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Biopotential electrodesA complex interface
Basics of Instrumentation, Measurement and Analysis 2011, 2012
3
metal electrolyte
M+
A-
e -I
the interface problem
To sense a signala current II must flow !
But no electron e- ispassing the interface!?
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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
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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
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half-cell voltageNo 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
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Nernst equationFor arbitrary concentration and temperature
E = RT/(zF)·ln(c/K)
E – electrode potential R = 8.314 J /(mol*K) – molar gas constantT – absolute temperaturez – valenceF = 96485 C/mol – Faraday’s constantc – concentration of metal ion in solutionK – “metal solution pressure”, or tendency to dissolve
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current influence
withwith currentcurrent flowing the half-cell voltage changes
this voltage change is calledoverpotentialoverpotential or polarization:polarization:
Vp = Vr + Vc + Va
activation, depends on direction of reaction
concentration (change in double layer)
ohmic (voltage drop)
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polarizable electrode
“perfectly” polarizable electrode:- only displacementdisplacement current, electrode behave like a capacitorcapacitor
example: noble metals like platinum Pt
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nonpolarizable electrode
“perfectly” nonpolarizable electrode:- current passes freely across interface,- no overpotentialoverpotential
examples: - silver/silver chloride (Ag/AgCl),- mercury/mercurous chloride (Hg/Hg2Cl2) (calomel)