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CH 104: TITRATIONS WITH PERMANGANATE

CH 104: TITRATIONS WITH PERMANGANATE. An analyte is the substance that is being analyzed. For example, the concentration of glucose in blood is commonly analyzed by diabetics. Glucose is the analyte.

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CH 104: TITRATIONS WITH PERMANGANATE

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  1. CH 104: TITRATIONS WITH PERMANGANATE • An analyte is the substance that is being analyzed. For example, the concentration of glucose in blood is commonly analyzed by diabetics. Glucose is the analyte. • A titrant is a solution of reagent that reacts with the analyte. The concentration of this reagent is accurately and precisely known. • In a titration, incremental volumes of titrant are added to the analyte until the reaction is complete. • A buret is often used to measure the volume of titrant added to the analyte.

  2. REQUIREMENTS OF A TITRATION • The reaction must be stoichiometric. For example, the net ionic equation for the reaction of potassium permanganate (KMnO4) and sodium oxalate (Na2C2O4) is quantitative. Exactly 2 moles of KMnO4 react with exactly 5 moles of Na2C2O4. • 2MnO4–(aq)+ 16H+(aq)+ 5C2O42–(aq) → 2Mn2+(aq)+ 8H2O(l)+ 10CO2(g) • The reaction should be rapid. • The reaction should be specific; that is, there should be no competing reactions. Systematic error caused by interferences must be eliminated or reduced.

  3. REQUIREMENTS OF A TITRATION • There should be a marked change when the reaction is complete. For example, this reaction is self-indicating. The titrant (KMnO4) is deep purple. The analyte (Na2C2O4) and products (Mn2+, H2O, and CO2) are nearly colorless. The titration is done when the first fraction of a drop of excess MnO4– changes the solution from nearly colorless to a faint and stable pink.

  4. EQUIVALENCE POINT, END POINT, AND INDICATORS • The equivalence point occurs when the volume of titrant added to the analyte is the exact stoichiometric amount that is needed to bring the reaction to completion. • The end point occurs when the indicator changes color. • We want to measure the equivalence point. We actually measure the end point. • Obviously, the faint pink MnO4– end point does not occur at the equivalence point. This end point occurs a fraction of a drop after the equivalence point. This error is small and can be corrected with a blank, or during standardization. • How would you use a blank to correct this error? • The volume of MnO4– used to reach the end point during the titration of distilled water (a blank) is subtracted from all standards and all samples. • How would you standardize to correct this error? • All standards and all samples are titrated to the same end point. We will do this today.

  5. EQUIVALENCE POINT, END POINT, AND INDICATORS • Titration using Permanganate as a Self-Indicator • When do you stop adding titrant to the analyte? • At the end point.

  6. STANDARDIZATION • Today we will standardize KMnO4 against Na2C2O4. • 2MnO4–(aq)+ 16H+(aq)+ 5C2O42–(aq) → 2Mn2+(aq)+ 8H2O(l)+ 10CO2(g) • This is an oxidation-reduction reaction. That is, electrons are transferred from 1 reactant to another reactant. • Oxidation is a loss of an electron or electrons by an atom or group of atoms. • Reduction is a gain of an electron or electrons by an atom or group of atoms.

  7. STANDARDIZATION • Today we will standardize KMnO4 against Na2C2O4. • 2MnO4–(aq)+ 16H+(aq)+ 5C2O42–(aq) → 2Mn2+(aq)+ 8H2O(l)+ 10CO2(g) • In this reaction, C2O42– is oxidized to CO2, and MnO4– to reduced to Mn2+. • What is the Lewis structure for C2O42–? • What is the Lewis structure for CO2? • Why is the C of CO2 more oxidized than the C of C2O42–? • The C in CO2 is in the +4 oxidation state • [(1 x +4) + (2 x –2) = 0]. Each C in C2O42– is in the +3 oxidation state [(2 x +3) + (4 x –2) = –2].Therefore, each C lost 1 electron and was oxidized during this reaction.

  8. STANDARDIZATION • Today we will standardize KMnO4 against Na2C2O4. • 2MnO4–(aq)+ 16H+(aq)+ 5C2O42–(aq) → 2Mn2+(aq)+ 8H2O(l)+ 10CO2(g) • What is the oxidation state of Mn in MnO4–? • The +7 oxidation state [(1 x +7) + (4 x –2) = –1]. • What is the oxidation state of Mn2+? • The +2 oxidation state [(1 x +2) = +2]. • Why is the Mn of Mn2+ more reduced than the Mn of MnO4–? • Mn2+ is in the +2 oxidation state. The Mn of MnO4– is in the +7 oxidation state. Therefore, Mn gained 5 electrons and was reduced during this reaction.

  9. STANDARDIZATION • Na2C2O4 is a primary standard. • For example, a solution was made by dissolving 1.095 g of Na2C2O4 in 100.0 mL of distilled water. The molar mass of Na2C2O4 is 134.0 g/mol. A 25.00 mL sample of this Na2C2O4 solution was titrated with 39.58 mL of a KMnO4 solution to a self-indicating end point. What is the molarity (M) of this KMnO4 solution?

  10. REQUIREMENTS OF A PRIMARY STANDARD • A primary standard should be 100.00% pure; although a 0.01% to 0.02% impurity is tolerable if it is accurately known. • A primary standard should be stable at drying temperatures, and it should be stable indefinitely at room temperature. (A primary standard is always dried before weighing, unless it is a hydrate.) • It should be readily available. • It should have a relatively large formula weight. Therefore, a relatively large mass of it will be weighed for titration. This will reduce error. • Explain this last point.

  11. OXIDATION–REDUCTION TITRATION WITH PERMANGANATE • After the KMnO4 is standardized, we will measure the Fe(II) content of an unknown salt. • MnO4–(aq) + 8H+(aq) + 5Fe2+(aq) → Mn2+(aq) + 4H2O(l) + 5Fe3+(aq) • What is oxidized? • The Fe is oxidized from +2 to +3. • What is reduced? • The Mn is reduced from +7 to +2.

  12. SAFETY • Give at least 1 safety concern for the following procedure. • Using oxidizing agents (KMnO4), reducing agents (Na2C2O4 and unknown Fe(II) salt), and acids (H2SO4 and H3PO4). • These are irritants. Wear your goggles at all times. Immediately clean all spills. If you do get either of these in your eye, immediately flush with water. • Your laboratory manual has an extensive list of safety procedures. Read and understand this section. • Ask your instructor if you ever have any questions about safety.

  13. SOURCES • Christian, G.D. 1986. Analytical Chemistry, 3rd ed. New York, NY: John Wiley & Sons, Inc. • Harris, D.C. 1999. Quantitative Chemical Analysis, 5th ed. New York, NY: W.H. Freeman Company. • McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall. • Petrucci, R.H. 1985. General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company.

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