A Redox Titration " Determining the percentage of iron in an iron ore

3. Standardization of diluted manganate(VII) solution

1. Wash a 25.0 cm3 pipette with distilled water and then with manganate(VII) solution.
2. Pipette 25.0 cm3 of manganate(VII) solution into a 250.0 cm3 of volumetric flask.
3. Repeat step(b) again.
4. Add deionized water upto the graduated mark of the volumetric flask.
5. Stopper the flask and invert it several times in order to mix the contents well.
6. Label the flask as ‘Titrant’.
7. Wash a burette with distilled water and then with ‘Titrant’.
8. Pour ‘Titrant’ into the burette and ensure the tip of burette is filled.
9. Record the initial burette reading.
10. Pipette 25.0 cm3 of ‘Standard’ into a conical flask.
11. Add 25.0 cm3 of 1 M sulphuric acid into the flask.
12. Run the solution from the burette into the conical flask, swirling the flask all the time.
13. Stop the titration when the colour of solution in the conical flask just changes from colourless to pink.
14. Record the final burette reading.
15. Repeat steps(i)-(n) three more times.

4. Determination of iron(II) in ‘B’

1. Wash a burette with distilled water and then with ‘Titrant’.
2. Pour ‘Titrant’ into the burette and ensure the tip of burette is filled.
3. Record the initial burette reading.
4. Pipette 25.0 cm3 of ‘B’ into a conical flask.
5. Add 25.0 cm3 of 1 M sulphuric acid into the flask.
6. Run the solution from the burette into the conical flask, swirling the flask all the time.
7. Stop the titration when the colour of solution in the conical flask just changes from colourless to pink.
8. Record the final burette reading.
9. Repeat steps(c)-(h) three more times.

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Results and Calculations

1a. Mass of iron(II) sulphate crystals = 5.000g

1b. No. of moles of iron(II) sulphate =  = 0.01800 mol

Molar concentration of iron(II) sulphate in ‘Standard’ =  = 0.07199 M

2a. Titration results from Procedure 3:

2b. By equation, no of moles of Fe2+ : MnO4- = 5 : 1

∴ no. of moles of MnO4- =  = mol

Molar concentration of MnO4- ion in the diluted ‘Titrant’ = = 0.04122 M

3a. Titration results from Procedure 4:

3b. By equation, no of moles of Fe2+ : MnO4- = 5 : 1

∴ no. of moles of Fe2+ =  = 0.1069 mol

Molar concentration of Fe2+ in Solution ‘B’ = = 0.1069 M

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4.  Mass of Fe2+ in Solution ‘B’ =  = 5.965g

The iron content of ‘B’ = = g dm-3

Percentage of iron in the ore = = 70.16%

Conclusion

Percentage of iron in the given spathic ore is 70.16%

Discussion

Sources of errors

• Iron(II) sulphate crystal absorbs moisture in air, which makes it becomes heavier.
  The calculated number of moles of crystal will then be inaccurate.
• Some solution maybe lost when it is transferred by a pipette.
  This may affect the experimental result.
• There may not be enough sulphuric acid added during the titrations.
  Manganate(VII) ions may reduced to manganese(IV) oxide, which is a brown solid.
  This many affect the calculated results.

Suggestion for the improvement

• Ammonium iron(II) sulphate crystal should be use instead of iron(II) sulphate crystal as
  it is more stable than in air and would not absorb moisture in air.
• When transferring solution, special attention should be paid to ensure no solution is lost during the process.
• Add more sulphuric acid during the titrations to ensure manganate(VII) ions undergoes the expected reduction.

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Questions

1. Iron(II) is oxidized.                                 
   As in acidic medium, manganate(VII) acts as an oxidizing agent and iron(II) acts as a reducing agent.

Fe2+(aq)   →  Fe3+(aq)   +  e-

2. Because during titration, manganate(VII) is reduced to manganese ions.

MnO4-(aq)   +  8H+(aq)   +  5e-   →   Mn2+(aq)   +  4H2O(l)

3. As potassium manganate(VII) solution itself is coloured, it serves as an indicator on its own. When it is reduced during titration, its colour changes from purple to pink, which is sharp enough to be observed.                
   However, for the previous experiment, sulphuric acid and the titrant are both colourless before and after the reaction. Thus, an indicator, for example, methyl orange, should be added to show the end point of the titration.

4a.  Apparatus used: a beaker, glass rod, 250.0 cm3 of volumetric flask and filter funnel.

     Steps:

1. Dissolve the crystal with about 100 cm3 of 1 M sulphuric acid in a beaker.
2. Stir the solution with a glass rod until all the crystal is dissolved.
3. Pour the solution into a 250.0 cm3 of volumetric flask through a filter funnel.
4. Rinse the beaker and glass rod with about 30 cm3 of 1 M sulphuric acid.
5. Pour the washing though the funnel into the volumetric flask.
6. Add 1 M sulphuric acid upto the graduated mark of the volumetric flask.
7. Stopper the flask and invert it several times in order to mix the contents well.
8. Label the flask as ‘Standard’.

4b.  Apparatus used: a pipette, pipette filler and 250.0 cm3 of volumetric flask.

     Steps:

1. Wash a 25.0 cm3 pipette with distilled water and then with manganate(VII) solution.
2. Pipette 25.0 cm3 of manganate(VII) solution into a 250.0 cm3 of volumetric flask.
3. Repeat step(b) again.
4. Add deionized water upto the graduated mark of the volumetric flask.
5. Stopper the flask and invert it several times in order to mix the contents well.
6. Label the flask as ‘Titrant’.