If you scan the potential from positive value and then you scan it back it seems quite obvious you observe an oxidation peak relative to the reversal of the reduction process.

In other words in the negative scan you reduce a specie and in the positive scan you oxidize it back.

Yes, but in this case reduction of hydrogen peroxide is not a reversible reaction. so i dont understand the oxidation peak.

Can you give some more info about your setup? Whats the reference electrode? What dies your WE consist of? Whats the exact composition of your electrolyte?

Hi!

The cyclic voltammogramm you have does not represent a reversible process.

Anodic and cathodic peaks are separated by about 1 V. A reversible process gives rise to cyclic voltammograms where the negative and positive peaks are separated by 60/n mV where n is the number of electrons exchanged in the reduction/oxidation process. If you want to see the mere reduction process you can use linear scan voltammetry or you can cycle the potential between +0.4 and -0.8 V.

Let me know if it works.

Thanks Lucia! in case of a non-redox reaction where there is no free electons produced, but only charged prodcuts (like -COO-, -NH3+), will the ip be still proportional to square root of scan rate?.

my RE was Ag/AgCl, CE: Pt electrode and electrolyte is phosphate r pH 7 with 1mM hydrogen peroxide.

My WE is ZnO thin film with immobilized horse radish peroxidase.

If you have movements of charged species different from electrons to and from the electrode surface you can observe voltammetric peaks. The proportionality of the current intensity with the square root of the scan rate is usually indicative of a diffusion controlled process.

I observe that in your plot you are using very similar scan rates varying from 20 to 50 mV/s. To ascertain if there is a real proportionality of the peak current with the square root of the scan rate you have to change the scan rate at least an order of magnitude. You can try 10 mV/s, 50mV/s, 100 mV/s. You will have greater differences and it would be easier to verify the current/scan rate relation

Thank you again and I have one last follow up question. If reaction is producing only charged species and no electrons, will it give me an amperometric callibration curve on successive addition of substrate. considering that this time i am stirring the solution and I am applying the potential Ep.

What mechanism you are referring to that produces charged species without involving electron transfer?

like the enzymatic reaction that may not necessarily produce a free electon always

eg: Glucose broken down to gluconic acid and hydrogen peroxideby enzyme glucose oxidase. will gluconic acid (Charged; -COO-)) contribute to current in an amperometric callibration curve?

I understand that you are detecting peroxide by amperometric titration and you wonder if the simultaneous production of gluconate is able do affect your amperometric measurements?

If this is the problem I do not think gluconate may interfere. Even if it should move towards the electrode its contribution to the current is not comparable with the faradaic one determined by your reaction.

May be the generation of negative charges may affect the local potential difference at the interphase, in other words if you are working at fixed applied potential the presence of negative charges accumulating during your reaction may induce a change in the local potential difference so influencing the electrochemical reaction you are observing.

Can you give me some more details about your experimental protocol?

What may be the reason for the appearence of a oxidation peak in an reduction reaction?

I am trying to study the reduction of peroxide by Horse radish peroxidase. but in the Cyclic voltammetry I am also getting an oxidation peak proportional to the root of scan rate. what might be the possible explanation for the oxidation peak? I am attaching the graph for reference.

Answer: In the figure you have attached, a problem of noise explain the split of your reduction peak, is not an oxidation peak close to the reduction one! It is possible that the ZnO immobilized on the electrode surface induced this noise. Normally you have not split peaks neither in the oxidation, nor in the reduction domain. You can try to increase/decrease the step of the potential scan rate, or to reduce the amount of ZnO on the electrode. I agree with Lucia Becucci, you must use a large domain of scan rates: 10, 25, 50, 100, 250 mV/s, for seeing the linear dependence of I vs. scan rate. Good luck! Graziella Turdean

Thanks Lucia once again. My doubts are clarified to a great extent because of your explanations.

Regarding my experimental system, I am immobilizing Dual enzyme glucose oxidase and peroxidase on ZnO thin film. the earlierone breaks down glucose to gluconaate and peroxide and latter one reduces peroxide. I am getting a reduction even with a single enzyme (Glucose oxidase) which indicates self reduction of peroxide. only problem is that there are many literature which shows peroxide oxidation and not reduction. so got alittle confused....

thanks again.

I am not so confident with this subject. The problem is:are you sure you are observing hydrogen peroxide oxidation or is there a possibility you are observing a different reaction? Moreover have you considered the possibility that your electrodic material could have a catalitic effect on peroxide electroreduction? Which is the reason for you have chosen this material and Which is the substrate you deposit ZnO on?