Thanks @Luca Bardini. In both these cases I am considering, the reactant should be free in the liquid phase, and that is why the k0 i calculated for the electrode surface was in the unit of cm/s. Regarding to the biofilm case, what i understand is the biofilm is immobilized on the solid surface, but we can take it as catalytic layer, and the reactant is also moving around the biofilm-liquid interface and within the biofilm.

Simply, i just want to confirm the idea that the kinetic rate would be slower if the electrode is coated with a layer of biofilm, it seems obvious in the lab experiment, but still want to see it from the mathematical point of view.

Hi, John,

Thanks. I calculated it based on the equations from Richard Nicholson, as the paper attached below. Also, I checked the book written by Allen J Bard (Electrochemical Methods: fundamentals and Applications, and it gives the unit of cm/s for k0. Maybe there is anything I misunderstood?

If possible, maybe I can send you emails to give a bit more details about what I want to calculate?

Cheers,

Bin

@ John   it will be the first condition for my case, and that is why i calculate my k0 in cm/s.

But the literature i was referring to is using the Raman spectrometry to get rate constant within the biofilm, the k1 and k2 as drawn in the attached figure. and the unit is in s-1.

my hypothesis is the kinetic with in the biofilm, basically the k1 and k2, is slower than the bulk solution, and if the electrode is coated with a layer of biofilm, all the kinetics will be slowing down.

Initially, what I trying to do is to convert the k1 and k2 to the same unit as the k0 i calculated, so i could directly compare their values. And this is the cole reason i am asking in this question.

But on the other hand, it seems it might be better to compare the mass flux instead? maybe use these k values to calculate the flux in the unit of mmol/cm2/s, which will be more feasible? How do you think?

Cheers,Bin

for the biofilm case, it can be considered as: step 1, the biofilm uptake the substrate in the bulk liquid and release electrons in terms of intracellular reduced redox cofactors; step 2, then electrons are transferring within the biofilm (k1) and finally the reduced redox cofactors get oxidized on the electrode-biofilm interface (k2).

the literature i attached above determined the value for k1 and k2, and the method used there is not an electrochemical method but based on spectroscopy. Then I get stuck of how to compare the literature value with the k0 I calculated based on the normal electrochemical procedure.  maybe I should determined the rate constant for the biofilm condition by myself, which would make the comparison easier and more feasible?

@ John    Thanks for the paper.

Units of 1/s are appropriate for a First-order rate constant. Units of Cm/s denotes a rate (or speed) and are appropriate for a Zero-order process.  Any good textbook on physical chemistry, or better on kinetics, should explain the difference between zero- order kinetics and first-order kinetics.  Also, you have to be clear about a rate versus a rate constant.  The Wikipedia entry entitled "Rate Equation" looks pretty good to me.  See

Hi Bin,

Do you have the title of this paper rateconstant-30-2-5-0407-11.pdf

Thanks

Hi, Chaker,

Sorry, I cannot remember which paper it refers to. I probably renamed the file already when I downloaded it and it is not possible to trace it back.

Cheers, Bin