Electrochemical diagnostics of dissolved oxygen diffusion

1. COST F2 Conference ”Electrochemical Sensors for Flow Diagnostics” Florence, Italy November 2001, 7th-9th Electrochemical diagnostics of dissolved oxygen diffusion Kamil Wichterle and Jana WichterlováDepartment of Chemistry, VSB-Technical University of Ostrava Ostrava, Czech Republic

2. O2 + 2 H2O + 4e- 4 OH-

3. Oxygen flow Electric current Area of the cathode Stoichiometric coefficient Faraday constant

4. 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

5. Convection in a shear flow layer (Lévēque) γ = dv/dx Shear rate x v Concentration c0 Velocity profile Circular cathode, zero concentration

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

7. Convection in a critical point (Levich) Concentration c0 Rotating disc electrode Rotation speed Ω Concentration 0

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

9. 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-

10. Diffusivity of oxygen • RDA measurement • ●water saturated by oxygen • ●water saturated by air

11. Unsteady diffusion to the semiinfinite space(Cotrel) Time t=0, concentration c0 everywhere Time t=0, switching the electrochemical cell - on Diffusion starts, decreasing electric current Time t>0, polarization, concentration c=0 at thecathode

12. Unsteady diffusion to the semiinfinite space(Cotrel) Diffusion coefficient Oxygen flow Initial concentration Time

13. Steady diffusion through a finite layer (Fick) Partial pressure p0*in the environment concentration c0*in the environment concentration c0at outer layer boundary h Diffusion coefficientD Permeability P concentration c=0 at thecathode Oxygen flow

14. oxygen sample tissue soaked by KCl solution comunicating with the anodic space Au cathode Determination of permeability by Fatt (thin samples)

15. D ~h2/ttransition c0 D ~i t1/2 Diffusion in the sample layer P p0*/h ~ i Diffusion through the sample layer Unsteady diffusion through a finite layer Fatt method Diffusion in the electrolyte layer

16. 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

17. Determination of permeability (thick samples) Oxygen Electrode driven oxygen diffusion

18. Determination of permeability (thick samples) Inert Nitrogen Oxygen Electrode and inert driven oxygen diffusion

19. body of the electrode resin insulation Au cathode sealing grid water saturated by oxygen Determination of permeability (thick samples) electrolyte 0.01-nK2SO4 saturated by nitrogen polyamide tissue sample

20. body of the electrode resin insulation Au cathode sealing grid water saturated by oxygen Determination of permeability (thick samples) electrolyte 0.01-nK2SO4 saturated by nitrogen polyamide tissue sample

21. Unsteady diffusion through a finite layer Time t<0 Time t>0 Partial pressure p0*in the environment p1* concentration c0*in the environment c1* c1 concentration c0at outer layer boundary h Diffusion coefficient D SAMPLE LAYER Permeability P concentration c=0 at thecathode Oxygen flow for t>0

22. t1/2 Diffusion coefficient D can be determined from the half time Unsteady diffusion through a finite layer t[min]

23. Why not oxygen ? • low current signal (and background currents) • variable concentration (temperature, pressure) • strange reactions (slow response, hysteresis) • electrode poisoning

24. 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 !)

25. Background reactions due to complicated mechanism of oxygen reduction ! due to trace of impurities !

26. 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 ?

27. O2 + 2 H2O + 4e- 4 OH- icorr = iOxygen - iNitrogen YES? NO ?

28. 2 H2O + 2e- H2 + 2 OH- O2 + 2 H2O + 4e- 4 OH- O2 + 2 H2O + 2e- H2O2 + 2 OH- Effect of OH- ions

29. body of the electrode resin insulation Au cathode sealing grid water saturated by oxygen High signal in inert atmosphere !!! electrolyte 0.01-nK2SO4 saturated by nitrogen Probably: 2 H2O + 2e- H2 + 2 OH- polyamide tissue In absence of: O2 + 2 H2O + 4e- 4 OH- sample

30. Electrode treatment • Gold? Platinum? Silver? • Acids? Bases? • Polarization +- ? • Emery paper?

31. Conclusions • Oxygen works ! • Less accurate results ! • Random impurities cause random behavior ! • Periodical checking of the system is strongly recommended !

32. Electrochemical diagnostics of oxygen mass transfer suitable for determination of : • oxygen concentration • oxygen diffusivity • oxygen permeability • oxygen solubility • essential properties of liquid flow

33. Thank you for your attention Kamil Wichterle and Jana WichterlováVSB-Technical University of Ostrava Ostrava, Czech Republic