electrochemical diagnostics of dissolved oxygen diffusion
DESCRIPTION
COST F2 Conference ”Electrochemical Sensors for Flow Diagnostics” Florence, Italy November 2001, 7 th -9 th. Electrochemical diagnostics of dissolved oxygen diffusion. Kamil Wichterle and Jana Wichterlová Department of Chemistry, VSB-Technical University of Ostrava Ostrava, Czech Republic. - PowerPoint PPT PresentationTRANSCRIPT
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Electrochemical diagnostics of dissolved oxygen diffusion
Kamil Wichterle and Jana WichterlováDepartment of Chemistry, VSB-Technical University of Ostrava
Ostrava, Czech Republic
COST F2 Conference
”Electrochemical Sensors for Flow Diagnostics”Florence, Italy
November 2001, 7th-9th
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O2 + 2 H2O + 4e- 4 OH-
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[C/mol]]m[[A]
smmol
22 FzSiM
Electric current
Faraday constant
Area of the cathode
Stoichiometric coefficient
Oxygen flow
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• 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
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Convection in a shear flow layer (Lévēque)
Concentration c0
Shear rate
Circular cathode, zero concentration
Velocity profile
vxγ = dv/dx
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31
32
0865.0
d
DM c
Diffusion coefficientConcentration
Shear rate
Cathode diameter
Oxygen flow
Convection in a shear flow layer (Lévēque)
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Convection in a critical point (Levich)
Concentration c0
Rotation speed Ω
Concentration 0
Rotating disc electrode
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Density
Convection in a critical point (Levich)
ConcentrationRotation speed
Viscosity
Rotating disc electrode
216
1
32
06205.0
DM c
Diffusion coefficient
Oxygen flow
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0
5
10
15
20
25
30
-1.2-1-0.8-0.6-0.4-0.20V (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-
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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
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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
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Unsteady diffusion to the semiinfinite space (Cotrel)
21
0564.0
tDM c
Initial concentration
Diffusion coefficient
Time
Oxygen flow
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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
hcDM 0
Oxygen flow
Partial pressure p0* in the environment
hpPM
*0
Permeability P
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oxygensample
tissue soaked by KCl solution comunicating with the anodic space
Au cathode
Determination of permeability by Fatt (thin samples)
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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
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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
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Determination of permeability (thick samples)
Electrode driven oxygen diffusion
Oxygen
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Determination of permeability (thick samples)
Electrode and inert driven oxygen diffusion
Oxygen
Inert Nitrogen
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Au cathode insulationresin
body of the electrodepolyamide tissue
sample
water saturated by oxygen
grid
sealing
electrolyte 0.01-n K2SO4 saturated by nitrogen
Determination of permeability (thick samples)
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Au cathode insulationresin
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)
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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 thDk
MMMM
SAMPLE LAYER
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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
21388.0
thD
Diffusion coefficient D can be determined from the half time
01
0
MMMM
t [min]
t1/2
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Why not oxygen ?•low current signal (and background currents)
•variable concentration (temperature, pressure)
•strange reactions (slow response, hysteresis)
•electrode poisoning
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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 !)
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Background reactions
due to complicated mechanism of oxygen reduction !
due to trace of impurities !
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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
?
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icorr = iOxygen - iNitrogen
YES ?NO ?
O2 + 2 H2O + 4e- 4 OH-
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0
5
10
15
20
-1.4-1.2-1-0.8-0.6-0.4-0.20V (SCE)
A/m2pH = 7
pH = 2 pH = 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
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Au cathode insulationresin
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-
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Electrode treatment
• Gold? Platinum? Silver?
• Acids? Bases?
• Polarization +- ?
• Emery paper?
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Conclusions
• Oxygen works !• Less accurate results !• Random impurities cause random
behavior !• Periodical checking of the system is
strongly recommended !
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Electrochemical diagnostics of oxygen mass transfer suitable for determination of :
• oxygen concentration
• oxygen diffusivity
• oxygen permeability
• oxygen solubility
• essential properties of liquid flow
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Thank you for your attention
Kamil Wichterle and Jana Wichterlová
VSB-Technical University of Ostrava Ostrava, Czech Republic