COD

COD

COD means chemical oxygen demand. COD is a measure of the oxygen equivalent to that portion of organic matter present in the waste matter sample that is suscsptible to oxidation by potassium dichromate.

Amount of oxygen required by organic matter in a sample of water for its oxidation by strong chemical. Oxidizing agent such as potassium dichromate. According to the ASTM, COD is defined as the amount of oxygen consumed under specified conditions in the oxidation of organic matter and oxidisable inorganic matter, corrected for the influence of chlorine.

COD is important and quickly measured parameter for stream, sewage and industrial waste samples to determine their pollutional strength.

Determination of COD

The principle involved in the determination of COD is that when the waste water sample is refluxed with a known excess of potassium dichromate in a 50% sulphuric acid solution in presence of AgSO4 and HgSO4, the organic matter of the sample is oxidized to water, CO2 and NH3. The excess dichromate remaining unreacted in the solution is titrated with a standard solution of ferrous ammonium sulphate. The COD of the sample is calculated as follows


(V2- V1) × N × 8 × 1000
COD = -----------------------------------------
X


Where, V1 and V2 are volumes of ferrous ammonium sulphate, N is normality run down in the blank and test experients respectively and X is the volume of the sample taken for the test.