To analyze an unknown iron (Fe+2) solution in an unknown sample by reduction oxidation titration with a standardized potassium permanganate solution.
Theory: In order for chemical analysis to be based on redox titrations under certain conditions. These conditions are that the reaction goes to completion, the reaction is kinetically fast enough to give immediate results, no side reactions occur, and a satisfactory indicator exists. The oxidation reagent chosen for this reaction is potassium permanganate. The potassium permanganate is first standardized according to the following reaction.
Oxidation half-reaction: [C2O42-→2CO2 + 2e-] x 5
[MnO4- + 8H+ + 5e-→Mn2+ + 4H2O] x 2
2MnO42- + 5C2O42- + 16H+→2Mn2+ + 10CO2 + 8H2O
Hence, five moles of oxalic acid react with 2 moles of KMnO4. in order to make this reaction proceed faster its is heated. Once the titration reaches completion the color change will be from purple to colourless. This is without the addition of an indicator due to the changing of the purple Mn7+ ions to colorless Mn+2 ions. However if the heated sodium oxalate is left open it will decompose to give hydrogen peroxide and water according to the following reaction:
C2O4-2 + O2 + 2H+ ↔ H2O2 + 2CO2
2H2O2 ↔ O2 + 2H2O
In order to prevent this the titration is started at room temperature and heated as the endpoint reached.
The reaction between potassium permanganate and iron is as follows:
Fe+2→Fe+3 + e-
MnO4- + 8H+ + 5e- →Mn+2 +4H2O
MnO4- + Fe+2 +8H+ →Mn2+ +Fe+3 +4H2O
One precaution that must be taken is to prevent any side reactions from taking place. This is done by keeping the solution acidic due to the production of MnO2 if it is neutral.
Part 1: A solution of 0.02m KMnO4 was filtered through a plug of glass wool into a 250 ml beaker. In order to prepare a 1000ml solution of 1M sulphuric acid 50ml of concentrated sulphuric acid were added to 950 ml of water in a 1000 ml flask.
Three samples of Na2C2O4 were weighed by difference using an analytical balance and were found to be 0.27g, 0.25g, and 0.29g. 250ml of 1M sulphuric acid were added to the three samples and then the contents were mixed with a stirring rod.
Using a burette 40 ml of permanganate solution were added to the oxalate in order to consume 90%-95% of the oxalate. Then the solution is heated and titrated.
For the first Na2C2O4 sample 36 ml of 0.02M KMnO4 were added, for the second sample 33.48ml added, and for the third sample 38.7ml were added.
Part 2: Two samples of iron ore were weighed using an analytical balance and placed in two 250ml flasks. The masses were found to be 1.4460g and 1.5200g. Then 10 ml of 3.0M sulphuric acid and 2ml of H3PO4 to the sample. Finally enough distilled water was added to dissolve the sample.
Each sample was then titrated against the standardized KMnO4 solution. The end point was considered to have been reached once the slightest pink color is observed and remains after swirling for about 15 seconds. The percent by weight of iron in the sample was determined by comparing the recorded sample number with known values.
Mass of the sodium oxalate samples and the volume of KMnO4 titrated against it.
Sample of sodium oxalate
Mass of Na2C2O4
Volume of KMnO4
The volume of required KMnO4 for each sample was calculated as follows: Sample 1:
0.27g Na2C2O4/133.98=2.02 x 10-3 moles
5moles C2O4→2moles MnO4-
2.02 x 10-3 moles C2O4→X moles MnO4-
X=8.07 x 10-4
8.07 x 10-4/0.02=0.04L =40ml KMnO4
40ml x 90%=36ml Similarly the volume of KMnO4 for the second and third samples was calculated and was found to be 33.48ml and 38.7ml respectively.
Using the data in the above table the molarity of potassium permanganate was determined: