Units and Symbols
There are a number of rate constants used in electrochemical literature. They vary in that they do or do not contain a potential term, etc. The letter k is used with various subscript modifiers. If you wonder about the units of the constant you determine them by dimensional analysis. Suppose you see the expression i = nFAkfCO Right away we can see that the letter i stands for current rather than current density because the area A is included on the right. So i is in amps or coul新ec-1. We solve for kf getting i/(nFACO). We know that n is a unitless number, F is the Faraday in coul搶ol-1. In terms of centimeters we can use cm2 for the area A and mol cm-3 for the the concentration CO of the oxidized species. Assembling the units we have
(coul新ec-1)/(coul搶ol-1搾m2搶ol搾m-3) = cm新ec-1.
This can be interpreted as passage of a volume per area as cm3搾m-2新ec-1. This is related to flux as in Fick's first law
which is in mol搾m-2新ec-1. The diffusion coefficient D is in cm2新ec-1.
Some current density equations contain a symmetry coefficient symbolized with a and then some use b. These depend on whether the reaction is written as a reduction or oxidation. If we write
then we might see
and
.
If written as an oxidation
then we have
and
.