🧪 COD — Chemical Oxygen Demand
Determination of chemical oxygen demand using sealed tubes (cuvettes) with potassium dichromate. A key parameter for assessing water and wastewater contamination with organic matter.
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
1. 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.
2. 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.
3. 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!
4. Mineralization in thermoreactor
Place cuvettes in thermoreactor preheated to 150°C. Mineralize for exactly 120 minutes. Do not open reactor during the process.
5. 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).
6. 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.
7. 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.
8. 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.