Standardization of KMnO₄ solution by titrating against a standard solution of either: Oxalic acid or Ferrous ammonium sulphate (Mohr’s salt)

Introduction

The standardization of potassium permanganate (KMnO₄) solution is a fundamental analytical chemistry experiment widely performed in chemistry laboratories across educational institutions. This redox titration process helps determine the exact concentration of KMnO₄ solution by titrating it against known standard solutions. The two most commonly used standard solutions are oxalic acid (H₂C₂O₄·2H₂O) and ferrous ammonium sulfate (Mohr's salt). This experiment is crucial for students pursuing chemistry and pharmacy courses as it demonstrates practical applications of volumetric analysis.

Aim

To determine the exact concentration (molarity) of a given potassium permanganate solution by titrating it against:

• Standard oxalic acid solution, or
• Standard ferrous ammonium sulfate solution

Apparatus Required

Glassware and Instruments:

• Burette (50 mL) - for dispensing KMnO₄ solution
• Pipette (25 mL) - for measuring standard solution
• Conical flask (250 mL) - for titration
• Beaker (100 mL) - for temporary storage
• Funnel - for filling burette
• Measuring cylinder (100 mL) - for measuring dilution volumes
• Burette stand - for supporting burette
• White tile - for better color observation
• Wash bottle - for rinsing

Chemicals Required:

• Potassium permanganate solution (KMnO₄) - approximately 0.02 M
• Oxalic acid (H₂C₂O₄·2H₂O) or Ferrous ammonium sulfate [(NH₄)₂Fe(SO₄)₂·6H₂O]
• Dilute sulfuric acid (H₂SO₄) - 2N
• Distilled water

Theory

Chemical Principles

Redox Titration: This experiment is based on redox reactions where KMnO₄ acts as a powerful oxidizing agent in acidic medium.

Reaction with Oxalic Acid:

2KMnO₄ + 3H₂SO₄ + 5H₂C₂O₄ → K₂SO₄ + 2MnSO₄ + 10CO₂ + 8H₂O

Half-reactions:

• Reduction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
• Oxidation: C₂O₄²⁻ → 2CO₂ + 2e⁻

Reaction with Ferrous Ammonium Sulfate:

10FeSO₄·(NH₄)₂SO₄·6H₂O + 8H₂SO₄ + 2KMnO₄ → 5Fe₂(SO₄)₃ + 2MnSO₄ + K₂SO₄ + 10(NH₄)HSO₄ + 68H₂O

Half-reactions:

• Reduction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
• Oxidation: Fe²⁺ → Fe³⁺ + e⁻

Self-Indicator Property

KMnO₄ solution acts as its own indicator. The endpoint is indicated by:

• Appearance of permanent pink color that persists for 30 seconds
• This pink color is due to excess MnO₄⁻ ions

Normality Calculations:

For Oxalic Acid Solution:

• Equivalent weight of H₂C₂O₄·2H₂O = 126/2 = 63 g equiv⁻¹

For Ferrous Ammonium Sulfate Solution:

• Equivalent weight of Fe²⁺ = 56 g equiv⁻¹

Procedure

Preparation of Standard Solutions:

Standard Oxalic Acid Solution (0.05N):

1. Weigh exactly 0.63 g of oxalic acid crystals
2. Transfer to a 100 mL volumetric flask
3. Dissolve in distilled water and make up to the mark
4. This gives 0.05N oxalic acid solution

Standard Ferrous Ammonium Sulfate Solution (0.05N):

1. Weigh exactly 1.96 g of Mohr's salt
2. Transfer to a 100 mL volumetric flask
3. Add 5 mL of dilute H₂SO₄ and dissolve
4. Make up to the mark with distilled water

Titration Procedure:

1. Filling Burette: Clean and rinse burette with KMnO₄ solution. Fill with KMnO₄ and remove air bubbles.

2. Preparing Conical Flask:

   • Pipette out 25 mL of standard solution into a conical flask
   • Add 10 mL of dilute H₂SO₄
   • Heat the solution to 60-70°C (for oxalic acid titration only)

3. Titration Process:

   • Place conical flask on white tile under burette
   • Add KMnO₄ solution slowly from burette in small increments
   • Swirl constantly to mix the solutions
   • Continue until permanent pink color appears

