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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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Presentation Transcript
slide1

Okay, let’s back up and think about what happens at

Electrode/Solution interface for O + ne- R:

Kads

M.T.

Osoln

Oads

Obulk

ne-

Kads

Rbulk

Rads

Rsdn

Rate of ET = F(E), ignoring other “stuff”

(also)

i is measure of flux

Sometimes better to normalize for A:

(current density)

Okay, if we are MT controlled,

1.) We can allow diffusion

2.) We can stir (hydrodynamic)

3.) We can allow migration

slide2

But, if we use S.E. (~100x [analyte])

- 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:

motor

Soln

Flow

Get Laminar flow solution flows parallel to electrode surface.

See page 337 of B&F.

slide3

But we have a stagnant layer due to frictional losses.

No stirring

motion

Di

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.

C(x)

C0*

Eapp = + 1.0V

0

X (mm)

0

slide4

Recall:

CO*

C(x)

CR* = 0

at w2

“old” delta at w1

+ 0.2V

H+ ½ H2 at Pt

ic

+1.0

~ + 0.528V

E vs. Ref (SCE)

ia

Now at 0.2 V, at ilim, so flux is max at this w:

0

0

x

For any geometry of

electrodes etc.

Rotate faster!

CO*

C(x)

CR*= 0

w2 > w1

0

x

0

slide5

At ,

C0*

C(x)

Be able

to draw all

cases!

CR* = 0

0

x

0

ic

ilim

E vs. Ref

E at ½ilim is E½

ia

or

See why

later

so

Mass-Transfer

Coefficient (cm/s)

or

slide6

Note: Flux of R away

= 0

What is general form of i equation when we are anywhere

on the i – E curve?

or

Solving for Conc of

O at Electrode:

Now for R:

slide7

at

substituting

Canceling common terms

Recall I said before:

slide8

So what is the deal?

Well:

ic

Reversible

Fe3+/2+

-

+

Eapp

?

ia

iff

But they often do not! So,

*

*

Slope is for

So:

Eapp

slide9

Also, if irreversible,

ic

Rev.

Irrev.

+

-

Eapp

ia

< ; 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)

Test Material