According to the Nernst equation: E =E0 +RT/naF ln[O]/[R] −2.303mRT/naF pH, where m is the number of protons participating in the electrochemical reaction, which could be determined from slopes of E vs. pH. Namely, dEp/dpH=−2.303mRT/nf. Genally, the number of protons participating in the electrochemical reaction was calculated for reversible or quasi-reversible electrode reaction process( a pair of redox voltammetric peak), but the irreversible electrode reaction processes (irreversible electrochemical response such as single oxidation or reduction voltammetric peak) also used this equation in many papers, why?
This kind of equation is valid only for reversible reaction. However, the reversibility of an electrochemical reaction depends on the time scale. With CV experiments, it is possible to obtain a reversible condition by lowering the scan rate.
The reversibility of an electrochemical reaction does not depend on the scan rate. The reversibility is an intrinsic property of a reaction.
Thanks very much, but i could not understand why Nernst equation for irreversible electrochemical process(electrochemical oxidation reaction or electrochemical reduction reaction), in irreversible electrochemical process, many papers (e.g. electroanalysis, organic electrochemistry electrochemical sensor) still use this equation for the number of protons participating in the electrochemical reaction.
if some people follow a wrong approach, this does make this approach correct. Nernst equation is valid only for the reversible proccesses
Dear Dr. Geletii,Thank you very much! i read many papers on irreversible electrochemical response (single oxidation or reduction voltammetric peak) of analytes in the past 40 years, but they did not detailedly described this equation for irreversible processes (irreversible electrochemical response), Moreover, their modifed equations (Only RT/naF ln[O]/[R] part is varied, 2.303mRT/naF pH part is constant) were also used (e.g. Analytica Chimica Acta 572 (2006) 259–264).However, i could still not understand why Nernst equation for irreversible electrochemical response.
Some links on the Nernst equation limitations[1-3] might help (more, Yangping Wen).
1. The Nernst equation works only in dilute ionic solutions http://www.chem1.com/acad/webtext/elchem/ec4.html#SIG
2. 24.4: The Nernst Equation https://chem.libretexts.org/Textbook_Maps/General_Chemistry_Textbook_Maps/Map%3A_Chem1_(Lower)/16%3A_Electrochemistry/24.4%3A_The_Nernst_Equation ; https://chem.libretexts.org/Textbook_Maps/General_Chemistry_Textbook_Maps/Map%3A_Chem1_(Lower)/16%3A_Electrochemistry/24.5%3A_Applications_of_the_Nernst_Equation
3. Using entropy and Gibbs energy http://www.thefullwiki.org/Nernst_equation
Dear Dr. Samaras , you links on the Nernst equation are a great help to my understanding, thank you very much!
Please note that electrochemical and chemical reversibility are different. Electrochemical reversibility has to do with whether the behavior during the experiment is Nernstian. Chemical reversibiliy is whether or not the forward reaction is reversed when the current reverses. The absence of a peak on the return sweep of a CV is indicative of chemical irreversibility. Whether or not the system is electrochemically reversible is irrelevant to that. It may still be Nernstian for the forward sweep (perhaps not likely, but that will, in fact depend on scan rate and the rates of electron transfer as well as those for any subsequent chemical reactions).
Dear Dr. Ketter, your answer are a great help to my understanding, Can you give me some references? thank you very much!
I agree with Jacob. Indeed, the electrochemical and chemical reversibility are different.
From thermodynamical point of view, CV experiments are irreversible process beacuse of the mass transport. Then, the competition between the mass tranport and the heterogeneous electron kinetics determines the reversibility level. Thus, the reversibility level depends upon on the time scale. One way to verify if an electrochemical reaction could be qualified "reversible" is to convert to CV response in semi-integrall CV or convolution CV . One can qualify reversible electrochemical response in the case where the semi-integral reponses for the forward and reverse scan are the same
see J. Electroanalytical Chemistry and Interfacial Electrochemistry, 52 (1974) 403-412 and cited references
Dear Dr. Randriamahazaka, your response are a great help to my understanding, thank you very much!
I want to ask how formal potential of an irreversible reaction is calculated in cyclic voltammetry?
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