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Biopotential electrodes. A complex interface. Basics of Instrumentation, Measurement and Analysis 2011, 2012. the interface problem. To sense a signal a current I must flow !. But no electron e - is passing the interface!. metal cation. leaving into the electrolyte. No current.
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Biopotential electrodes A complex interface Basics of Instrumentation, Measurement and Analysis 2011, 2012
the interface problem To sense a signala currentImust flow ! But no electron e- ispassing the interface!
metal cation leaving into the electrolyte 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.
metal cation joining the metal No current One cation M+out of the electrolyte becomes one neutral atom M taking off one free electron from the metal.
metal: Li Al Fe Pb H Ag/AgCl Cu Ag Pt Au Vh / Volt -3,0negativ 00,223positiv1,68 half-cell voltage No current, 1M salt concentration, T = 25ºC
Nernst equation For arbitrary concentration and temperature E = RT/(zF)·ln(c/K)E – electrode potential R = 8.314 J /(mol*K) – molar gas constantT – 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
electrode double layer No current
concentration (change in double layer) ohmic (voltage drop) current influence • withcurrent flowing the half-cell voltage changes • this voltage change is calledoverpotential orpolarization: Vp = Vr + Vc + Va activation, depends on direction of reaction
polarizable electrode • “perfectly” polarizable electrode:- only displacement current, electrode behave like a capacitor • example: noble metals like platinum Pt
nonpolarizable electrode • “perfectly” nonpolarizable electrode:- current passes freely across interface,- no overpotential • examples: - silver/silver chloride (Ag/AgCl),- mercury/mercurous chloride (Hg/Hg2Cl2) (calomel)
chemical reactions silver / silver chloride
electrical behaviour equivalent circuit
equivalent circuit electrode-electrolyte
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