Determination of Iron by Reaction with Permanganate—A Redox Titration Potassium permanganate, KMnO4, is widely used as an oxidizing agent in volumetric analysis. In acid solution, MnO4- ion undergoes reduction to Mn2+ ion as shown in the equation: 8 H+(aq) + MnO4-(aq) + 5e- Mn2+(aq) + 4 H2O(l). Since the MnO4- ion is violet and the Mn2+ ion is nearly colorless, the end point in titrations using MnO4- as the titrant can be taken as the first permanent pink color that appears in the solution being titrated. KMnO4 will be employed in this experiment to determine the percentage of iron in an unknown containing iron(II). The titration, which involves the oxidation of Fe2+ ion to Fe3+ by permanganate ion, is carried out in sulfuric acid solution to prevent the air-oxidation of Fe2+. The end point of the titration is sharpened markedly if phosphoric acid is present because the Fe3+ ion produced in the titration forms an essentially colorless complex with this acid; thus, phosphoric acid is also added during the titration. The number of moles of potassium permanganate used in the titration is equal to the product of the molarity of the KMnO4 and the volume used. The number of moles of iron(II) present in the sample is obtained from the balanced equation for the reaction and the moles of permanganate used. The mass of iron in the unknown sample is then calculated from the number of moles and the atomic mass of iron and, finally, the corresponding percentage by weight of iron in the unknown is determined. Experimental Procedure 1. Obtain a buret from the stockroom and an unknown sample containing iron(II) from your instructor. 2. Weigh out three samples of your unknown, each containing the amount specified on the sample vial (± 0.1 g), into three separate clean 250-mL Erlenmeyer flasks. Record all of the digits from the balance. 3. Obtain about 100 mL of the standard KMnO4 solution in a clean dry beaker. 4. Clean your buret thoroughly as described by your instructor, rinse it with distilled water and then rinse it three times with a few milliliters of the KMnO4 solution. Drain the buret into a beaker (not into the sink!) and fill it with the KMnO4 solution. Record the volume in the buret to 0.01 mL. 5. Prepare 150 mL of 1 M H2SO4 by pouring 25 mL of 6 M H2SO4 into 125 mL of distilled water while stirring. 6. Add 50 mL of this 1 M H2SO4 solution to one of the three iron samples. The sample should dissolve completely. 7. Immediately titrate this iron solution with the KMnO4 solution. When a light yellow color develops in the iron solution during the titration, add 3 mL of 85% H3PO4. Caution: Caustic reagent. Continue the titration until you obtain the first pink color that persists for 15-30 seconds after swirling. Record the volume in the buret to 0.01 mL. 8. Repeat the titration with the other two samples. Be sure to start each titration with enough solution in the buret to complete the titration. Don’t forget the H2SO4. Dispose of your titrated solutions and any unused KMnO4 in the waste container. Name ______________________________________ Lab Day ____________ Determination of Iron by Permanganate Titration Data and Calculations Show calculations for items market with an asterisk (*). Molarity of standard KMnO4 solution _______________ Unknown number __________ Sample I Sample II Sample III Mass of sample Initial buret reading Final buret reading Volume KMnO4 used Moles KMnO4 used (*) Moles Fe2+ in sample (*) Mass Fe2+ in sample (*) Percentage Fe2+ in sample (*) Average Percentage of Fe in the sample Calculations (use back if needed): Name _____________________________ Lab Day ____________ Determination of Iron by Permanganate Titration Advance Study Assignment 1. Use the half reaction method to write the balanced net ionic equation for the reaction between MnO4- and Fe2+ ion in acid solution. The unbalanced equation is Fe2+(aq) + MnO4-(aq) Fe3+(aq) + Mn2+(aq). (aq) 2. How many moles of Fe2+ ion can be oxidized by 0.012 moles MnO4- ion in the reaction in Question 1? Show work. 3. A solid sample containing some Fe2+ ion weighs 1.923 g. It requires 36.44 mL of 0.0244 M KMnO4 to titrate the Fe2+ in the dissolved sample to a pink end point. Calculate the grams of iron in the sample and the percentage of iron in the sample. Show work with units. 4. What is the percentage of iron in iron(II) ammonium sulfate hexahydrate? Show work. Hint – this is obtained from the formula of the compound.