Biopotential electrodesA complex interface

Basics of Instrumentation, Measurement and Analysis 2011, 2012

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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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electrode double layerNo current

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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)

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chemical reactions silver / silver chloride

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electrical behaviour

equivalent circuit

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equivalent circuitelectrode-electrolyte

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more precise approximation of double layer – Randles circuit

electrode-electrolyte

Rct – active charge transfer resistance W – Warburg element reflecting diffusionwith impedance ZW = AW/(jω)0.5

AW – Warburg coefficient