Titration of Oxalate with Permanganate
Chem 128 September 20, 2002
Web page edited: September 17, 2002
Goal: To determine the oxalate content of a sample
(the green salt synthesized previously)
Method: Titration using an Oxidizing Agent, Permanganate ion
The permanganate ion, MnO4-, is a powerful oxidizing agent.
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
- 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
Combining, and getting the electrons to match
- 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
- 2 MnO4- + 16 H+ + 5 C2O42- -----10e-----> 2 Mn+2 + 8 H2O + 10 CO2 (g)
Finding the Endpoint:
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.
Reading the Buret
- 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.
- 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.
- 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 80oC 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
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