After reading many journal articles, most state that increasing the scan rate in voltammetry should increase the oxidation potential values of the analyte. However, in my CVs of synthetic cannabinoids, I lowered the scan rates from 100 mV s-1 to 80 mV s-1 and 50 mV s-1. The 50 mV s-1 presented with a greater oxidation peak when compared to the scan rate of 80 mV s-1. What error or possibility can explain this situation?
Could this be due to a diffusion vs absorption controlled process? What parameters could I change in the future to assess the reliability of this part of my study?
Do you have images of the CVs, so that we could get a sense of the increase? Does the CV peak location shift?
Potential is a thermodynamics concept (Faraday's law to convince). Thermodynamics implies equilibrium theoretically. So this can be approached, in my understanding, only by the lowest scan rate values experimentally reachable.
Scan rate is instead a kinetics concept.
Variation of oxidation potential with scan rate only determines a threshold where scan rate influences the balance of the reaction occurring at the electrode. The reasons can be various, and affect all the parameters involved in the reaction studied (concentration, diffusion, temperature, ...). In your case having a greater oxidation peak at lower scan rate may suggest a transport related parameter. But what's the coulometry in both cases ?
Amen
I propose to perform CVs at continuously increasing sweep rates. Begin from very low sweep rates and track the sweep rate dependence of peak current and peak position. The obtained dependence reveals the dominant mass transfer mechanism in every sweep rate span. Perhaps, a change from a mass transfer controlled regime to a charge transfer or adsorption controlled one occurs upon increasing the sweep rate that can illustrate the contradiction you have observed. Try using lower concentration of electroactive material to reduce the adsorption and replicate the experiments.
At first oxidation potential remains fix, its peak potential which shifts with increasing scan rate for quasireversible behavior of electron transfer. For reversible behavior peak potential remains constant. Sometime solution resistance also changes peak potential.
2nd, draw a plot between sq.root of scan rate and peak current. If this graph is linear and have the regression 0.99 then reaction is diffusion controled.
3rd draw a graph between scan rate and Ip, if graph is linear with 0.99 regression there is adsorption on electrode surface.
4rth draw a graph between log of scan rate with log of peak potential. If slope of this line is 0.5 then reaction is diffusion control. If slop is 1, then there is complete adsorption of analyte and if it is between 0.5-0.9, its mean there is mixed controlled reaction.
5th, usually at low scan rate, adsorption prevails and high scan rate diffusion.
For all detail see attached paper.
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