Introduction
This titration involves the reaction between acidified potassium permanganate (KMnO₄) and iron(II) ions (Fe²⁺). It is a standard redox titration in NECTA Chemistry practicals and is used to determine the concentration of Fe²⁺ in a solution.
🔹 Reaction Principle
KMnO₄ acts as a strong oxidizing agent in acidic medium and oxidizes Fe²⁺ to Fe³⁺. KMnO₄ is self-indicating due to its purple color, which fades upon reduction.
Balanced Ionic Equation:
🔹 Theory
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KMnO₄ is purple; reduced to Mn²⁺ (colorless/pale pink)
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Fe²⁺ is oxidized to Fe³⁺ (colorless in dilute acid)
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H₂SO₄ is used to acidify the solution (not HCl, to avoid interference)
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The endpoint is a faint pink color persisting after the final drop
🔹 Apparatus and Chemicals
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Burette, pipette, conical flask, white tile
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KMnO₄ solution (in burette)
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FeSO₄ or Fe²⁺ solution (in flask)
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Dilute H₂SO₄ (for acidification)
🔹 Procedure
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Rinse burette with KMnO₄, fill, and record initial volume
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Pipette 25.00 mL of Fe²⁺ solution into a conical flask
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Add ~10 mL of dilute H₂SO₄ to acidify the solution
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Titrate by adding KMnO₄ slowly with constant swirling
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Stop when the pale pink color persists for 30 seconds
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Record final burette reading and calculate volume used
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Repeat until concordant results are obtained (within ±0.10 mL)
🧠 Calculation Example
Given:
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25.00 mL of Fe²⁺ is titrated
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Average volume of 0.0200 M KMnO₄ used = 22.60 mL
Balanced equation mole ratio:1 MnO₄⁻ : 5 Fe²⁺
Step 1: Calculate moles of KMnO₄ used
Step 2: Moles of Fe²⁺ (from mole ratio)
Step 3: Concentration of Fe²⁺
✅ Answer: Concentration of Fe²⁺ = 0.0904 mol/dm³
🔹 Observations and Color Change
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Before endpoint: Solution is colorless (Fe²⁺ only)
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Endpoint: First permanent pale pink color
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After endpoint: Pale pink becomes darker if KMnO₄ is in excess
✅ Summary
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KMnO₄ oxidizes Fe²⁺ in acidic solution
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Reaction ratio is 1:5 (MnO₄⁻ : Fe²⁺)
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KMnO₄ is self-indicating
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Use only H₂SO₄ for acidification
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Observe and record results to 2 decimal places for NECTA