acde

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

2. 2
metal electrolyte
M+
A-
e-
I
the interface problem
To sense a signal
a current I must flow !
But no electron e- is
passing the interface!
?

3. 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

4. 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

5. 8
half-cell voltage
No current, 1M salt concentration, T = 25ºC
metal: Li Al Fe Pb H Ag/AgCl Cu Ag Pt Au
Vh / Volt -3,0 negativ 0 0,223 positiv 1,68

6. 9
Nernst equation
For arbitrary concentration and temperature
E = RT/(zF)·ln(c/K)
E – electrode potential
R = 8.314 J /(mol*K) – molar gas constant
T – absolute temperature
z – valence
F = 96485 C/mol – Faraday’s constant
c – concentration of metal ion in solution
K – “metal solution pressure”,
or tendency to dissolve

7. 13
electrode double layer
No current

8. 16
current influence
 with current flowing
the half-cell voltage changes
 this voltage change is called
overpotential or polarization:
 Vp = Vr + Vc + Va
activation, depends on direction of reaction
concentration (change in double layer)
ohmic (voltage drop)

9. 18
polarizable electrode
 “perfectly” polarizable electrode:
- only displacement current,
electrode behave like a capacitor
 example: noble metals like platinum Pt

10. 20
nonpolarizable electrode
 “perfectly” nonpolarizable electrode:
- current passes freely across
interface,
- no overpotential
 examples:
- silver/silver chloride (Ag/AgCl),
- mercury/mercurous chloride
(Hg/Hg2Cl2) (calomel)

11. 22
chemical reactions
silver / silver chloride

12. 26
electrical behaviour
equivalent circuit

13. 27
equivalent circuit
electrode-electrolyte

14. 28
more precise approximation of
double layer – Randles circuit
electrode-electrolyte
Rct – active charge transfer resistance
W – Warburg element reflecting diffusion
with impedance ZW = AW/(jω)0.5
AW – Warburg coefficient