Titration of Oxalate with Permanganate

(the green salt synthesized previously)

Method: Titration using an Oxidizing Agent, Permanganate ion

• The permanganate ion, MnO₄-, is a powerful oxidizing agent.
  • It reacts to form Mn²⁺ ion
    • MnO₄- + 8H⁺ +5e ----> Mn²⁺ + 4 H₂O
  • So the Manganese goes from an oxidation state of +7 to +2
  • Each MnO₄- ion will therefore consume 5 electrons
  • It will oxidize another species, stealing the 5 electrons in the process
• Since H+ is also consumed, the reaction will need to be done in an acidic solution.
• The Oxalate ion is stable, but can be oxidized
  • C₂O₄2- --------> 2 CO₂ + 2e-
  • The C in oxalate is in an oxidation state of +5
  • The C in CO₂ has an oxidation state of +4
  • So each carbon will release 1 electron
  • Each oxalate will release 2 electrons
• Combining, and getting the electrons to match
  • 2 MnO₄- + 16 H⁺ + 5 C₂O₄2- -----^10e-----> 2 Mn⁺² + 8 H₂O + 10 CO₂ (g)

• In addition to being a strong oxidizing agent, Permanganate has another important property. It is a compound with an intense purple color. Even one drop of our titrant is enough to produce a strong pink color. In this case we simply titrate until the solution remains pink.
• Our sample will be colored (yellow green) so the titrant appears rather orangish in the flask. As you near the endpoint the solution becomes colorless and the endpoint is a clear pink.

• The reaction of permanganate tends to be relatively slow. It can take 30-60 seconds. This would normally make titration impractical.
  • However, the reaction is much faster in te presence of a catalyst.
    • A catalyst is a species that causes a reaction to go much faster, but it does not affect the final outcome.
  • In this case, Mn⁺² is a good catalyst
  • So, after the first small addition of MnO₄- the reaction will speed up and proceed at a comfortable rate.
    • This is an example of a phenomenon known as autocatalysis

• The permanganate ion is so strongly colored that it will be impossible to see the meniscus in the buret.
• We simply sight across the top of the liquid.
• Of course, it is still important that your eye be level with the liquid layer to avoid parallax errors.

Safety Concerns:

• 3M H₂SO₄ can case chemical burns.
• The heated solution can erupt if it reaches boiling.
• Contact with permanganate can lead to brown stains.
  • These can often be removed by treating with Hydrogen Peroxide.

Experimental procedure

• 1. Collect 200 ml of 0.02M permanganate solution from the reagent shelf.
  • use the same supply for all titrations.
  • use a clean, dry* beaker or flask
  • be sure to record the concentration (different bottles will have different concentration)
    • (*) if your container is wet, rinse it carefully with three small rinses of the KMnO₄ solution
• 2. Clean and set up your buret
  • rinse the buret with three 10 ml batches of the titrant
  • place the buret in the titration stand
  • fill the buret and drain to the calibrated region
  • 3. Weigh a sample of the salt into a 250 ml Erlenmeyer flask (or beaker)
  • the sample should weigh between 0.25 - 0.35 g.
  • weigh to the nearest 0.001 gram
• 4. Add about 80 ml. of water
• 5. Add 25 ml of 3M H₂SO₄
• 6. Heat to 80^oC on a hot plate
  • be careful not to contaminate the sample with the thermometer or to lose sample on the thermometer.
• 7. Now titrate
  • record (+ 0.01 ml) the initial buret reading
  • add titrant while swirling the flask to mix the contents
    • a strap made from a paper towel can help you hold the neck of the heated flask.
  • you can add the first 10 ml of titrant in one rapid step.
  • titrate until the solution remains pink for 15 seconds after addition of a drop
    • if you are unsure of the end point, record the buret reading. The add another drop and verify that the titration is really complete.
  • finally record the final buret volume.
• 8. Repeat
  • we'd like three titrations of the same compound

• We showed earlier that the stoichiometry is 2 MnO₄- to 5 Oxalates
• # moles of MnO₄- = (concentration of titrant) x (volume of titrant, liters)
• # moles of oxalate = (5/2) (# moles of permanganate)
  • report moles of oxalate per gram of sample
  • compare to the value expected for the salt, K₃Fe(C₂O₄)₃ 3 H₂O

• Links
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  • Ahref=calcium.htm go to titration of Ca by EDTA
  • A href=ion_exchange.htm go to titration of Possium, using ion exchange
  • A href=ch128.htm retun to Chemeistry 128 home page