Instrumental Lecture 20B Electrochem

Instrumental Lecture 20B Electrochem - Chapter 22:...

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Unformatted text preview: Chapter 22: Introduction to Electroanalytical Chemistry Two general categories: 1) Potentiometric Systems measure voltage (i.e., potential) of a galvanic cell (produces electricity spontaneously) 2) Voltammetric Systems control potential & usually measure current in an electrolytic cell (consumes power to cause an electrochemical reaction to occur) Potentiometry Determine concentrations by measuring the potential (i.e., voltage) of an electrochemical cell (galvanic cell) Two electrodes are required 1) Indicator Electrode potential responds to activity of species of interest 2) Reference Electrode chosen so that its potential is independent of solution composition. Representation of Electrochemical Cell Anode Salt Bridge For ion transfer Representation of Electrochemical Cell V E electronic conductor + ionic conductor electronic conductor - electrode Anode or Reference electrode Cathode or Indicator V - Represents device to measure potential (voltage) without drawing significant current i.e potentiometer or electrometer (high input impedance > 100 M (mega ohms) d b a c Ecell = Eind Eref (+ EJ) EJ = junction potential, a non-ideal potential which develops across the interface between two dissimilar solutions Ecell = Eind Eref (+ EJ) Nernst Equation RT [Red] E = Eo - ------ ln --------nF [Ox] Where R = gas constant T = absolute temperature n = number of electrons in reaction F = Faraday's constant E = potential Eo = standard potential [Red] = molar concentration of reduced form of species [Ox] = molar concentration of oxidized form of species Reference Electrodes The Normal Hydrogen Electrode (NHE) is important historically and could serve as a reference electrode today, however, it is impractical, requiring a source of H2 gas at constant pressure, and is highly flammable. Reference Electrodes The Calomel Electrode or Saturated Calomel Electrode (SCE) is the next most important reference electrode historically and was used almost exclusively for many decades as the reference electrode of choice Calomel is the insoluble compound Hg2Cl2 The electrode half reaction is 2 Hg + 2Cl- Eo = 0.242 v Hg2Cl2 + 2 e- Calomel Reference Electrode Calomel Reference Electrode Can use 1 M or 0.1 M KCl rather than a saturated solution E for reference changes slightly with any change in concentration from the Nernst Eq Temperature coefficient of reference electrode is less with 1 M or 0.1 M than for SCE SCE often gets clogged if solution dries out ...
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This note was uploaded on 02/13/2012 for the course UML 84.314 taught by Professor Ryan during the Fall '11 term at UMass Lowell.

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