Electrochemical diagnostics of dissolved oxygen diffusion

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Electrochemical diagnostics of dissolved oxygen
diffusion
COST F2 Conference Electrochemical Sensors for
Flow Diagnostics Florence, Italy November 2001,
7th-9th

• Kamil Wichterle and Jana WichterlováDepartment
  of Chemistry, VSB-Technical University of
  Ostrava Ostrava, Czech Republic

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O2 2 H2O 4e- ? 4 OH-
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Oxygen flow
Electric current
Area of the cathode
Stoichiometric coefficient
Faraday constant
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• Convection in a shear flow layer (Léveque)
• 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 (Léveque)
? dv/dx
Shear rate
x
v
Concentration c0
Velocity profile
Circular cathode, zero concentration
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Convection in a shear flow layer (Léveque)
Shear rate
Oxygen flow
Concentration
Diffusion coefficient
Cathode diameter
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Convection in a critical point (Levich)
Concentration c0
Rotating disc electrode
Rotation speed O
Concentration 0
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Convection in a critical point (Levich)
Rotating disc electrode
Density
Oxygen flow
Concentration
Rotation speed
Diffusion coefficient
Viscosity
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Rotating disc electrode (RDE)
O2 2 H2O 4e- ? 4 OH-
H2O2 2e- ? 2 OH-
O2 2 H2O 2e- ? H2O2 2 OH-
2 H2O 2e- ? H2 2 OH-
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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 t0, concentration c0 everywhere
Time t0, switching the electrochemical cell -
on Diffusion starts, decreasing electric current
Time tgt0, polarization, concentration c0 at
the cathode
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Unsteady diffusion to the semiinfinite
space (Cotrel)
Diffusion coefficient
Oxygen flow
Initial concentration
Time
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Steady diffusion through a finite layer (Fick)
Partial pressure p0 in the environment
concentration c0 in the environment
concentration c0 at outer layer boundary
h
Diffusion coefficient D
Permeability P
concentration c0 at the cathode
Oxygen flow
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Determination of permeability by Fatt (thin
samples)
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Unsteady diffusion through a finite layer Fatt
method
Diffusion in the electrolyte layer
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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)
Oxygen
Electrode driven oxygen diffusion
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Determination of permeability (thick samples)
Inert Nitrogen
Oxygen
Electrode and inert driven oxygen diffusion
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Determination of permeability (thick samples)
electrolyte 0.01-n K2SO4 saturated by nitrogen
polyamide tissue
sample
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Determination of permeability (thick samples)
electrolyte 0.01-n K2SO4 saturated by nitrogen
polyamide tissue
sample
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Unsteady diffusion through a finite layer
Time tlt0
Time tgt0
Partial pressure p0 in the environment
p1
concentration c0 in the environment
c1
c1
concentration c0 at outer layer boundary
h
Diffusion coefficient D
SAMPLE LAYER
Permeability P
concentration c0 at the cathode
Oxygen flow for tgt0
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Unsteady diffusion through a finite layer
t min
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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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O2 2 H2O 4e- ? 4 OH-
icorr iOxygen - iNitrogen YES ?
NO ?
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Effect of OH- ions
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High signal in inert atmosphere !!!
electrolyte 0.01-n K2SO4 saturated by nitrogen
Probably 2 H2O 2e- ? H2 2 OH-
polyamide tissue
In absence of O2 2 H2O 4e- ? 4 OH-
sample
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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 WichterlováVSB-Technical University of
Ostrava Ostrava, Czech Republic