PerManganoMeTry

Experiment No.___________________ Date ___________________ OXIDATION-REDUCTION TITRATIONS-Permanganometry INTRODUCTION Potassium permanganate, KMnO4, is probably the most widely used of all volumetric oxidizing agents. It is a powerful oxidant and readily available at modest cost. The intense color of the permanganate ion, MnO4-, is sufficient to detect the end point in most titrations. Depending upon reaction conditions permanganate ion is reduced to manganese in the 2+, 3+, 4+ or 6+ state. In solutions that are 0.1 M or greater in mineral acid the common reduction product is manganese (II) ion MnO4- + 8H+ + 5e- ↔ Mn2+ + 4H2O E0 = 1.51 V This is the most widely used of the permanganate reactions. In solutions that are weakly acidic (above pH 4) neutral, or weakly alkaline manganese dioxide is the most common reduction product MnO4- + 4H+ + 3e- ↔ MnO2(s) + 2H2O E0 = 1.70 V Titration in which manganese dioxide is the product suffer from the disadvantage that the slightly soluble brown oxide obscures the end point; time must be allowed for the solid to settle before an excess of the permanganate can be detected. Some important volumetric analyses based on permanganate involve reduction to manganese ion according to the half reaction given below; MnO4- + e- ↔ MnO42- E0 = 0.56 V This stoichiometry tends to predominate in solutions that are greater than 1 M in sodium hydroxide. Alkaline oxidations with permanganate have proved to be most useful in the determination of organic compounds. 1 M AgNO3 (in droppers) Blue band filter paper (2 for each student) Glass wool buret Watch glass 250 mL conical flasks x2 ( for each student) 100 mL graduated cylinder PROCEDURE A) Preparation and Standardization of KMnO4 Solution 1) 2) Prepare 1. Store the solution in a clean. Pour into a 1. Remove MnO2 by filtration using glass wool.0 L of 0. SnCl2 (already prepared) Mercury (II) Chloride.020 M KMnO4 in distilled water for 4 students. H2SO4. H2O2 for unknowns Potassium permanganate. 25 mL of 6.0 L of 0.0 L beaker.  5) Particles of MnO2 should be removed since these particles catalyze further decomposition of the solution.7H2O. KMnO4: 1. HCl.4H2O. NH3. glass-stoppered amber bottle and keep in the dark when not in use. . 500 mL of 0. HgCl2 (already prepared) Preventive solution (or Zimmermann-Reinhart reagent) (already prepared) Sulfuric acid.0 M (for each student) Sulfuric acid. 25 mL of 6.H2O (solid in the balance room) Methyl orange (in droppers) Stannous chloride. Na2C2O4 (primary standard in desiccator) Ammonium oxalate.020 M (for 4 students) Sodium oxalate.  Distilled water may contain organic matter which will reduce MnO4-2 ion. Filter paper can not be used for filtering because permanganate ion reacts with it to form additional manganese dioxide. reducing agent  MnO  oxidation product (in neutral solution) 4 2 3) 4) Cover and let stand overnight. 250 mL of 3. (NH4)2C2O4. Keep the solution at a gentle boil for about 1 hour. Ca(NO3)2. The solution is heated in order to hasten the oxidation of this material and coagulate the colloidal precipitate of MnO2 which forms as a reduction product: MnO 2 . H2SO4.0 M (for 2 students) 0.75 M (for 2 students) Hydrochloric acid.REAGENTS AND APPARATUS                    FeSO4.0 M (for each student) Ammonia. B) Standardization of Permanganate Solution with Sodium Oxalate 1) 2) 3) Weigh 0. do not boil the solution. . 6) C) Determination of Iron 1) Take your unknown sample into a 250 mL Erlenmeyer flask and place it in a steam bath and evaporate to a volume of about 10 mL.3 g (0. The reaction is given below: 2Fe3  Sn2  2Fe2  Sn 4 4) 5) Add two drops of SnCl2 solution in excess after the color change is observed. add SnCl2 solution a drop at a time until color changes to light green.  Finely divided MnO2 will form along with Mn2+ if the KMnO4 is added too rapidly and will cause the solution to acquire a faint brown discoloration. The solution will probably show the yellow color of ferric ion at this stage.  