this presentation showsChemical oxygen demand (mg O2 / lit.) which is the amount of oxygen required for reacting with the organic (harmful) matter present in waste water, both soluble or insoluble (suspended) matters, producing CO2 and H2O. In this experiment, organic compounds are oxidized to carbon dioxide and water by a boiling acid dichromate solution Materials Waste water sample. Distilled water. Potassium dichromate (K2Cr2O7). Sulfuric acid (H2SO4). Silver sulfate (Ag2SO4). Mercuric sulfate (HgSO4). and procedures Take a sample of waste water (2.5 ml) in a standard test tube. Add K2Cr2O7 (1.5 ml) to the above sample. Add 3.5 ml of a solution containing H2SO4, Ag2SO4 and HgSO4 to the above mixture. This solution is known as "digestion solution" which is prepared by adding Ag2SO4 and HgSO4 to 1 kg of H2SO4. Repeat the above procedure with a sample of distilled water (2.5 ml) in another test tube. Heat the two test tubes in the reactor for 2 hrs. at a temperature of 150 ºC and after that leave them to cool. Use the spectrophotometer to detect the COD (in mg/lit.) value for the waste water sample. some notes K2Cr2O7 is used as an oxidizing agent (source of oxygen needed to react with organic matters). H2SO4 is a digesting agent which helps in decomposing the organic matters to be easily reacted with oxygen. Ag2SO4 is used to reduce the volatility of the organic matters exist in the waste water sample and keep them in liquid phase. If those matters vaporized, the measured value of COD will be incorrect. HgSO4 is used to avoid oxidation of 〖𝐶𝑙〗^− if it exists in wastewater as salt. This will lead to high misleading value of COD since 〖𝐶𝑙〗^− is oxidized by K2Cr2O7 into Cl2. The distilled water sample is used as a blank sample which allows the calibration of the spectrophotometer. The COD value for this sample is zero. also shows Biological oxygen demand (mg O2 / lit.) is the amount of oxygen required to be used up by bacteria so as to decompose the waste matters in a liter of wastewater. This test may need at least 3 months to be finished: the standard test defines it as BOD5 as it is performed within 5 days only. During those 5 days, about 70 – 80% of degradation is achieved. In the COD test we completely oxidize the wastes, whether biodegradable (i.e. can be decomposed by bacteria) or non – biodegradable. In the BOD test we oxidize the biodegradable wastes only. Determination of the BOD5 of undiluted samples of sewage containing high levels of industrial pollutants may be considerably impaired(damaged) by the presence of inhibitors or toxic substances. Measurements can only be carried out after the sample has been diluted with dilution water that contains a sufficient amount of nutrients and microorganisms in order to reduce the interfering substances to an acceptable level. Determination of the BOD5 of undiluted samples of sewage containing high levels of industrial pollutants may be considerably impaired(damaged) by the presence of CO