⚗️ Oxidizability (permanganate index)
In short
Determination of water permanganate index (oxidizability) by oxidation with KMnO₄ in acidic medium, with reverse titration with sodium oxalate. The test is performed according to PN-EN ISO 8467:2001; Measurement range: 0.5–10 mg O₂/L (higher after dilution). The procedure comprises 10 steps (about 51–59 min in total); it is used for: 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.
At a glance
- Standard: PN-EN ISO 8467:2001
- Category: Physicochemistry
- Procedure steps: 10
- Total time of stages: 51 min–59 min
- 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)
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
-
Water bath preparation
Fill water bath and boil to 100°C. Maintain vigorous boiling throughout analysis.
-
Sample measurement
Measure 100 mL water sample with cylinder into 250 mL conical flask. For water with oxidizability >10 mg O₂/L—dilute.
-
Sulfuric acid addition
Add 5 mL H₂SO₄ (~4 mol/L) to sample. Mix.
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Potassium permanganate addition
Add exactly 5.0 mL KMnO₄ 0.01 mol/L solution with pipette. Mix. Solution will turn pink/violet.
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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.
-
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).
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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).
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Blank
Repeat entire procedure with 100 mL deionized water (blank). KMnO₄ consumption in blank: V₀.
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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.
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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.
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 8467:2001 — “Water quality — Determination of permanganate index”.
How does this method work?
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.
What is the measuring range and accuracy?
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).
How long does the test take?
The times given for the individual stages add up to approximately 51 min–59 min. The procedure comprises 10 steps.
What equipment is required?
Boiling water bath (100°C), Burette 25 mL (class A), Erlenmeyer flasks 250–300 mL, Volumetric pipettes (5, 10, 25 mL), Graduated cylinder 100 mL, Stopwatch / timer. Indicative cost of the core instrument (Boiling water bath (100°C)): 3,000–10,000 PLN.
Where is this test used?
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.
What safety precautions apply?
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.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 8467.