Estimate the volume comparison with 10 mL of water in a similar beaker.0 mL mercuric chloride. Correct the titration data for the blank. The net reaction in the titration can be written as follows: 2MnO 42. The solution must be free of MnO2 at the end point.75 M H2SO4.2 to 0. Your partners will do the standardization experiment twice. Heat the solution to 80 to 90 C and titrate with KMnO4.0 mL of 0.1 mg) dry primary standard Na2C2O4 into 250 mL Erlenmeyer flasks. Reduction of stannous chloride must be carried out in a relatively concentrated solution in order the point at which reaction is complete can be determined by the change in color. Precipitate formation is not a serious problem as long as sufficient oxalate remain to reduce the MnO 2 to Mn2+. HgCl2 solution. the titration is simply discontinued until the brown color disappears. 3) While still hot. 5C 2 O42  16H   2Mn 2  10CO2  8H2O  Promptly wash any KMnO4 that spatters on the walls of the beaker into the bulk of the liquid with a stream of water. End point is permanent pink color (~30 s).75 M H2SO4. Dissolve the sodium oxalate in 75. 5) Determine a blank by titrating an equal volume of 0. 4) Repeat the titration with one additional sample. Record the results of all titrations (totally 8 titrations) and calculate the molarity of the KMnO 4 solution for eigth replicates and at the end calculate the average of molarity of KMnO4 solution. Cool the solution to room temperature and pour in rapidly 20. 2) Treat each unknown solution individually. The first drop added may be sufficient to reduce all Fe3+ present. If no precipitate appears.0 mL of mercuric chloride. there may be a local excess of stannous ion which will cause the reduction of HgCl2 to Hg rather than Hg2Cl2 mercurous chloride. The oxidation of chloride ion during a titration is believed to involve a direct reaction between this species and the manganese(II) ions that form as an intermediate in the reduction of permanganate ion by iron (II). The presence of manganese(II) in the Zimmermann-Reinhart reagent is belived to inhibit the formation of chlorine by decreasing the potential of the manganese(III)/manganese(II) couple. it means that not enough stannous chloride has been added. A grayish precipitate indicates reduction to mercury. Phosphate ion is believed to exert a similar effect by forming stable manganese(III) complexes. Moreover. The slightly soluble mercury (I) chloride (Hg2Cl2) does not reduce permanganate. which contains manganese (II) in a fairly concentrated mixture of sulfuric and phosphoric acid. The net reaction in the titration can be written as follow: 5Fe 2  MnO   8H   5Fe 3  Mn 2  4H O 4 2  8) 9) The reaction of iron(II) with permanganate is smooth and rapid. Sn  2 2  HgCl2  Hg(l)  Sn 4  2Cl  The precipitate should be small in amount and of a pure white color. Note the volume of permanganate needed to give the same color as the end point reached in the titrations. . The excess reducing agent is eliminated by addition HgCl2. induces the oxidation of chloride ion by permanganate. phosphate ions react with iron(III) to form nearly colorless complexes so that the yellow color of the iron(II)/chloro complexes doe not interfere with the end point. Its effects can be eliminated through removal of the hydrochloric acid by evaporation with sulfuric acid or by the addition of Zimmermann-Reinhart reagent. Elemental Hg(l) reacts with permanganate and causes the results of the analysis to be high. Allow to stand 2 minutes and then add 100 mL distilled water. then proceed with addition of 20. the sample is spoiled and should be discarded. a reaction that does not ordinarily proceed rapidly enough to cause serious error.0 mL preventive solution and titrate immediately with permanganate. Add 20. Sn  6) 7)  2HgCl2  Hg2Cl2 (s)  Sn 4  2Cl  If the reagent is not added all at once. If a small white precipitate is not obtained.0 mL of preventive solution and 100 mL of distilled water just in the titration of a sample. Determine a blank by adding two drops of stannous chloride to 100 mL of distilled water in a 250 mL Erlenmeyer flask. however. High results are obtained if this parasitic reaction is not controlled. 20. Subtract the blank from the total volume used to obtain the net volume for each portion of unknown sample. This reacts slowly with permanganate and will not give true titration values. nor does the excess mercury (II) chloride (HgCl2) reoxidize iron (II). The presence of iron (II) in the reaction mixture. then introduce 6.0 mL of distilled water and 50.0 M HCl. End point is the first pink color that persists for 2 minutes. Titrate with standardized KMnO4 solution with filter paper.H2O. 5) Add 3 drops of methyl orange indicator. 