Article — Chemical Oxygen Demand (COD) Calculator
Chemical Oxygen Demand (COD) Calculator: Dichromate Titration in mg O₂/L
Chemical oxygen demand (COD) is the amount of oxygen, in mg O₂ per litre, needed to chemically oxidise all organic and oxidisable inorganic matter in a water sample. The titrimetric formula is COD = (V_blank − V_sample) × N_FAS × 8000 / V_sample, where V values are ferrous-ammonium-sulfate titration volumes in mL, N_FAS is the FAS normality, and 8000 combines the oxygen equivalent (8 g/eq) with the mL-to-L conversion. Domestic sewage runs 250 to 400 mg O₂/L; treatment-plant effluent must hit below 125 mg O₂/L under the EU Urban Waste Water Directive. The EPA Method 410.3 and ISO 6060 standards both use potassium dichromate digested at 150 °C for 2 hours with silver sulfate catalyst, then back-titration with FAS using ferroin indicator.
This calculator handles the titrimetric formula directly. Enter the FAS volume for blank and sample, the FAS normality, and the original sample volume. Output gives the COD in mg O₂/L with a water-quality tag (clean, moderate, polluted, wastewater).
What is chemical oxygen demand
Chemical oxygen demand is the total amount of oxygen (in mg per litre) required to chemically oxidise the organic and oxidisable inorganic matter in a water sample using a strong oxidant. COD is the principal indicator of organic pollution in wastewater monitoring, alongside biochemical oxygen demand (BOD) and total organic carbon (TOC).
The COD test uses potassium dichromate (K₂Cr₂O₇), a powerful oxidant in hot sulfuric acid, to attack the sample. About 95% of organic compounds are oxidised to CO₂ and water. The remaining 5% — including pyridine, benzene, toluene, and some surfactants — resist dichromate oxidation. Silver sulfate is added as a catalyst to push slower compounds like fatty acids and straight-chain alcohols to react.
The COD test predates the BOD test as a wastewater indicator and remains preferred for industrial effluents because BOD relies on viable bacteria. Heavily polluted or toxic samples (high heavy metals, low pH, antibiotic-laden) suppress the microbial activity needed for BOD, giving falsely low readings. COD uses a purely chemical oxidant that does not care about toxicity, making it the more reliable benchmark for monitoring industrial discharge.
COD formula from the dichromate method
The COD formula from the dichromate method is COD (mg O₂/L) = (V_blank − V_sample) × N_FAS × 8000 / V_sample. The 8000 factor combines the equivalent mass of oxygen (8 g/eq, half of the 16 g/mol molar mass because O₂ accepts 4 electrons per molecule) with the unit conversion from mL of titrant per L of sample (× 1000). N_FAS is the normality of the ferrous ammonium sulfate titrant, typically 0.025 to 0.25 N depending on the COD range.
The difference V_blank − V_sample measures how much dichromate was reduced by the sample. A blank that uses 20 mL of FAS and a sample that uses 12 mL means the sample consumed dichromate equivalent to 8 mL of FAS, which the formula converts to mg O₂/L.
Factor 8000 O₂ eq × 1000FAS normality 0.025 – 0.25 NDigestion 150 °C, 2 hIndicator FerroinEU limit 125 mg/LCOD vs BOD and how they differ
COD measures all chemically oxidisable matter; BOD measures only what microbes oxidise in 5 days at 20 °C. COD is always greater than or equal to BOD. The ratio BOD₅/COD reveals biodegradability: domestic sewage runs 0.5 to 0.7 (mostly biodegradable), industrial brewery waste 0.5 to 0.6, paper mill effluent 0.2 to 0.4 (largely recalcitrant lignin and cellulose). A low BOD₅/COD ratio means biological treatment alone will not meet discharge limits — advanced oxidation or chemical treatment is needed.
COD test procedure step by step
The COD test procedure step by step: (1) Take a representative water sample, typically 10 to 50 mL. (2) Add concentrated sulfuric acid containing silver sulfate catalyst. (3) Add a measured volume of standard potassium dichromate. (4) Add mercuric sulfate to mask any chloride. (5) Reflux at 150 °C for 2 hours. (6) Cool, then back-titrate the unreacted dichromate with ferrous ammonium sulfate, using ferroin as colour-change indicator (blue-green to reddish brown). (7) Run a blank in parallel. (8) Compute COD with the formula above.
Typical COD values by water source
Typical COD values by water source span six orders of magnitude. Treated drinking water has COD below 5 mg O₂/L. Unpolluted rivers run 5 to 15 mg/L. Polluted rivers reach 30 to 100. Raw domestic sewage runs 250 to 400, and treatment-plant effluent should fall below 100. Industrial effluents vary widely: brewery waste 1,000 to 2,000, dairy 1,500 to 6,000, paper mill 2,000 to 10,000, landfill leachate 5,000 to 80,000.
COD regulatory limits for effluents
COD regulatory limits for effluents vary by region and discharge point. The EU Urban Waste Water Treatment Directive (91/271/EEC) sets 125 mg O₂/L for treatment plants serving more than 2,000 population equivalents. The US Clean Water Act regulates COD indirectly through total organic carbon (TOC) and BOD limits. Many industrial sectors have category-specific COD limits — pulp mills, food processing, slaughterhouses — typically in the 200 to 1,000 mg/L range depending on the receiving water.
Modern routine COD monitoring uses sealed-vial colorimetric kits (APHA 5220D) instead of open-flask titration. Sealed vials are pre-loaded with all reagents, digested in a block thermostat, and read directly on a spectrophotometer at 420 nm (low-range) or 600 nm (high-range). The result agrees with the titrimetric method to within ±5%.
Interferences in COD measurement
The main interference in COD measurement is chloride, which is oxidised by dichromate to Cl₂ and falsely raises COD readings. Mercuric sulfate (HgSO₄) at 10:1 mass ratio to chloride masks it as undissociated HgCl₂. Other interferences include nitrites (rare in raw sewage; if present, add sulfamic acid), volatile organics (lost during open-flask digestion; use sealed vials), and reduced inorganics (sulfide, ferrous iron) which add to the measured COD.
Common COD test mistakes
The most common COD test mistake is skipping the silver sulfate catalyst, which underestimates COD by 25 to 35% for samples rich in straight-chain fatty acids or alcohols. Second mistake: missing the chloride correction for seawater or salty effluents, which over-estimates COD by 10 to 30%. Third: digesting at the wrong temperature — below 148 °C and dichromate does not fully react; above 152 °C and the dichromate itself starts to decompose. Fourth: storing samples too long without preservation. Add sulfuric acid to pH below 2 and refrigerate; analyse within 28 days.
The classic COD test produces mercury-containing waste from the chloride-masking step. Many labs have switched to chloride-free sample preparation (dilution, removal by precipitation as silver chloride before digestion) to avoid hazardous waste streams. The newer mercury-free COD kits available from major vendors meet the same ISO 6060 performance specs without HgSO₄.