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

• 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.

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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.

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REQUIREMENTS OF A TITRATION

1. 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.

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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.

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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.

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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.

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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.

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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.

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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?

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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.

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OXIDATIONREDUCTION 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.

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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.

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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.