4. Reading: Note the initial and final burette readings.

5. Repeat: Perform the titration 3-4 times for concordant values.

Observation Table

Calculations

For Oxalic Acid Standardization:

N₁V₁ (KMnO₄) = N₂V₂ (Oxalic acid)
N₁ × 22.37 = 0.05 × 25
N₁ = (0.05 × 25)/22.37 = 0.0559 N

Molarity of KMnO₄ = Normality/5 = 0.0559/5 = 0.0112 M

For Ferrous Ammonium Sulfate Standardization:

N₁V₁ (KMnO₄) = N₂V₂ (FAS)
N₁ × 22.37 = 0.05 × 25
N₁ = (0.05 × 25)/22.37 = 0.0559 N

Molarity of KMnO₄ = Normality/5 = 0.0112 M

Result

The exact concentration of the given potassium permanganate solution is determined as:

• Normality: 0.0559 N
• Molarity: 0.0112 M

Precautions

1. Cleanliness: Ensure all glassware is thoroughly cleaned with distilled water
2. Heating: Heat oxalic acid solution to 60-70°C only; excessive heating converts oxalic acid to carbonate
3. Acid Addition: Always add dilute H₂SO₄ before titration; never use HCl or HNO₃
4. Slow Addition: Add KMnO₄ dropwise near endpoint to avoid overshooting
5. Endpoint Recognition: Brown color indicates incomplete reaction; continue titration
6. Burette Reading: Take readings at eye level to avoid parallax error
7. Temperature Control: Maintain appropriate temperature throughout the experiment
8. Proper Mixing: Continuous swirling ensures complete reaction

Viva Questions and Answers

Q1: Why is dilute sulfuric acid added in this titration?

A: Dilute H₂SO₄ provides acidic medium which is essential for the redox reaction. H⁺ ions are required for MnO₄⁻ to be reduced to Mn²⁺. Also, if HCl is used, it will be oxidized by KMnO₄, giving inaccurate results.

Q2: Why KMnO₄ cannot be used as a primary standard?

A: KMnO₄ is not a primary standard because:

• It is not available in pure form
• It is a powerful oxidizing agent that oxidizes organic matter in glassware
• It decomposes on exposure to light and heat

Q3: What is the role of temperature in oxalic acid titration?

A: The oxalic acid solution is heated to 60-70°C because:

• The reaction is slow at room temperature
• Heating increases the reaction rate
• But overheating must be avoided as it may decompose oxalic acid

Q4: Why does the pink color appear slowly in the beginning and rapidly near endpoint?

A: Initially, Fe²⁺ or C₂O₄²⁻ ions are present in excess. As these ions are oxidized, MnO₄⁻ ions are reduced to colorless Mn²⁺. Near endpoint, all reducing agents are consumed, so excess MnO₄⁻ gives instant pink color.

Q5: Why can't we use HCl or HNO₃ instead of H₂SO₄?

A:

• HCl: Gets oxidized by KMnO₄ to Cl₂, consuming extra KMnO₄
• HNO₃: It is itself an oxidizing agent and may interfere with the reaction

Q6: What is the significance of equivalent weight in this titration?

A: Equivalent weight helps in calculating normality and determining the amount of substance that will react with one equivalent of another substance, making calculations easier in volumetric analysis.

Q7: What is meant by concordant readings?

A: Concordant readings are those titration values that differ by 0.1 mL or less. These values are considered for calculating mean titre value.

Q8: Why should the burette not be rinsed with KMnO₄ before filling?

A: Actually, burette should be rinsed with KMnO₄ solution to remove any water or impurities. This ensures accurate concentration of KMnO₄ in the burette.

Conclusion

The standardization of KMnO₄ solution is a classic example of redox titration that demonstrates fundamental principles of analytical chemistry. This experiment is invaluable for developing skills in:

• Accurate measurements using volumetric glassware
• Understanding redox reactions and their applications
• Endpoint detection using self-indicators
• Calculation skills in volumetric analysis

The experiment's success depends on careful execution of procedures, proper temperature control, and accurate observation of color changes. This titration method finds applications in various industries including pharmaceuticals, water treatment, and quality control laboratories for determining the concentration of reducing agents.