals and applications", Bard and Faulkner, 2001, p. 293-298. There are two figures that are extremely intuitive for this discussion (Fig. 7.3.14 and 7.3.15), that were extracted from J. Osteryoung and J.J. O'Dea, Electroanal.Chem., 14, 209 (1986). Here you can se that even when you scan in the negative (or the positive) direction you have anodic and cathodic currents (in a reversible system). You sample the current at the end of the forward pulse, that is in the direction of the staircase scan (if) and at the end of the reverse pulse, which is in the opposite direction (ir). A difference current is calculated if-ir, and plotted: your square wave voltammogram.

If you put all the forward current samples together you get a voltammogram (let's say "forward") and if you put all the reverse current samples you get another scan (let's say "reverse"). The difference (Bard and Faulkner talk about difference) is the net current (higher because since one is positive and the other negative, you obtain the addition). It is true that most of the software allow to visualize all the three, "forward", "reverse" and this with the net current.

This is why is considered a diagnostic technique as CV, because it keeps all the information!

I encourage you to check these figures. They are very intuitive!

Best,

Teresa

M.Teresa Fernández-Abedul

Teresa, please observe the two records (same experiment) provided by Mahedi (in different posts): in the first one the Difference is between two currents with opposite signs, but in the second one the Difference is between two currents with the same sign. can you comment on this situation?

Mahedi Hasan

Mahedi, have you altered the sign convention in these records?

No. I did not change the sign conventions.

Fig. 7.3.16 in "Electrochemical methods:Fundamentals and applications", Bard and Faulkner, 2001, p. 298 may be an answer to my previous comment.