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

O H 2 O + 4e -  4 OH -

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

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

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

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

Rotating disc electrode (RDE) H 2 O 2 + 2e -  2 OH - O H 2 O + 2e -  H 2 O OH - O H 2 O + 4e -  4 OH - 2 H 2 O + 2e -  H OH -

Diffusivity of oxygen RDA measurement ● water saturated by oxygen ● water saturated by air

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

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

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

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

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

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 K 2 SO 4 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 K 2 SO 4 saturated by nitrogen Determination of permeability (thick samples)

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

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

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

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

O H 2 O + 2e -  H 2 O OH - 2 H 2 O + 2e -  H 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 K 2 SO 4 saturated by nitrogen High signal in inert atmosphere !!! Probably: 2 H 2 O + 2e -  H OH - In absence of: O H 2 O + 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