Biopotential electrodes A complex interface Basics of Instrumentation, Measurement and Analysis 2011, 2012

1 the interface I To sense a signal a current I must flow !

2 the interface problem I To sense a signal a current I must flow ! But no electron e - is passing the interface!

3 metal cation No current What’s going on? leaving into the electrolyte

4 metal cation No current One atom M out of the metal is oxidized to form one cation M + and giving off one free electron e - to the metal. leaving into the electrolyte

5 metal cation What’s going on? No current joining the metal

6 metal cation One cation M + out of the electrolyte becomes one neutral atom M taking off one free electron from the metal. No current joining the metal

7 half-cell voltage No current

8 half-cell voltage No current, 1M salt concentration, T = 25ºC metal: Li Al Fe Pb H Ag/AgCl Cu Ag Pt Au V h / Volt -3,0 negativ 0 0,223 positiv 1,68

9 Nernst equation For arbitrary concentration and temperature E = RT/(zF)·ln(c/K) E – electrode potential R = J /(mol*K) – molar gas constant T – absolute temperature z – valence F = C/mol – Faraday’s constant c – concentration of metal ion in solution K – “metal solution pressure”, or tendency to dissolve

10 electrode double layer No current ? ??

11 electrode double layer No current ? ?

12 electrode double layer No current ?

13 electrode double layer No current

14 current influence n withcurrent n with current flowing the half-cell voltage changes n n

15 current influence n withcurrent n with current flowing the half-cell voltage changes overpotentialpolarization: n this voltage change is called overpotential or polarization: n

16 current influence n withcurrent n with current flowing the half-cell voltage changes overpotentialpolarization: n this voltage change is called overpotential or polarization: n V p = V r + V c + V a activation, depends on direction of reaction concentration (change in double layer) ohmic (voltage drop)

17 polarizable electrode displacement capacitor n “perfectly” polarizable electrode: - only displacement current, electrode behave like a capacitor n

18 polarizable electrode displacement capacitor n “perfectly” polarizable electrode: - only displacement current, electrode behave like a capacitor n example: noble metals like platinum Pt

19 nonpolarizable electrode overpotential n “perfectly” nonpolarizable electrode: -current passes freely across interface, -no overpotential n

20 nonpolarizable electrode overpotential n “perfectly” nonpolarizable electrode: -current passes freely across interface, -no overpotential n examples: -silver/silver chloride (Ag/AgCl), -mercury/mercurous chloride (Hg/Hg 2 Cl 2 ) (calomel)

21 Question! n How is the current doing this to “pass freely”? Electrons can’t live in liquids! Can they?

22 chemical reactions silver / silver chloride

23 electrical behaviour equivalent circuit

24 electrical behaviour equivalent circuit ??

25 electrical behaviour equivalent circuit ?

26 electrical behaviour equivalent circuit

27 equivalent circuit electrode-electrolyte

28 more precise approximation of double layer – Randles circuit electrode-electrolyte R ct – active charge transfer resistance W – Warburg element reflecting diffusion with impedance Z W = A W /(jω) 0.5 A W – Warburg coefficient