OXIDATION-REDUCTION TITRATIONS-Permanganometry

Transcription

1 Experiment No. Date OXIDATION-REDUCTION TITRATIONS-Permanganometry INTRODUCTION Potassium permanganate, KMnO, 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, MnO -, is sufficient to detect the end point in most titrations. Depending upon reaction conditions permanganate ion is reduced to manganese in the +, 3+, + or 6+ state. In solutions that are 0.1 M or greater in mineral acid the common reduction product is manganese (II) ion MnO - + 8H + + 5e - Mn + + H O E 0 = 1.51 V This is the most widely used of the permanganate reactions. In solutions that are weakly acidic (above ph ) neutral, or weakly alkaline manganese dioxide is the most common reduction product MnO - + H + + 3e - MnO (s) + H O E 0 = 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; MnO - + e - MnO - E 0 = 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.

2 REAGENTS AND APPARATUS FeSO.7H O, Ca(NO 3).H O, H O for unknowns Potassium permanganate, KMnO : 1.0 L of 0.00 M (for students) Sodium oxalate, Na C O (primary standard in desiccator) Ammonium oxalate, (NH ) C O.H O (solid in the balance room) Methyl orange (in droppers) Stannous chloride, SnCl (already prepared) Mercury (II) Chloride, HgCl (already prepared) Preventive solution (or Zimmermann-Reinhart reagent) (already prepared) Sulfuric acid, H SO, 500 ml of 0.75 M (for students) Hydrochloric acid, HCl, 5 ml of 6.0 M (for each student) Ammonia, NH 3, 5 ml of 6.0 M (for each student) Sulfuric acid, H SO, 50 ml of 3.0 M (for students) 0.1 M AgNO 3 (in droppers) Blue band filter paper ( for each student) Glass wool buret Watch glass 50 ml conical flasks x ( for each student) 100 ml graduated cylinder PROCEDURE A) Preparation and Standardization of KMnO Solution 1) Prepare 1.0 L of 0.00 M KMnO in distilled water for students. Pour into a 1.0 L beaker. ) Keep the solution at a gentle boil for about 1 hour. Distilled water may contain organic matter which will reduce MnO - ion. The solution is heated in order to hasten the oxidation of this material and coagulate the colloidal precipitate of MnO which forms as a reduction product: MnO - reducing agent MnO oxidation product (in neutral solution) 3) Cover and let stand overnight. ) Remove MnO by filtration using glass wool. Particles of MnO should be removed since these particles catalyze further decomposition of the solution. Filter paper can not be used for filtering because permanganate ion reacts with it to form additional manganese dioxide. 5) Store the solution in a clean, glass-stoppered amber bottle and keep in the dark when not in use.

3 B) Standardization of Permanganate Solution with Sodium Oxalate 1) Weigh 0. to 0.3 g (0.1 mg) dry primary standard Na C O into 50 ml Erlenmeyer flasks. ) Dissolve the sodium oxalate in 75.0 ml of 0.75 M H SO. 3) Heat the solution to 80 to 90 C and titrate with KMnO, do not boil the solution. End point is permanent pink color (~30 s). The net reaction in the titration can be written as follows: - MnO 5CO 16H Mn 10CO 8HO Promptly wash any KMnO that spatters on the walls of the beaker into the bulk of the liquid with a stream of water. Finely divided MnO will form along with Mn + if the KMnO is added too rapidly and will cause the solution to acquire a faint brown discoloration. Precipitate formation is not a serious problem as long as sufficient oxalate remain to reduce the MnO to Mn + ; the titration is simply discontinued until the brown color disappears. The solution must be free of MnO at the end point. ) Repeat the titration with one additional sample. 5) Determine a blank by titrating an equal volume of 0.75 M H SO. Correct the titration data for the blank. 6) Your partners will do the standardization experiment twice. Record the results of all titrations (totally 8 titrations) and calculate the molarity of the KMnO solution for eigth replicates and at the end calculate the average of molarity of KMnO solution. C) Determination of Iron 1) Take your unknown sample into a 50 ml Erlenmeyer flask and place it in a steam bath and evaporate to a volume of about 10 ml. 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. 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. ) Treat each unknown solution individually. 3) While still hot, add SnCl solution a drop at a time until color changes to light green. The first drop added may be sufficient to reduce all Fe 3+ present. The reaction is given below: Fe 3 Sn Fe Sn ) Add two drops of SnCl solution in excess after the color change is observed. 5) Cool the solution to room temperature and pour in rapidly 0.0 ml mercuric chloride, HgCl solution.

