COD = (a − b) × N × 8000 / V
Chemical Oxygen Demand (COD) measures the total quantity of oxygen required to oxidize all organic material in a water sample into CO₂ and H₂O. It is one of the most important indicators of water quality and pollution level. The standard dichromate reflux method uses potassium dichromate (K₂Cr₂O₇) as the oxidizing agent under acidic conditions at 150 °C for 2 hours. Excess dichromate is back-titrated with ferrous ammonium sulfate (FAS). The formula COD = (a−b) × N × 8000 / V converts the titration volume difference into mg/L of oxygen. Typical COD values: clean river water 1-5 mg/L, domestic wastewater 250-800 mg/L, industrial wastewater 500-60,000 mg/L. COD is always higher than BOD because chemical oxidation is more complete than biological oxidation.