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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, 7 th -9 th

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O 2 + 2 H 2 O + 4e - 4 OH -

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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 c 0 Shear rate Circular cathode, zero concentration Velocity profile v x γ = dv/dx

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Diffusion coefficient Concentration Shear rate Cathode diameter Oxygen flow Convection in a shear flow layer (Lévēque)

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Convection in a critical point (Levich) Concentration c 0 Rotation speed Ω Concentration 0 Rotating disc electrode

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Density Convection in a critical point (Levich) Concentration Rotation speed Viscosity Rotating disc electrode Diffusion coefficient Oxygen flow

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Rotating disc electrode (RDE) H 2 O 2 + 2e - 2 OH - O 2 + 2 H 2 O + 2e - H 2 O 2 + 2 OH - O 2 + 2 H 2 O + 4e - 4 OH - 2 H 2 O + 2e - H 2 + 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 t=0, concentration c 0 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) Initial concentration Diffusion coefficient Time Oxygen flow

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Steady diffusion through a finite layer (Fick) h Diffusion coefficient D concentration c=0 at the cathode concentration c 0 * in the environment concentration c 0 at outer layer boundary Oxygen flow Partial pressure p 0 * in the environment Permeability P

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oxygen sample 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 layer Fatt method Diffusion in the electrolyte layer D ~h 2 /t transition Diffusion in the sample layer c 0 D ~i t 1/2 Diffusion through the sample layer P p 0 */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 insulation resin body of the electrode polyamide tissue sample water saturated by oxygen grid sealing electrolyte 0.01-n K 2 SO 4 saturated by nitrogen Determination of permeability (thick samples)

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Au cathode insulation resin body of the electrode polyamide tissue sample water saturated by oxygen grid sealing electrolyte 0.01-n K 2 SO 4 saturated by nitrogen Determination of permeability (thick samples)

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Unsteady diffusion through a finite layer h Diffusion coefficient D concentration c=0 at the cathode concentration c 0 * in the environment concentration c 0 at outer layer boundary Oxygen flow for t>0 Partial pressure p 0 * in the environment Permeability P Time t<0Time t>0 p1*p1* c1*c1* c1c1 SAMPLE LAYER

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Unsteady diffusion through a finite layer Diffusion coefficient D can be determined from the half time t [min] t 1/2

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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/m 3 from air ~ 1.25 mol/m 3 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 ? i corr = i Oxygen - i Nitrogen ?

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i corr = i Oxygen - i Nitrogen YES ? NO ? O 2 + 2 H 2 O + 4e - 4 OH -

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O 2 + 2 H 2 O + 2e - H 2 O 2 + 2 OH - 2 H 2 O + 2e - H 2 + 2 OH - Effect of OH - ions

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Au cathode insulation resin body of the electrode polyamide tissue sample water saturated by oxygen grid sealing electrolyte 0.01-n K 2 SO 4 saturated by nitrogen High signal in inert atmosphere !!! Probably: 2 H 2 O + 2e - H 2 + 2 OH - In absence of: O 2 + 2 H 2 O + 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

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