SCH4C                       Lab #23 - Redox Titration
Introduction:  In practise, a redox tittration is very similar to an acid-base titration.  Solutions  of an oxidizing agent o reducing agent are combined to reach an end point.  One of the solutions is of known concentration and the other is unknown.  The objective of the titration being to determine the concentation of the unknown, or the number of moles present, or the percentage of a particular substance within a mixture.
     In this activity, you will be given a sample containing iron in the form of iron(II) ions, and the objective will be to determine the percentage of ion in the sample.  A common oxiding agent, potassium permanganate, KMnO4, will be the other reactant.  In acid solution, the permanganate ion is reduced to Mn2+ according to the equation. 

8 H+1  +  MnO4-1  +  5 Fe2+   ---->  Mn2+  +  5 Fe3+  + 4 H2O

The iron(II) is oxidized to iron(III).  Notice the 1-to-5 ratio between the permanganate and the Fe2+.  A major advantage of using potassium permanganate is its strong purple colour, whereas the Mn2+ ion are nearly colourless.  Even with very dilute, aqueous potassium permanganate ions are noticeably pink, and so as soon as all the Fe+2 ions have reacted , one or two drops of potassium permanagante will result in a persistent pink colour.

Problem:   What is the percentage of iron in an unknown sample?
Apparatus:   safety goggles, burette, burette clamp, beakers, Erlenmeyer flask, waste beaker, retort stand, filter paper, balance
Materials:  1 M H2SO4, 85% H3PO4, KMnO4(aq)(standard), Unonwn iron sample
 
Procedure:
1.  Your teacher will provide you with the solid to be tested.  Transfer about 1 gram to a piece filter paper, and determine the combined mass as accurately as possible.  Carefully add the iron sample to a clean 250 mL Erlenmeyer flask, and determine the mass of the filter paper alone.  The difference is, of course, the mass of the solid that went into the flask.
2. Clean a 50 mL buret and set it up for titration.  Obtain about 100 mL of standard KMnO4 solution in a clean dry beaker.  Record its concentration in your data table.
3. Use about 10 mL of the KMnO4 solution to rinse the buret; do this at least twice.  Fill the buret and remove any air bubbles from the tip.  Record the buret reading.  This is your initial reading for sample #1.
4. To the unknown sample #1, add 50 mL of 1 M H2SO4, and swirl the flask to dissolve the solid completely.  Add 4 or 5 mL of 85% H3PO4 and titrate immediately with the permanganate solution.   At the end point, the faint pink colour should persist for about 30 seconds.  Record the final buret reading in the data table.
5. Repeat Steps 1 to 4 for two more samples of unknown.   Do not dissolve the next samples until you are completley done with the first titration.

Data Table:
 

Mass of Sample Plus Paper Mass of paper alone Calculated Sample Mass Final Buret Reading Initial Buret Reading Volume of KMnO4 used
Sample #1            
Sample #2            
Sample #3            

Molarity of the standard KMnO4 solution: _________

Sample #1:
Moles of KMnO4 used: __________

Moles of Iron(II) ______________    Grams of Iron(II):____________

Percentage of Iron in Sample: ________
 

Sample #2:
Moles of KMnO4 used: __________

Moles of Iron(II) ______________    Grams of Iron(II):____________

Percentage of Iron in Sample: ________

Sample #3:
Moles of KMnO4 used: __________

Moles of Iron(II) ______________    Grams of Iron(II):____________

Percentage of Iron in Sample: ________
 

Calculations:
Molarity = moles X litres, calcualte the Moles of KMnO4 solution used in each titration.
From the stoichiometrically balanced equation we can see that MnO4-1 reacts with Fe2+ in a 1-to-5 ratio.  Calculate the moles of iron(II) for each sample, convert to grams, and calculate the percentage of iron for each sample.
For your conclusion, average your results and report your percentage answer.