421-821-chapter-25

421-821-chapter-25 - VOLTAMMETRY A.) Comparison of...

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VOLTAMMETRY A.) Comparison of Voltammetry to Other Electrochemical Methods 1.) Voltammetry: electrochemical method in which information about an analyte is obtained by measuring current (i) as a function of applied potential - only a small amount of sample (analyte) is used Instrumentation Three electrodes in solution containing analyte Working electrode: microelectrode whose potential is varied with time Reference electrode: potential remains constant (Ag/AgCl electrode or calomel) Counter electrode: Hg or Pt that completes circuit, conducts e - from signal source through solution to the working electrode Supporting electrolyte: excess of nonreactive electrolyte (alkali metal) to conduct current
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Apply Linear Potential with Time Observe Current Changes with Applied Potential 2.) Differences from Other Electrochemical Methods a) Potentiometry : measure potential of sample or system at or near zero current. voltammetry – measure current as a change in potential b) Coulometry: use up all of analyte in process of measurement at fixed current or potential voltammetry – use only small amount of analyte while vary potential
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3.) Voltammetry first reported in 1922 by Czech Chemist Jaroslav Heyrovsky (polarography). Later given Nobel Prize for method. B.) Theory of Voltammetry 1.) Excitation Source: potential set by instrument (working electrode) - establishes concentration of Reduced and Oxidized Species at electrode based on Nernst Equation: - reaction at the surface of the electrode E electrode = E 0 - log 0.0592 n (a R ) r (a S ) s (a P ) p (a Q ) q Apply Potential
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Current is just measure of rate at which species can be brought to electrode surface Two methods: Stirred - hydrodynamic voltammetry Unstirred - polarography (dropping Hg electrode) Three transport mechanisms: (i) migration – movement of ions through solution by electrostatic attraction to charged electrode (ii) convection mechanical motion of the solution as a result of stirring or flow (iii) diffusion motion of a species caused by a concentration gradient
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421-821-chapter-25 - VOLTAMMETRY A.) Comparison of...

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