15Electrode Double Layer No currentOxidation or reduction reactions at the electrode-electrolyte interface lead to a double-charge layer
16Contact (Half Cell) Potential Depends on:The metal,Concentration of ions in solution andTemperature.Half cell potential cannot be measured without a second electrode.The half cell potential of the standard hydrogen electrode has been arbitrarily set to zero.
17Measuring Half Cell Potential Note: Electrode material is metal + salt or polymer selective membrane
18Half Cell Potential (Vh) Iron mVLead mVCopper mVPlatinum mVCompare to electrophysiological Signals ???Two Similar electrodes ??? (Ag/Agcl 5 mV and steel 100mV)
19Difference bet. observed and zero-current half cell potentials PolarizationIf there is a current between the electrode and electrolyte, the observed half cell potential is often altered due to polarization.OvervoltageDifference bet. observed and zero-current half cell potentialsResistanceCurrent changes resistanceof electrolyte and thus,a voltage drop results.ConcentrationChanges in distributionof ions at the electrode-electrolyte interfaceActivationThe activation energybarrier depends on thedirection of current anddetermines kinetics
20Polarizable and Non-Polarizable Electrodes Perfectly Polarizable ElectrodesNo actual charge crosses the electrode-electrolyte interface when a current is applied. (e.g Platinum electrode)Perfectly Non-Polarizable ElectrodeCurrent passes freely across the electrode-electrolyte interface. These electrodes see no overpotentials. (e.g. Ag/AgCl Electrode)Example: Ag-AgCl is used in recording while Pt is use in stimulation
21Ag/AgCl Electrode Fabrication of Ag/AgCl electrodes Electrolytic deposition of AgClSintered AgCl: process forming pellet electrodes