Calculate COD from titration data, estimate organic load, convert between COD/BOD/TOC, and assess wastewater treatment requirements.
Chemical Oxygen Demand (COD) is a critical parameter in environmental and water chemistry that measures the total amount of oxygen required to chemically oxidize both organic and inorganic substances in a water sample. It's expressed in milligrams of oxygen per liter (mg O₂/L) and serves as a key indicator of water pollution and organic load.
The standard COD test uses potassium dichromate (K₂Cr₂O₇) as the oxidizing agent in a strongly acidic solution, heated under reflux conditions. The excess dichromate is then determined by back-titration with ferrous ammonium sulfate (FAS), and the amount consumed is used to calculate the COD. This calculator automates the computation from your titration data, converting between COD, BOD₅, and TOC values.
Engineers and environmental scientists use COD measurements to assess wastewater treatment efficiency, design treatment plants, monitor industrial discharge compliance, and evaluate natural water body health. This tool helps you quickly process lab data and compare results against regulatory discharge standards from EPA, EU, and other jurisdictions.
This calculator eliminates manual COD computation errors from titration data, instantly converts between COD/BOD/TOC parameters, and provides regulatory context with discharge standards — essential for environmental labs processing multiple samples daily. This chemical oxygen demand calculator helps you compare outcomes quickly and reduce avoidable mistakes when making day-to-day care decisions. Use the estimate as a planning baseline and confirm final decisions with a qualified professional when risk is high.
COD (mg/L) = [(B − S) × N × 8000] / V, where B = mL FAS for blank, S = mL FAS for sample, N = normality of FAS, V = sample volume (mL). 8000 = milliequivalent weight of oxygen × 1000.
Result: COD = 2160 mg/L
With a blank titration of 25.0 mL, sample titration of 14.2 mL, 0.25 N FAS, and 10 mL sample: COD = (25.0 − 14.2) × 0.25 × 8000 / 10 = 2160 mg O₂/L.
Chemical Oxygen Demand is used at every stage of wastewater treatment — from characterizing raw influent to monitoring treated effluent. In the primary treatment stage, COD values help size settling tanks. In biological treatment (activated sludge, trickling filters), the BOD/COD ratio determines whether the waste is amenable to biological degradation. Tertiary treatment requirements are often triggered by COD thresholds in discharge permits.
The standard method involves adding a known excess of potassium dichromate to an acidified water sample, refluxing at 150°C for 2 hours, then titrating the unreacted dichromate with ferrous ammonium sulfate using ferroin indicator. The color change from blue-green to reddish-brown marks the endpoint. Closed (sealed tube) methods are now common and produce less hazardous waste than open reflux.
Discharge limits vary by jurisdiction: US EPA typically requires <125 mg/L COD for secondary treatment effluent, the EU Urban Wastewater Directive sets 125 mg/L (or 75% removal), and many developing countries allow higher limits. Industrial pretreatment programs may set site-specific limits. Regular COD monitoring with proper calibration ensures compliance and avoids regulatory penalties.
COD measures total chemically oxidizable matter (2-3 hour test); BOD measures only biologically degradable matter (5-day test). COD is always ≥ BOD. The BOD/COD ratio indicates biodegradability.
For domestic wastewater: 0.3–0.8. For highly biodegradable waste: >0.5. Values <0.3 indicate poor biodegradability, suggesting chemical or advanced treatment may be needed.
Potassium dichromate oxidizes about 95-100% of organic compounds, providing a more complete and reproducible result than other oxidants like permanganate. This keeps planning practical and lowers the chance of preventable errors.
Clean rivers: <20 mg/L, domestic wastewater: 250-1000 mg/L, industrial wastewater: 1000-100,000+ mg/L depending on the industry. This keeps planning practical and lowers the chance of preventable errors.
The general relationship is COD/TOC ≈ 2.67 for fully oxidized organic carbon. Actual ratios vary from 2.0 to 4.0 depending on the chemical nature of the organics.
For low-range COD (<150 mg/L): 20 mL sample with 0.025 N FAS. For mid-range (100-900 mg/L): 10 mL with 0.25 N FAS. For high-range: dilute first.