Standardization of KMnO4 and Determination of Fe+2 Content of an Unknown

Part I - Standardization of KMnO4

1. The MnO2 present in the KMnO4 solution will interfere with the standardization. More MnO2 may also form as the KMnO4 comes into contact with contaminants in dilution water and on the glassware, particularly in the presence of light. Fortunately, the MnO2 is insoluble, and so after allowing enough time for complete oxidation of these contaminants, the solution must be filtered to remove all of the solid MnO2. The cleaned solution should then be stored in the dark.

2. The end-point of this is poor at room temperature because the reaction proceeds slowly. The titration is therefore done between 60 and 90°C. but even then, the reaction is still slow until it begins to be catalyzed by the manganese II ion which is produced as the reaction proceeds. Thus, at the beginning of the titration some time is required to decolorize the oxalic acid solution but the reaction becomes very rapid as more Mn2+ ion is produced.

3. The solution of 0.02M KMnO4 was prepared for you at least one week in advance of this laboratory. Filter the KMnO4 solution through a plug of glass wool into a clean, dry 250 mL beaker and mix thoroughly.

4. Make 1000 mL of 1M H2SO4. Slowly add 55 mL of concentrated H2SO4 (by TA), with stirring, to 900 mL of water in a 1000 mL Erlenmeyer flask. Top up the solution with distilled water to a total volume of 1000 mL.

5. Using your dry weighing bottle, obtain some dried Na2C2O4 from your demonstrator.

6. Using an analytical balance, accurately weigh by difference (see techniques 1) three samples of Na2C2O4 of approximately 0.2500 to 0.3000 grams each into three separate 500 mL Erlenmeyer flasks.

7. To each sample add 250 ml of the 1.0M H2SO4 you made in step 2 and swirl to dissolve the Na2C2O4. Large chunks may have to be broken up with a stirring rod.

8. Introduce, from a burette, sufficient permanganate to consume 90 to 95% of the oxalate. Let stand, swirling occasionally, until the solution is decolorized.

9. Warm to 55-60°C and then complete the titration, taking the first pale pink color that persists for 30 seconds as the end point.

10. Testing the end point of a blank solution: Add a single drop of KMnO4 to the remaining 250 mL of 1.0 M H2SO4. If the solution does not turn pink with a single drop, continue adding KMnO4 until it does. Record the required volume and subtract this value from all your titration values.

1. Obtain a numbered sample of an iron ore (it has been prepared for you with the iron reduced to the ferrous state). Record the number.

2. Using an analytical balance, accurately weigh by difference two samples of ore, weighing about 1.5 g each, into two separate 250 mL Erlenmeyer flasks. Record the weight of the sample to 4 decimal places.

3. To each sample add 10 mL of 3M H2SO4, 2 mL of H3PO4, and enough distilled water to dissolve the sample.

4. Titrate each sample with standardized KMnO4 solution, approaching the end-point drop wise. Take as the endpoint the faintest shade of pink that remains after swirling for about 15 seconds.

5. Calculate your percent iron(II) for each sample. Obtain the actual value from your demonstrator and calculate the percent error (relative error) in your average value.