Redox Titration: Potassium Permanganate vs Mohr's Salt
What you will learn
Determine the molarity and strength of Potassium Permanganate (KMnO4) solution by titrating against a standard 0.05 M solution of Mohr's Salt [FeSO4·(NH4)2SO4·6H2O] in acidic medium.
Materials needed
- Standard 0.05 M Mohr's Salt Solution mLWhat is it?Standard 0.05 M Mohr's Salt Solution mL₹50
Mohr's Salt (Ferrous Ammonium Sulfate hexahydrate, FeSO4·(NH4)2SO4·6H2O) AR primary standard crystals.
Specifications- Purity
- ≥ 99.5% AR grade
- Formula Weight
- 392.14 g/mol
- Appearance
- Light blue-green crystalline powder
- Stability
- Resistant to atmospheric air oxidation
- Potassium Permanganate (KMnO4) Solution (~0.02 M) mLWhat is it?Potassium Permanganate (KMnO4) Solution (~0.02 M) mL₹95
Potassium Permanganate (KMnO4) volumetric analytical reagent solution.
Specifications- Concentration
- Approx 0.02 M / 0.01 M
- Purity
- AR Analytical Grade
- Appearance
- Deep purple / violet liquid
- Storage
- Amber glass bottle away from organic matter
- Dilute Sulfuric Acid (2 M H2SO4) mLWhat is it?Dilute Sulfuric Acid (2 M H2SO4) mL₹140
Class A 50 mL burette with white porcelain backing strip for easy meniscus reading.
Specifications- Capacity
- 50.0 mL
- Graduation
- 0.1 mL intervals
- Backing
- Schellbach blue stripe on white enamel
- Tolerance
- ± 0.05 mL
- 50 mL Burette with Burette StandWhat is it?50 mL Burette with Burette Stand₹185
Standard laboratory apparatus used in Redox Titration: Potassium Permanganate vs Mohr's Salt.
Specifications- Category
- Chemistry Apparatus
- Grade
- CBSE / NEP 2020 Laboratory Standard
- 20 mL Volumetric PipetteWhat is it?20 mL Volumetric Pipette₹230
Standard laboratory apparatus used in Redox Titration: Potassium Permanganate vs Mohr's Salt.
Specifications- Category
- Chemistry Apparatus
- Grade
- CBSE / NEP 2020 Laboratory Standard
- Conical Flask (250 mL) & White TileWhat is it?Conical Flask (250 mL) & White Tile₹275
Standard laboratory apparatus used in Redox Titration: Potassium Permanganate vs Mohr's Salt.
Specifications- Category
- Chemistry Apparatus
- Grade
- CBSE / NEP 2020 Laboratory Standard
- Distilled water wash bottleWhat is it?Distilled water wash bottle₹320
Standard laboratory apparatus used in Redox Titration: Potassium Permanganate vs Mohr's Salt.
Specifications- Category
- Chemistry Apparatus
- Grade
- CBSE / NEP 2020 Laboratory Standard
Tick items as you collect them. Tap + to see the product, specifications and buy options.
Stay safe
- For dark purple KMnO4, read the upper meniscus of the burette level at eye line.
- Always use dilute H2SO4; never use HCl (oxidized to chlorine) or HNO3 (competing oxidizing agent).
- Mohr's salt solution must be freshly prepared to avoid aerial oxidation of Fe2+ to Fe3+.
- Keep endpoint strictly to faint pink; dark purple indicates over-titration error.
Steps
- 1Rinse and fill the burette with the dark purple KMnO4 solution. Read the upper meniscus level.Observation
Prepare standard M/20 (0.05 M) Mohr's salt solution by dissolving 4.90 g in 250 mL volumetric flask with 5 mL dilute H2SO4.
SafetyHandle concentrated H2SO4 with utmost care; always add acid to water, never water to acid.
- 2Pipette out 20.0 mL of standard 0.05 M Mohr's salt solution into a clean conical flask.Observation
Rinse and fill burette with dark purple KMnO4 solution; take initial reading at UPPER meniscus.
SafetyKMnO4 stains skin and clothes dark brown (MnO2); clean any spills immediately.
- 3Add one full test tube (~20 mL) of 2 M dilute sulfuric acid into the conical flask to ensure acidic medium.Observation
Pipette 20.00 mL Mohr's salt solution into conical flask; add one full test tube (~20 mL) of 2M H2SO4.
SafetyNever use HCl or HNO3 as acid medium; HCl oxidizes to toxic Cl2 gas and HNO3 is an oxidizing agent.
- 4No external indicator is needed since KMnO4 is a self-indicator.Observation
Titrate at room temperature without heating; purple drops decolorize instantaneously upon swirling.
SafetySwirl continuously to ensure complete stoichiometric reduction of permanganate.
- 5Titrate by adding KMnO4 dropwise while swirling. The purple color discharges immediately upon contact with Fe2+.Observation
Endpoint is marked by appearance of permanent faint pink coloration persisting for 30 seconds.
SafetyDo not overshoot endpoint; stop when a single drop gives the faintest detectable pink hue.
- 6Continue titrating until a single drop gives a permanent, distinct pale pink color that persists for 30 seconds.Observation
Three concordant readings of 20.20 mL yield KMnO4 molarity = 0.0099 M (~0.01 M).
SafetyDispose of manganese waste in designated heavy-metal laboratory waste receptacle.
- 7Note the concordant volume of KMnO4 and apply the redox equation: 5 Fe2+ + MnO4- + 8 H+ -> 5 Fe3+ + Mn2+ + 4 H2O.
Tap + on a step to see what you will observe and any safety tips.
Record your observations
| Burette Trial | Initial Reading (mL) | Final Reading (mL) | Volume KMnO4 Used (mL) | Concordance |
|---|---|---|---|---|
| Rough Pilot | 0.00 mL | 20.60 mL | 20.60 mL | Pilot estimate |
| Trial 1 | 0.00 mL | 20.20 mL | 20.20 mL | Recorded |
| Trial 2 | 20.20 mL | 40.40 mL | 20.20 mL | Concordant |
| Trial 3 | 0.00 mL | 20.20 mL | 20.20 mL | Concordant (V = 20.20 mL) |
| Trial 4 | 20.20 mL | 40.40 mL | 20.20 mL | Concordant confirmed |
Tap a highlighted cell to write your own observation.
Result and conclusion
KMnO4 acts as its own self-indicator (Mn7+ reduced to colorless Mn2+). Concordant endpoint reached when excess drop produces a persistent faint pink color, yielding KMnO4 molarity of 0.020 M.
| Observation | What it means |
|---|---|
| Self-Indicator Action | KMnO4 acts as its own indicator; purple MnO4⁻ reduces to colorless Mn²⁺ until the first excess drop imparts permanent faint pink. |
| Upper Meniscus Reading | Because KMnO4 is darkly colored, readings are strictly taken from the upper meniscus edge for both initial and final levels. |
| Acid Medium Requirement | One test tube of dilute H2SO4 (approx 20 mL) is essential to prevent formation of brown MnO2 precipitate. |
| Concordant Volume Molarity | Concordant titrant volume 20.20 mL gives molarity of KMnO4 solution = M(Mohr)·V(Mohr) / (5 · V_KMnO4) = (0.05 × 20.0) / (5 × 20.20) = 0.0099 M (~0.01 M). |
Sample readings. Replace with your own results.