Chem 3253 Introduction to Electrochemistry March 31, 2004

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Chem 3253Introduction to ElectrochemistryMarch
31, 2004
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Four Electroanalytical Methods

• 1) Potentiometric
• 2) Voltammetric, Polarographic, Amperometric
• 3) Electrolysis (including electrogravimetric
  and coulometric)
• 4) Conductiometric
• We will consider the details of each of these
• separately.

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Potentiometry

• 1) Potentiometric - The electrical potential of a
  galvanic
• (spontaneous cell) is related to the
  concentration of the
• analyte by the Nernst Equation.
• E Eo RT/nF log Q
• where Eo is the standard voltage for the cell,
• R is the gas constant (8.314 J/mol K)
• T is the absolute temperature (K)
• n is the number of electrons in the net redox
  equation
• F is the charge of 1 mole of electrons, and
• Q is a term like Keq except the actual
  concentrations and
• pressures are used instead of equilibrium values.

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Potentiometry

• In order to make potentiometric measurements one
  must have
• both a reference and an indicator electrode. The
• measurement is the voltage difference between the
  two
• electrodes.
• Reference Electrodes Primary Reference
  Electrode
• The point of beginning for the construction and
  definition of
• reference electrodes is with the standard
  hydrogen electrode
• (SHE). It is based on the half-cell reaction
• H(aq, 1 M) e ½H2 (g, 1 atm).

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Standard Hydrogen Electrode
Standard Cu2/Cu half-cell
Experimental setup to measure the potential of a
half-cell vs. the standard hydrogen electrode.
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Reference Electrodes Primary Reference Electrode

• For the SHE when the H 1.00 m and pH2
  1.00 atm this
• electrode has a defined potential of 0.0000
  volts. Unfortunately
• the SHE is not a very convenient reference
  electrode. It is quite
• fragile, has the hazard of using hydrogen gas,
  and the standard
• acid solution is difficult to maintain at 1.00 m.
  Although it is not
• used in the laboratory for measurements, it is
  the primary
• reference electrode against which all other
  electrodes are
• ultimately standardized.

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Secondary Reference Electrode Saturated Calomel

• There are 2 commonly used secondary reference
  electrodes.
• 1) The first is the saturated calomel electrode
  (SCE) whose
• half-cell reaction is
• Hg2Cl2(s) 2e 2Hg(l) 2Cl?(aq)
• This half-cell is measured against the SHE with
  saturated KCl as
• the electrolyte its potential is 0.2445 volts _at_
  25oC. The potential
• is also dependent of the concentration of
  chloride, so the
• saturated KCl is the most commonly used because
  it is
• easiest to maintain. The literature gives the
  potential at other
• temperatures, as well as with other
  concentrations of KCl.

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The Calomel Reference Electrode
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Secondary Reference Electrode Silver-silver
Chloride

• 2) The other widely used laboratory reference
  electrode is
• the silver-silver chloride electrode whose
  half-cell reaction is
• AgCl(s) e Ag(s) Cl?(aq).
• The potential of the Ag-AgCl system is 0.2223
  volts _at_ 25oC and
• a saturated solution of KCl is used as its
  electrolyte. As with the
• calomel reference electrode, the literature gives
  the potential at
• other temperatures.

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The Silver-Silver Chloride Reference Electrode
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Indicator Electrodes

• Indicator electrodes may be classified according
  to the process
• that produces the electrode potential.
• Metal indicator electrodes develop a potential
  dependent on
• the position of the equilibrium of the redox
  half-reaction at the
• surface of the electrode.
• Membrane indicator electrodes develop a potential
  
• determined by the difference in the concentration
  of particular
• ions across a special thin layer known as a
  membrane.

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Metal Indicator Electrodes

• Metal indicator electrodes are classified as
  either first-order or
• second-order electrode.
• A first order electrode involves the metal in
  contact with
• Its own ions, such as Ag, Ag or Zn, Zn2. Only a
  few metals
• give reproducible potentials as first-order
  electrodes due to
• crystalline irregularities on their surfaces and
  the ease of forming
• oxides.
• A second-order electrode is one that responds to
  the
• presence of a precipitating or complexing ions.
  For example, a
• silver wire could serve as the indicator
  electrode for chloride.
• The use of second-order indicator electrodes is
  somewhat
• limited.

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Inert Electrodes

• The most commonly used indicator electrodes are
  known as
• inert. These electrodes are not involved in the
  half-cell reactions
• of the electrochemical species. Typical inert
  electrodes are
• platinum, gold, and carbon. Inert electrodes are
  responsive to
• any reversible redox system these are widely
  used in
• potentiometric work.

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An inert electrode of Platinum Responding to the
Sn4/Sn2 half-cell
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Membrane Indicator Electrodes

• There are several different types of membrane
  indicator
• electrodes according to the type of membrane and
  the
• specific use of that electrode. Membranes may be
  made of
• glass, polymers, or crystals.

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The Glass Electrode

• The first and most widely used membrane indicator
  electrode is
• the glass electrode, most commonly used as the pH
  electrode.
• The membrane is a very thin layer of specially
  composed glass,
• generally containing CaO, BaO, Li2O and Na2O as
  well as SiO2.
• The internal solution is of known constant
  hydrogen ion activity,
• such as pH 7.0. When this system is placed in an
  external
• solution of other hydrogen ion activity, a
  Nernstian potential
• develops across the glass membrane. Although the
  response
• follows the Nernst equation
• E 0.05916/1 log (Hexternal /Hinternal)
• there are three important facts that must be
  considered.