Electrochemistry-- Other Electrodes

December 6, 2002

• In the previous part we dealt with the simplest form of an half cell
  • A metal in a solution of the metal ion
    • The metal serves both as a reactant and the conductive path for the electrons
    • The reduction reaction is M⁺ⁿ + n e^- ---> M
    • The voltage will depend on the concentration of the metal ion

• The Redox electrode is an important form for a half cell
  • perhaps the simplest form is Pt | Fe²⁺, Fe³⁺
    • The reduction reaction is Fe³⁺ + 1e^- ---> Fe²⁺ E⁰=____
  • Since neither Fe²⁺ nor Fe³⁺ can serve as a conductive path
    • we need to supply an inert metal electrode (usually platinum or Gold)
  • The voltage will depend on the ratio of [Fe²⁺]/Fe³⁺]

• Experimental: It's easy to make this electrode
  • The solution is a mixture of both Fe²⁺ and Fe³⁺ ions
    • We will supply bottles with mixtures of Fe²⁺ and Fe³⁺
  • The electrode wire is an inert conductor
  • Gold and Platinum are good choices
  • If we go cheap, a graphite rod (a pencil lead) will work

• Note: we definitely do not want an Iron wire here
  • We'd have an electrode with three possible reactions
    • Fe²⁺ +2e ---> Fe
    • Fe³⁺ +3e ----> Fe
    • Fe³⁺ 1e ----> Fe²⁺
  • We also want to avoid copper wire which reacts
    • Cu + 2Fe³⁺ --> Cu²⁺ + 2Fe²⁺

• Anticipating a later part of this experiment: Monitoring a Titration
  • we can use an inert electrode to monitor the Fe2+/Fe3+ ratio
  • if we titrate a sample of Fe2+ with Permanganate
• initally the ratio is extremely large
  • after we begin the titration
    • the Fe2+/Fe3+ ratio begins to decrease
    • at the midpoint, the ratio is 1
    • as we near the endpoint the ratio becomes very tiny
  • the voltage at this electrode will rise sharply
    • at the start of the titration
    • and as we reach the endpoint

• Earlier in the year we did a demonstration kinetics experiment-- the oscillating clock reaction
  • solution changes from colorless...
    • to golden (as I2 forms)
    • to blue (as I- is produced)
    • and back to colorless (as both I2 and I- are eliminated)
• We will show you this reaction again
  • this time with a Pt Redox electrode
    • to measure the voltage of an Pt|I₂(s)|I- electrode
      • we will send this voltage either to a computer or a recorder
      • the color changes should track the voltage changes
    • we really monitor the relative concentration of I2 vs. I-

• The Ag|AgCl(s)|Cl- electrode is often used
  • this is a silver wire (one phase)
  • with a thin layer of AgCl (a second solid phase, insoluble solid)
  • in a solution of Cl- ,usually KCl (a third phase, a liquid)
• This can be used as an electrode for measuring chloride ion concentration

• It is often used as a reference electrode
  • a general purpose second electrode in a voltaic cell
  • all cells require two electrodes
    • life is simpler if only one electrode voltage depends on the contents of the solution
    • our pH electrodes have built in silver chloride reference electrodes
  • reference electrodes are often filled with saturated potassium chloride solution
    • this guarantees that the Cl- concentration stays constant

• Experimental: You can easily create a Ag | AgCl | Cl- electrode
• We have several silver metal strips
  • these have been coated with AgCl
  • we made them the anode in a small cell with KCl and they get coated with AgCl
• The electrode is inserted into a bottle of KCl solution

• pH electrode (used in the acid-base experiments)
• K+ electrode
• Ca⁺² electrode (used in equilibrium experiments)
• F^- electrodes (used routinely to monitor fluoride level in most municipal water systems)