4 The excess reducing agent is eliminated by addition HgCl. The slightly soluble mercury (I) chloride (Hg Cl ) does not reduce permanganate, nor does the excess mercury (II) chloride (HgCl ) reoxidize iron (II). Sn HgCl Hg Cl (s) Sn Cl If the reagent is not added all at once, there may be a local excess of stannous ion which will cause the reduction of HgCl to Hg rather than Hg Cl mercurous chloride. Elemental Hg(l) reacts with permanganate and causes the results of the analysis to be high. Sn HgCl Hg(l) Sn Cl The precipitate should be small in amount and of a pure white color. A grayish precipitate indicates reduction to mercury. This reacts slowly with permanganate and will not give true titration values. If no precipitate appears, it means that not enough stannous chloride has been added. If a small white precipitate is not obtained, the sample is spoiled and should be discarded. 6) Allow to stand minutes and then add 100 ml distilled water. 7) Add 0.0 ml preventive solution and titrate immediately with permanganate. The net reaction in the titration can be written as follow: 5Fe MnO 8H 5Fe 3 Mn H O The reaction of iron(ii) with permanganate is smooth and rapid. The presence of iron (II) in the reaction mixture, however, induces the oxidation of chloride ion by permanganate, a reaction that does not ordinarily proceed rapidly enough to cause serious error. High results are obtained if this parasitic reaction is not controlled. Its effects can be eliminated through removal of the hydrochloric acid by evaporation with sulfuric acid or by the addition of Zimmermann-Reinhart reagent, which contains manganese (II) in a fairly concentrated mixture of sulfuric and phosphoric acid. 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. Phosphate ion is believed to exert a similar effect by forming stable manganese(iii) complexes. Moreover, 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. 8) Determine a blank by adding two drops of stannous chloride to 100 ml of distilled water in a 50 ml Erlenmeyer flask; then proceed with addition of 0.0 ml of mercuric chloride, 0.0 ml of preventive solution and 100 ml of distilled water just in the titration of a sample. Note the volume of permanganate needed to give the same color as the end point reached in the titrations. 9) Subtract the blank from the total volume used to obtain the net volume for each portion of unknown sample.

5 10) Report the result as mg iron. D) Determination of Hydrogen Peroxide 1) Take your unknown sample into a 50 ml Erlenmeyer flask and add 100 ml distilled water. ) Treat each unknown solution individually. 1) Add 7.0 ml of 3.0 M H SO. 1) Titrate with standard KMnO solution. End point is the first pink color that persists for minutes. 1) Report the result as mg H O. The net reaction in the titration is 5H O MnO Mn 6H 5O 8H O PRE-LAB STUDIES Read pages from the textbook (9 th Ed) 1) Why KMnO is preferred as an oxidizing agent in redox titrations? ) What is the aim of heating KMnO during its preparation step? Explain and write the related reaction equations. 3) What is the importance of filtering KMnO solution and why do we use glass-wool instead of filter-paper? ) Explain the importance of heating the solution to 80 to 90 C in the standardization of KMnO. 5) Why should the medium be acidic in KMnO standardization? 6) What is the aim of adding SnCl in Fe determination? Write the related reaction equations. 7) What are the function(s) of preventive solution? Explain. 8) Why an indirect method is applied for Ca + determination? POST-LAB STUDIES 1) Why do we perform the titrations slowly? ) During standardization of KMnO if you observe a brown precipitate, 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 KMnO standardization? ) Why do we add HgCl and why do we add it quickly? Write the related reaction equations. 5) If you add HgCl slowly what will you observe and how does this affect your result? 6) Why should the ph of the medium be about in Ca + determination? How do we adjust this ph? 7) Why do we use NH 3 but not NaOH for ph adjustment in Ca + determination?

6 Name surname: Section: Date: REPORT SHEET FOR PERMANGANOMETRY B. Standardization of Permanganate Solution with Sodium Oxalate Replicates 1 Group Members: Name, Surname Mass of Na C O, g Blank corrected volume of KMnO, ml Concentration of KMnO, M Concentration of KMnO, M (X s) C. Determination of Iron Replicates Blank corrected volume of KMnO, ml Mass of iron, mg Mean mass, mg (X s) True mass of iron, mg % Relative Error 1 D. Determination of Hydrogen Peroxide Replicates Volume of KMnO, ml Mass of H O, mg Mean mass, mg (X s) True mass of H O, mg % Relative Error 1 The following information (true values) will be sent to your address: Concentration of H O in the unknown solution= Volume of H O unknown, ml= Concentration of iron in the unknown solution= Volume of iron unknown, ml= TA`s Name and Signature: