Redox Titration Curves

December 6, 2002

• We will monitor the titration of Fe²⁺
  • It will be oxidized by Cerium(IV) Ion, Ce⁴⁺
  • We could have used KMnO₄ as we did previously
• This relies on the Pt | Fe³⁺, Fe²⁺ electrode you studied in part II of this experiment
• The voltage of this electrode varies with the Fe²⁺/Fe³⁺ ratio
  • E = 0.77 - 0.059 log₁₀ ([Fe²⁺]/[Fe³⁺] )
• We will use a Calomel reference electrode
  • calomel is Hg₂Cl₂, mercurous chloride
  • the electrode is Hg | Hg₂Cl₂(s) | Cl (sat'd KCl)
  • this acts as the require second electrode
  • it adds a fixed voltage to the measured potential
  • the Calomel potential is +0.244 volts.
• The expected cell voltage is therefore
  • E = 0.53 - 0.059 log₁₀ ([Fe²⁺]/[Fe³⁺] )

Experimental:

• We will use the same titration stand as the acid base titration curves
  • A peristaltic pump will deliver Ce⁴⁺ titrant
  • A platinum electrode will monitor the Fe²⁺/Fe³⁺ ratio
  • A calomel electrode is included

• place the Fe²⁺ sample in a 100 ml beaker
  • (volume and conc. data at the apparatus)
  • add a magnetic stir bar
  • lower the electrode assembly

• start the program and enter information
  • start data collection when you turn on the pump
  • record and plot the titration curve
  • determine the endpoint

Remarks:

• It would seem that the Fe²⁺/Fe³⁺ ratio is infinite at the start of the experiment
  • the Fe³⁺ concentration is presumably zero
• In practice, a small fraction of the Fe²⁺ will be air oxidized
• Still the ratio will be quite large
• Once we begin the titration, the ratio becomes relatively well behaved
  • 100:1 when the titration is 1% complete
  • 10:1 when the titration is 10% complete
  • 1:1 when the titration is 50% complete
  • 1:10 as we near the endpoint and have titrated 90% of the Fe²⁺
  • 1:100 as we get within 1% of the endpoint

• Also, note that the Ce⁴⁺ is an oxiding agent and it has a potential on the table
  • Ce⁺⁴ + e- ---> Ce³⁺ E^o= +1.44 V (in 1M H₂SO₄)
  • This large voltage means it is a very strong oxidizing agent
• We are really using it as a titrant and we are not thinking of it as electrode material

• However, as we exceed the endpoint, we really end up with a solution that contains both Ce⁴⁺ (unreacted titrant) and Ce³⁺ (reduced titrant)
  • The voltage of the redox electrode is forced by the Ce³⁺/Ce⁴⁺ ratio
    • when we go 10% beyond the endpoint the Ce³⁺/Ce⁴⁺ ratio is 0.1
    • so the electrode voltage will be
      • E= (1.44-0.24) - 0.059 log₁₀( [Ce³⁺] / [Ce⁴⁺ ])
      • notice that the Fe potentials are around 0.5 volts but after the endpoint the Ce potentials will raise the voltage to the neighborhood of 1.2 volts
      • that's a big jump in voltage at the endpoint