6) Allow the solution to stand half an hour. 5) The net reaction in the titration is   6H   5O  2Mn 2  8H O 5H2O2  2MnO4 2 2 E) Determination of Calcium 1) Take your unknown sample into a 250 mL Erlenmeyer flask and add 100 mL distilled water 2) Treat each unknown solution individually. CaC O (s)  2H 2 4   Ca 2 H C O 2 2 4 10) Heat to 80 to 90 C to dissolve the precipitate.10) Report the result as mg iron. Report the result as mg H2O2. 3) 4) Add 7. 7) Filter the solution through a blue band filter paper.0 mL of 3.0 M H2SO4.ions can be removed. 3) Add 10.0 M NH3 dropwise from pipette until the color changes from red to yellow. Add 50. 4) Heat to 60 C to 80 C and then add 3. The net reaction in the titration is given below: 2   2 5H C O  2MnO  6H  2Mn  10CO  8H O 2 2 4 2 2 4 11) Report the result as mg Ca. Titrate with standard KMnO4 solution.0 g of (NH4)2C2O4. After enough washings almost all Cl.0 mL of 6. 8) Wash the beaker and precipitate with 10 to 20 mL portions of distilled water until the washings show a faint cloudiness when tested with an acidified AgNO3 solution. The temperature of the solution should not be allowed to drop below 60 C. 9) Place the filter in which the precipitate was formed in a 250 mL Erlenmeyer flask. Treat each unknown solution individually. Take as an end point the first pink color that persists for 15 to 20 sec.0 mL of 3.0 M H2SO4. . D) Determination of Hydrogen Peroxide 1) 2) Take your unknown sample into a 250 mL Erlenmeyer flask and add 100 mL distilled water. 5) Why should the medium be acidic in KMnO4 standardization? 6) What is the aim of adding SnCl2 in Fe determination? Write the related reaction equations. 7) What are the function(s) of preventive solution? Explain.PRE-LAB STUDIES Read pages 510-531 from the textbook (9th Ed) 1) Why KMnO4 is preferred as an oxidizing agent in redox titrations? 2) What is the aim of heating KMnO4 during its preparation step? Explain and write the related reaction equations. 3) What is the importance of filtering KMnO4 solution and why do we use glass-wool instead of filter-paper? 4) Explain the importance of heating the solution to 80 to 90 C in the standardization of KMnO4. what does this indicate? Do you continue the titration in this situation or stop and repeat it? 3) Why do we perform blank analysis in KMnO4 standardization? 4) Why do we add HgCl2 and why do we add it quickly? Write the related reaction equations. 5) If you add HgCl2 slowly what will you observe and how does this affect your result? 6) Why should the pH of the medium be about 4 in Ca2+ determination? How do we adjust this pH? 7) Why do we use NH3 but not NaOH for pH adjustment in Ca2+ determination? . 8) Why an indirect method is applied for Ca2+ determination? POST-LAB STUDIES 1) Why do we perform the titrations slowly? 2) During standardization of KMnO4 if you observe a brown precipitate. Name surname: Section: Date: REPORT SHEET FOR PERMANGANOMETRY B. ̅  ?) mg (? True mass of iron. g Blank corrected volume of KMnO4. mg 1 2 The following information (true values) will be sent to your e-mail address: Concentration of iron in the unknown solution= Volume of iron unknown. M Concentration of KMnO4. mL Concentration of KMnO4. Surname Mass of Na2C2O4. mg Mean mass. mL Mass of iron. mL= TA`s Name and Signature: % Relative Error . M ̅  ?) (? 1 2 3 4 5 6 7 8 C. Standardization of Permanganate Solution with Sodium Oxalate Replicates Group Members: Name. Determination of Iron Replicates Blank corrected volume of KMnO4. ̅  ?) mg (? True mass of H2O2. mg Mean mass. mL= E. mL Mass of calcium. ̅  ?) mg (? True mass of Ca.D. mg % Relative Error 1 2 The following information (true values) will be sent to your e-mail address: Concentration of H2O2 in the unknown solution= Volume of H2O2 unknown. Determination of Calcium Replicates Volume of KMnO4. mL= TA`s Name and Signature: % Relative Error . Determination of Hydrogen Peroxide Replicates Volume of KMnO4. mL Mass of H2O2. mg 1 2 The following information (true values) will be sent to your e-mail address: Concentration of calcium in the unknown solution= Volume of calcium unknown. mg Mean mass.