🧪 COD — Chemical Oxygen Demand
In short
Determination of chemical oxygen demand using sealed tubes (cuvettes) with potassium dichromate. The test is performed according to PN-EN ISO 15705:2005; Measurement range: 0–1000 mg O₂/L (without dilution). The procedure comprises 8 steps (about 2 h 53 min–3 h 8 min in total); it is used for: Treated wastewater quality control (water permit, limit 125 mg O₂/L), Monitoring technological processes at wastewater treatment plants, Industrial wastewater testing (chemical, food, pharmaceutical).
At a glance
- Standard: PN-EN ISO 15705:2005
- Category: Physicochemistry
- Procedure steps: 8
- Total time of stages: 2 h 53 min–3 h 8 min
- Measurement range: 0–1000 mg O₂/L (without dilution)
- Detection limit: 6 mg O₂/L (photometry at 600 nm)
- Mineralization temperature: 150 ± 2°C
Overview
Chemical Oxygen Demand (COD) is one of the most important parameters in water and wastewater analysis. It determines the amount of oxygen (in mg O₂/L) required for the chemical oxidation of organic substances and certain inorganic reducing compounds present in a water sample.
Unlike BOD5 (biological oxygen demand), COD encompasses all oxidizable substances — both biodegradable and resistant to biological decomposition. The BOD5/COD ratio allows assessing wastewater biodegradability (>0.5 = easily biodegradable).
COD is a mandatory parameter in water permits, municipal and industrial wastewater monitoring, and wastewater treatment plant efficiency evaluation. The permissible COD value in treated wastewater discharged to surface waters is 125 mg O₂/L (Regulation of the Polish Ministry of Environment). The ISO 15705 method (small-scale, sealed tubes) is preferred due to lower reagent consumption and improved work safety compared to the classical reflux method.
Method principle
A water sample is oxidized in a sealed tube (cuvette) with a strong oxidizing mixture: potassium dichromate (K₂Cr₂O₇) in concentrated sulfuric acid (H₂SO₄), in the presence of silver sulfate (Ag₂SO₄) as a catalyst for oxidizing difficult-to-decompose compounds and mercury(II) sulfate (HgSO₄) as a chloride masking agent.
Mineralization proceeds at 150°C for 2 hours. During the reaction, Cr⁶⁺ (orange) is reduced to Cr³⁺ (green). The amount of reduced chromium is measured spectrophotometrically at 600±20 nm (for range up to 1000 mg/L O₂) — Cr³⁺ absorbance is proportional to the sample's COD.
COD [mg O₂/L] = spectrophotometer reading after calibration with standard solutions (KHP — potassium phthalate).
Applications
- Treated wastewater quality control (water permit, limit 125 mg O₂/L)
- Monitoring technological processes at wastewater treatment plants
- Industrial wastewater testing (chemical, food, pharmaceutical)
- Biodegradability assessment (BOD5/COD ratio)
- Surface water monitoring (Water Framework Directive)
- Industrial discharge control to sewerage
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0–1000 mg O₂/L (without dilution) |
| Detection limit | 6 mg O₂/L (photometry at 600 nm) |
| Mineralization temperature | 150 ± 2°C |
| Mineralization time | 120 minutes (2 hours) |
| Max. chloride concentration | 1000 mg/L Cl⁻ (without interference) |
| Precision (CV) | 5–10% (depends on concentration) |
Standard
- Standard number
- PN-EN ISO 15705:2005
- Title (PL)
- Jakość wody -- Oznaczanie indeksu chemicznego zapotrzebowania tlenu (SP-ChZT) -- Metoda zminiaturyzowana z zastosowaniem szczelnych probówek
- Title (EN)
- Water quality — Determination of the chemical oxygen demand index (ST-COD) — Small-scale sealed-tube method
Step-by-step procedure
-
Sample preparation
Mix the wastewater sample thoroughly (suspensions affect the result). For samples with COD >1000 mg/L, dilute with deionized water. Bring sample to room temperature.
-
Adding sample to cuvette
Open the test cuvette (e.g., Hach LCK 514). Pipette precisely the required sample volume (usually 2.0 mL) into the cuvette with reagents. Close tightly with cap.
-
Mixing
Invert the cuvette several times (min. 3×) to mix the sample thoroughly with reagents. WARNING: cuvette heats up strongly (exothermic reaction with H₂SO₄) — hold by the cap!
-
Mineralization in thermoreactor
Place cuvettes in thermoreactor preheated to 150°C. Mineralize for exactly 120 minutes. Do not open reactor during the process.
-
Cooling
After mineralization, turn off reactor. Leave cuvettes in the block for 15–20 minutes for initial cooling. Then remove and place in rack. Invert several times. Cool to room temperature (~30 min).
-
Spectrophotometric measurement
Wipe the cuvette with a clean cloth (no streaks, fingerprints). Insert into spectrophotometer. Measure absorbance at 600 nm (Cr³⁺). Read COD result from calibration curve or built-in program.
-
Quality control
With each batch, run: blank sample (deionized water), KHP standard solution (e.g., 500 mg O₂/L). Acceptable deviation: ±10% of certified value.
