CH 104: TITRATIONS WITH PERMANGANATE

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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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A titrant is a solution of reagent that reacts with the analyte. The concentration of this reagent is accurately and precisely known.

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In a titration, incremental volumes of titrant are added to the analyte until the reaction is complete.

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A buret is often used to measure the volume of titrant added to the analyte.

REQUIREMENTS OF A TITRATION 1. The reaction must be stoichiometric. For example, the net ionic equation for the reaction of potassium permanganate (KMnO 4 ) and sodium oxalate (Na 2 C 2 O 4 ) is quantitative. Exactly 2 moles of KMnO 4 react with exactly 5 moles of Na 2 C 2 O 4 .

2MnO 4 – (aq) + 16H + (aq) + 5C 2 O 4 2 – (aq) → 2Mn 2+ (aq) + 8H 2 O (l) + 10CO 2(g) 2. The reaction should be rapid.

3. The reaction should be specific; that is, there should be no competing reactions. Systematic error caused by interferences must be eliminated or reduced.

REQUIREMENTS OF A TITRATION 4. There should be a marked change when the reaction is complete. For example, this reaction is self-indicating. The titrant (KMnO 4 ) is deep purple. The analyte (Na 2 C 2 O 4 ) and products (Mn 2+ , H 2 O, and CO 2 ) are nearly colorless. The titration is done when the first fraction of a drop of excess MnO 4 – changes the solution from nearly colorless to a faint and stable pink.

EQUIVALENCE POINT, END POINT, AND INDICATORS

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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.

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The end point occurs when the indicator changes color.

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We want to measure the equivalence point. We actually measure the end point.

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Obviously, the faint pink MnO 4 – 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 MnO 4 – 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.

EQUIVALENCE POINT, END POINT, AND INDICATORS Titration using Permanganate as a Self-Indicator

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When do you stop adding titrant to the analyte?

At the end point.

STANDARDIZATION

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Today we will standardize KMnO 4 against Na 2 C 2 O 4 .

2MnO 4 – (aq) + 16H + (aq) + 5C 2 O 4 2 – (aq) → 2Mn 2+ (aq) + 8H 2 O (l) + 10CO 2(g)

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This is an oxidation-reduction reaction. That is, electrons are transferred from 1 reactant to another reactant.

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Oxidation is a loss of an electron or electrons by an atom or group of atoms.

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Reduction is a gain of an electron or electrons by an atom or group of atoms.

STANDARDIZATION

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Today we will standardize KMnO 4 against Na 2 C 2 O 4 .

2MnO 4 – (aq) + 16H + (aq) + 5C 2 O 4 2 – (aq) → 2Mn 2+ (aq) + 8H 2 O (l) + 10CO 2(g)

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In this reaction, C 2 O 4 2 – reduced to Mn 2+ .

is oxidized to CO 2 , and MnO 4 – to

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What is the Lewis structure for C 2 O 4 2 – ?

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What is the Lewis structure for CO 2 ?

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Why is the C of CO 2 more oxidized than the C of C 2 O 4 2 – ?

The C in CO 2 is in the +4 oxidation state [(1 x +4) + (2 x –2) = 0]. Each C in C 2 O 4 2 – 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.

STANDARDIZATION

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Today we will standardize KMnO 4 against Na 2 C 2 O 4 .

2MnO 4 – (aq) + 16H + (aq) + 5C 2 O 4 2 – (aq) → 2Mn 2+ (aq) + 8H 2 O (l) + 10CO 2(g)

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What is the oxidation state of Mn in MnO 4 – ?

The +7 oxidation state [(1 x +7) + (4 x –2) = –1].

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What is the oxidation state of Mn 2+ ?

The +2 oxidation state [(1 x +2) = +2].

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Why is the Mn of Mn Mn 2+ 2+ more reduced than the Mn of MnO 4 – ?

is in the +2 oxidation state. The Mn of MnO 4 – is in the +7 oxidation state. Therefore, Mn gained 5 electrons and was reduced during this reaction.

STANDARDIZATION

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Na 2 C 2 O 4 is a primary standard.

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For example, a solution was made by dissolving 1.095 g of Na 2 C 2 O 4 Na 2 C 2 O 4 in 100.0 mL of distilled water. The molar mass of is 134.0 g/mol. A 25.00 mL sample of this Na 2 C 2 O 4 solution was titrated with 39.58 mL of a KMnO 4 solution to a self-indicating end point. What is the molarity (M) of this KMnO 4 solution?

REQUIREMENTS OF A PRIMARY STANDARD 1. A primary standard should be 100.00% pure; although a 0.01% to 0.02% impurity is tolerable if it is accurately known.

2. 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.) 3. It should be readily available.

4. It should have a relatively large formula weight. Therefore, a relatively large mass of it will be weighed for titration. This will reduce error.

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Explain this last point.

OXIDATION –REDUCTION TITRATION WITH PERMANGANATE

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After the KMnO 4 is standardized, we will measure the Fe(II) content of an unknown salt.

MnO 4 – (aq) + 8H + (aq) + 5Fe 2+ (aq) → Mn 2+ (aq) + 4H 2 O (l) + 5Fe 3+ (aq)

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What is oxidized?

The Fe is oxidized from +2 to +3.

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What is reduced?

The Mn is reduced from +7 to +2.

SAFETY

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Give at least 1 safety concern for the following procedure.

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Using oxidizing agents (KMnO 4 ), reducing agents (Na 2 C 2 O 4 and unknown Fe(II) salt), and acids (H 2 SO 4 and H 3 PO 4 ).

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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.

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Your laboratory manual has an extensive list of safety procedures. Read and understand this section.

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Ask your instructor if you ever have any questions about safety.

SOURCES

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Christian, G.D. 1986. Analytical Chemistry, 3rd ed. New York, NY: John Wiley & Sons, Inc.

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Harris, D.C. 1999. Quantitative Chemical Analysis, 5th ed. New York, NY: W.H. Freeman Company.

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McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall.

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Petrucci, R.H. 1985. General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company.