Electrochemical diagnostics of dissolved oxygen diffusion Kamil Wichterle and Jana Wichterlová...

Electrochemical diagnostics of dissolved oxygen diffusion

Kamil Wichterle and Jana Wichterlová

Department of Chemistry, VSB-Technical University of Ostrava Ostrava, Czech Republic

COST F2 Conference

”Electrochemical Sensors for Flow Diagnostics”Florence, Italy

November 2001, 7th-9th

O2 + 2 H2O + 4e- 4 OH-

[C/mol]]m[

[A]

sm

mol22 FzS

iM

Electric current

Faraday constant

Area of the cathode

Stoichiometric coefficient

Oxygen flow

• Convection in a shear flow layer (Lévēque)

• Convection in a critical point (Levich)• Unsteady diffusion to the semiinfinite

space (Cotrel)• Steady diffusion through a finite layer• Unsteady diffusion through a finite layer

Convection in a shear flow layer (Lévēque)

Concentration c0

Shear rate

Circular cathode, zero concentration

Velocity profile

vxγ = dv/dx

31

32

0865.0

d

DM

c

Diffusion coefficient

Concentration

Shear rate

Cathode diameter

Oxygen flow

Convection in a shear flow layer (Lévēque)

Convection in a critical point (Levich)

Concentration c0

Rotation speed Ω

Concentration 0

Rotating disc electrode

Density

Convection in a critical point (Levich)

ConcentrationRotation speed

Viscosity

Rotating disc electrode

216

1

32

06205.0

DM c

Diffusion coefficient

Oxygen flow

0

5

10

15

20

25

30

-1.2-1-0.8-0.6-0.4-0.20

V (SCE)

A/m2

3000 RPM

2000 RPM

1000 RPM

400 RPM

100 RPM

Rotating disc electrode (RDE)

H2O2 + 2e- 2 OH-

O2 + 2 H2O + 2e- H2O2 + 2 OH-

O2 + 2 H2O + 4e- 4 OH-

2 H2O + 2e- H2 + 2 OH-

1

2

3

4

0 10 20 30 40 50 60T [oC]

D [10-9

m2/s]

Diffusivity of oxygen

RDA measurement

● water saturated by oxygen

● water saturated by air

Unsteady diffusion to the semiinfinite space (Cotrel)

Time t=0, concentration c0 everywhere

Time t>0, polarization, concentration c=0 at the cathode

Time t=0, switching the electrochemical cell - on

Diffusion starts, decreasing electric current

Unsteady diffusion to the semiinfinite space (Cotrel)

21

0564.0

tD

M c

Initial concentration

Diffusion coefficient

Time

Oxygen flow

Steady diffusion through a finite layer(Fick)

hDiffusion coefficient D

concentration c=0 at the cathode

concentration c0* in the environment

concentration c0 at outer layer boundary

h

cDM 0

Oxygen flow

Partial pressure p0* in the environment

h

pPM

*0

Permeability P

oxygensample

tissue soaked by KCl solution comunicating with the anodic space

Au cathode

Determination of permeability by Fatt (thin samples)

Unsteady diffusion through a finite layerFatt method

1

10

100

1000

0.01 0.1 1 10 100 1000t [s]

i [A]

Diffusion in the electrolyte layer

D ~h2/ttransition

Diffusion in the sample layer

c0 D ~i t1/2

Diffusion through the sample layer

P p0*/h ~ i

Thin samples

• + high current signal

• + short time if saturation

• - significant effect of electrolyte layer

Thick samples• + minor effect of electrolyte layer

• - low current signal

• - long time if saturation

• - inhomogeneous concentration field

Determination of permeability (thick samples)

Electrode driven oxygen diffusion

Oxygen

Determination of permeability (thick samples)

Electrode and inert driven oxygen diffusion

Oxygen

Inert Nitrogen

Au cathode insulation

resin

body of the electrode

polyamide tissue

sample

water saturated by oxygen

grid

sealing

electrolyte 0.01-n K2SO4 saturated by nitrogen

Determination of permeability (thick samples)

Au cathode insulation

resin

body of the electrode

polyamide tissue

sample

water saturated by oxygen

grid

sealing

electrolyte 0.01-n K2SO4 saturated by nitrogen

Determination of permeability (thick samples)

Unsteady diffusion through a finite layer

hDiffusion coefficient D

concentration c=0 at the cathode

concentration c0* in the environment

concentration c0 at outer layer boundary

Oxygen flow for t>0

Partial pressure p0* in the environment

Permeability P

Time t<0 Time t>0

p1*c1*c1

02

22

01

0 exp)1(21k

k th

DkMM

MM

SAMPLE LAYER

Unsteady diffusion through a finite layer

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20t [min]

i *

2/1

2

1388.0th

D

Diffusion coefficient D can be determined from the half time

01

0

MM

MM

t [min]

t1/2

Why not oxygen ?

•low current signal (and background currents)

•variable concentration (temperature, pressure)

•strange reactions (slow response, hysteresis)

•electrode poisoning

Low current signal

due to limited concentration of oxygen

solubility of oxygen at normal pressure :

~ 0.25 mol/m3 from air

~ 1.25 mol/m3 from pure oxygen

(100 times lower than for common salts !)

Background reactions

due to complicated mechanism of oxygen reduction !

due to trace of impurities !

Does the reduction of oxygen correspond to the difference of signals given for mass transfer driven by oxygen and blind current without oxygen ?

icorr = iOxygen - iNitrogen

?

icorr = iOxygen - iNitrogen

YES ?NO ?

O2 + 2 H2O + 4e- 4 OH-

0

5

10

15

20

-1.4-1.2-1-0.8-0.6-0.4-0.20

V (SCE)

A/m2

pH = 7

pH = 2pH = 3

pH = 11

pH = 12

O2 + 2 H2O + 4e- 4 OH-

O2 + 2 H2O + 2e- H2O2 + 2 OH-

2 H2O + 2e- H2 + 2 OH-

Effect of OH- ions

Au cathode insulation

resin

body of the electrode

polyamide tissue

sample

water saturated by oxygen

grid

sealing

electrolyte 0.01-n K2SO4 saturated by nitrogen

High signal in inert atmosphere !!!

Probably:2 H2O + 2e- H2 + 2 OH-

In absence of:O2 + 2 H2O + 4e- 4 OH-

Electrode treatment

• Gold? Platinum? Silver?

• Acids? Bases?

• Polarization +- ?

• Emery paper?

Conclusions

• Oxygen works !

• Less accurate results !

• Random impurities cause random behavior !

• Periodical checking of the system is strongly recommended !

Electrochemical diagnostics of oxygen mass transfer suitable for determination of :

• oxygen concentration

• oxygen diffusivity

• oxygen permeability

• oxygen solubility

• essential properties of liquid flow

Thank you for your attention

Kamil Wichterle and Jana Wichterlová

VSB-Technical University of Ostrava Ostrava, Czech Republic