-
Waste disposal
Used cuvettes contain mercury and chromium — treat as hazardous waste. Collect in labeled containers. Transfer to laboratory waste disposal company. Do not pour into sewage!
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Thermoreactor (heating block) | Hach DRB200, WTW CR 4200, Macherey-Nagel Nanocolor Vario C2 | 4 000–12 000 PLN |
| VIS/UV-VIS spectrophotometer | Hach DR3900, Hach DR6000, WTW photoLab 7600 | 15 000–45 000 PLN |
| Test cuvettes (ready-to-use reagents) | Hach LCK 314 (15–150 mg/L), LCK 514 (100–2000 mg/L), LCK 014 (1000–10000 mg/L) | 250–450 PLN / 25 pcs |
| Automatic pipettes | Eppendorf Research Plus 0.5–5 mL, Brand Transferpette S | 800–2 000 PLN |
| Cuvette rack | Dedicated rack for cooling after mineralization | 100–300 PLN |
| Deionized water | Millipore Milli-Q system or Hydrolab, conductivity <0.1 mS/m | system 15 000–40 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Potassium dichromate K₂Cr₂O₇ | 7778-50-9 | Oxidizing solution in concentrated H₂SO₄, concentration 0.01667 mol/L. Contained in ready-to-use test cuvettes. |
| Sulfuric acid H₂SO₄ (concentrated) | 7664-93-9 | Reaction medium, concentration >95%. Highly corrosive, exothermic mixing with water. |
| Silver sulfate Ag₂SO₄ | 10294-26-5 | Catalyst for oxidation of difficult-to-decompose compounds (aliphatic, aromatic). 5.5 g/kg H₂SO₄. |
| Mercury(II) sulfate HgSO₄ | 7783-35-9 | Chloride masking (Cl⁻ complexation). 33.3 g/L mineralizing solution. Mercury waste requires special disposal. |
| Potassium phthalate (KHP) | 877-24-7 | Calibration standard, 1.000 g KHP = 1.176 g O₂ (theoretical COD). For preparing calibration curve. |
| COD standard solution 1000 mg/L | — | Ready certified solution (CRM) for calibration verification and quality control. |
Health and safety (OHS)
- Potassium dichromate (Cr⁶⁺) — carcinogenic (category 1B), mutagenic, toxic. Avoid contact and inhalation. Use fume hood.
- Concentrated sulfuric acid — highly corrosive, causes severe burns. Exothermic mixing with water. Never add water to acid!
- Mercury sulfate — highly toxic (Hg), hazardous to the environment. Mercury waste requires specialized disposal.
- Hot cuvettes after mineralization (150°C) — risk of burns. Use thermal gloves.
- Safety goggles, nitrile gloves, and laboratory coat mandatory.
- Used cuvettes — hazardous waste (Cr + Hg). Collect separately, do not pour into sink.
Training and video materials
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 15705:2005 — “Water quality — Determination of the chemical oxygen demand index (ST-COD) — Small-scale sealed-tube method”.
How does this method work?
A water sample is oxidized in a sealed tube (cuvette) with a strong oxidizing mixture: potassium dichromate (K₂Cr₂O₇) in concentrated sulfuric acid (H₂SO₄), in the presence of silver sulfate (Ag₂SO₄) as a catalyst for oxidizing difficult-to-decompose compounds and mercury(II) sulfate (HgSO₄) as a chloride masking agent. Mineralization proceeds at 150°C for 2 hours.
What is the measuring range and accuracy?
Measurement range: 0–1000 mg O₂/L (without dilution); Detection limit: 6 mg O₂/L (photometry at 600 nm); Mineralization temperature: 150 ± 2°C; Mineralization time: 120 minutes (2 hours).
How long does the test take?
The times given for the individual stages add up to approximately 2 h 53 min–3 h 8 min. The procedure comprises 8 steps.
What equipment is required?
Thermoreactor (heating block), VIS/UV-VIS spectrophotometer, Test cuvettes (ready-to-use reagents), Automatic pipettes, Cuvette rack, Deionized water. Indicative cost of the core instrument (Thermoreactor (heating block)): 4 000–12 000 PLN.
Where is this test used?
Treated wastewater quality control (water permit, limit 125 mg O₂/L); Monitoring technological processes at wastewater treatment plants; Industrial wastewater testing (chemical, food, pharmaceutical); Biodegradability assessment (BOD5/COD ratio); Surface water monitoring (Water Framework Directive); Industrial discharge control to sewerage.
What safety precautions apply?
Potassium dichromate (Cr⁶⁺) — carcinogenic (category 1B), mutagenic, toxic. Avoid contact and inhalation. Use fume hood.; Concentrated sulfuric acid — highly corrosive, causes severe burns. Exothermic mixing with water. Never add water to acid!; Mercury sulfate — highly toxic (Hg), hazardous to the environment. Mercury waste requires specialized disposal.; Hot cuvettes after mineralization (150°C) — risk of burns. Use thermal gloves.; Safety goggles, nitrile gloves, and laboratory coat mandatory.; Used cuvettes — hazardous waste (Cr + Hg). Collect separately, do not pour into sink.
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 15705.