⚗️ Oxidizability (permanganate index)
Determination of water permanganate index (oxidizability) by oxidation with KMnO₄ in acidic medium, with reverse titration with sodium oxalate. Result in mg O₂/L.
Overview
Oxidizability (permanganate index, COD-Mn) is a sum parameter conventionally determining the content of organic substances and easily oxidizable inorganic substances in water. It is expressed in mg O₂/L—the amount of oxygen equivalent to the amount of KMnO₄ consumed for oxidizing reducing substances in the sample under specified conditions.
The PN-EN ISO 8467 standard describes a method for determining permanganate index for drinking water, mineral water, well water, and pool water. The method involves boiling the sample with excess KMnO₄ in sulfuric acid medium for 10 minutes, then determining remaining KMnO₄ by titration (reverse—excess oxalate is back-titrated with KMnO₄).
Permissible permanganate index value in drinking water: 5 mg O₂/L (Journal of Laws 2017 item 2294). Unpolluted waters have values of 1–3 mg O₂/L. It is a simple, inexpensive, and widely used test in drinking water analysis—performed in practically every water laboratory in Poland.
Method principle
The water sample is boiled in a water bath (100°C) for 10 minutes with a measured amount of potassium permanganate (KMnO₄) solution in sulfuric acid (H₂SO₄) medium. Part of the KMnO₄ is reduced (MnO₄⁻ → Mn²⁺) by oxidizable substances contained in the sample. After 10 minutes, excess sodium oxalate solution (Na₂C₂O₄) is added, which reduces remaining KMnO₄ (solution decolorizes). Then the excess oxalate is back-titrated with hot KMnO₄ solution to persistent light pink color. From the KMnO₄ balance, the permanganate index is calculated in mg O₂/L.
Applications
- Drinking water quality control (Journal of Laws 2017 item 2294—limit 5 mg O₂/L)
- Mineral, spring, and table water testing
- Groundwater and surface water intake monitoring
- Water treatment process efficiency control (filtration, activated carbon sorption)
- Pool water testing
- Rapid assessment of water organic pollution
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.5–10 mg O₂/L (higher after dilution) |
| Drinking water limit | 5 mg O₂/L |
| Reaction temperature | 100°C (boiling water bath) |
| Reaction time | 10 minutes (strictly) |
| Accuracy | ±0.5 mg O₂/L |
| Max chloride concentration | <300 mg/L Cl⁻ (no interference) |
Standard
- Standard number
- PN-EN ISO 8467:2001
- Title (PL)
- Jakość wody — Oznaczanie indeksu nadmanganianowego
- Title (EN)
- Water quality — Determination of permanganate index
Step-by-step procedure
1. Water bath preparation
Fill water bath and boil to 100°C. Maintain vigorous boiling throughout analysis.
2. Sample measurement
Measure 100 mL water sample with cylinder into 250 mL conical flask. For water with oxidizability >10 mg O₂/L—dilute.
3. Sulfuric acid addition
Add 5 mL H₂SO₄ (~4 mol/L) to sample. Mix.
4. Potassium permanganate addition
Add exactly 5.0 mL KMnO₄ 0.01 mol/L solution with pipette. Mix. Solution will turn pink/violet.
5. Boiling in water bath
Place flask in water bath (100°C). Start stopwatch. Boil for EXACTLY 10 minutes. Flask must be immersed to liquid level inside.
6. Sodium oxalate addition
After 10 minutes remove flask. Immediately add 5.0 mL Na₂C₂O₄ 0.01 mol/L solution. Solution will decolorize (oxalate reduces KMnO₄).
7. Excess oxalate titration
Hot solution (~60–80°C) titrate from burette with KMnO₄ 0.01 mol/L solution until persistent (min. 30 s) light pink color appears. Record V (mL KMnO₄).
8. Blank
Repeat entire procedure with 100 mL deionized water (blank). KMnO₄ consumption in blank: V₀.
9. Calculations
Permanganate index [mg O₂/L] = ((V - V₀) × c_KMnO₄ × 8000) / V_sample, where V = KMnO₄ volume used in titration, V₀ = blank, 8 = molar mass O₂/2.
10. Quality control
Duplicates—difference max ±0.5 mg O₂/L. Check that sample retained pink color after boiling (if decolorized—repeat with smaller sample volume).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Boiling water bath (100°C) | Memmert WNE 10, Julabo TW 20, GFL 1003 | 3,000–10,000 PLN |
| Burette 25 mL (class A) | Brand, Hirschmann, with PTFE stopcock | 200–600 PLN |
| Erlenmeyer flasks 250–300 mL | DURAN / Simax borosilicate, with ground joint (for bath) | 15–40 PLN/piece |
| Volumetric pipettes (5, 10, 25 mL) | Class A, Brand, Hirschmann | 20–60 PLN/piece |
| Graduated cylinder 100 mL | Class A, borosilicate | 30–60 PLN |
| Stopwatch / timer | Any laboratory stopwatch—10 min time must be strictly controlled | 30–100 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Potassium permanganate (KMnO₄) 0.01 mol/L (1/5 KMnO₄) | 7722-64-7 | Titrant solution: 0.316 g KMnO₄ in 1 L water. Standardize against Na₂C₂O₄. Store in dark bottle. Validity: 1 month. |
| Sodium oxalate (Na₂C₂O₄) 0.01 mol/L (1/2 Na₂C₂O₄) | 62-76-0 | Standard solution: 0.6700 g Na₂C₂O₄ (dried 105°C, 2h) in 1 L water. Primary standard for KMnO₄ standardization. |
| Diluted sulfuric acid (H₂SO₄) (~4 mol/L) | 7664-93-9 | Dilute 1:4 (v/v) concentrated H₂SO₄ with water (CAREFULLY—add acid to water!). For acidifying reaction medium. |
| Deionized water (free from organic substances) | — | Freshly boiled or from purification system. Blank <0.5 mg O₂/L. |
Health and safety (OHS)
- Potassium permanganate (KMnO₄)—oxidizer, hazardous to environment (H272, H302, H410). Brown stains on skin and clothing. Use gloves.
- Concentrated sulfuric acid (H₂SO₄)—strongly corrosive (H314). Exothermic reaction with water! Add acid to water, not vice versa.
- Sodium oxalate (Na₂C₂O₄)—harmful if swallowed (H302). Use gloves.
- Water bath (100°C)—burn risk from hot water and steam. Use heat-insulating gloves.
- Hot flasks and solutions—handle with tongs or flask holders.
- Safety glasses, gloves, and lab coat required.