Okay, let’s back up and think about what happens at Electrode/Solution interface for O + ne - R:. K ads. M.T. O soln. O ads. O bulk. n e -. K ads. R bulk. R ads. R sdn. Rate of ET = F( E ), ignoring other “stuff”. (also). i is measure of flux.
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Electrode/Solution interface for O + ne- R:
Rate of ET = F(E), ignoring other “stuff”
i is measure of flux
Sometimes better to normalize for A:
Okay, if we are MT controlled,
1.) We can allow diffusion
2.) We can stir (hydrodynamic)
3.) We can allow migration
- Reduce Rs (tdl and iRs)
- Reduce migration
So, we are left with 1) Diffusion + 2) Convection
1 Occurs only when a quiet solution is inspected
2 + 1 Occur when we stir, why?
We will rotate electrode:
Get Laminar flow solution flows parallel to electrode surface.
See page 337 of B&F.
This layer is the “Nernst Diffusion Layer” and is also called
the “diffusion layer.”
Its thickness is denoted as d, in cm or mm, usually.
Look at Fe3+ concentration gradient at +1.0V vs. SCE
if Fe3+ + e- Fe2+ and no Fe2+ in solution initially.
Eapp = + 1.0V
CR* = 0
“old” delta at w1
H+ ½ H2 at Pt
~ + 0.528V
E vs. Ref (SCE)
Now at 0.2 V, at ilim, so flux is max at this w:
For any geometry of
w2 > w1
to draw all
CR* = 0
E vs. Ref
E at ½ilim is E½
What is general form of i equation when we are anywhere
on the i – E curve?
Solving for Conc of
O at Electrode:
Now for R:
Canceling common terms
Recall I said before:
But they often do not! So,
Slope is for
< ; it takes more to get reaction going
at same rate of E. T. !
For any O/R couple:
Check out the other cases:
1. O+R present (Do i-E plot for them)
2. R insoluble (metal deposits)