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Chemical Oxygen DemandPrepared by :Lamiaa Farghally AhmedChemical Oxygen DemandDefinition Chemical Oxygen Demand (COD) is test used to measure the amount of oxygen in water consumed to chemical oxidation of pollutants and oxidize the organic matter in waste water sample, under specific conditions of oxidizing agent , temperature, and time It is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution . The ratio between COD and BODis useful to assess the amenability of waste for biological treatment. Ratio of BOD to COD greater than or equal to 0.8 indicates that wastewater highly polluted and amenable to the biological treatment.it is useful to assess strength of wastes, which contain toxins and biologically resistant organic substances BOD value is always lower than COD value. For domestic and some industrial wastewater, COD value is about 2.5 times BOD value

Uses(COD) is used to measure organic matter in 1-wastewater . 2-treated effluent. 3-receiving waters. 4-organic constituents .organic pollutants found in surface water (e.g. lakes and rivers), making COD a useful measure of water quality .SourcesSources of COD in water are varied 1-soluble organic compounds 2-Residual food waste from bottles and cans. 3- antifreeze, emulsified oils . 4-industrial stormwater 5-All aquatic plants and animals through their metabolism and excretion of waste products. 6-Dissolution of dead organisms contributes to the organic carbon, as well as surrounding humus and peatHazards Adverse human impacts: High levels of COD in water with threats to human health including toxic algae blooms bacteria from organic wastes and seafood contamination.

2-Adverse impacts on the environment COD levels decrease the amount of dissolved oxygen available for aquatic organisms. Low (generally under 3 mg/L) dissolved oxygen, or hypoxia, causes1- reduced cell functioning,2- disrupts circulatory fluid balance in aquatic species and can result in death of individual organismsas well as large dead zones.3- Hypoxic water can also release pollutants stored in sediment. A high COD-Cr load in water may signify oxygen deficiency, and fish and other aquatic species are consequently repelled.Method of Measurement of COD

Potassium dicromatlc method (titrimetric method)Spectrophotometery(NIR)Frist (titrimetric method) The basis for the COD test is that nearly all organic compounds can be fully oxidized tocarbon dioxidewith a strong oxidizing agentunderacidicconditions. The amount ofoxygenrequired to oxidize an organic compound to carbon dioxide, ammonia, and water is given by:

This expression includes the oxygen demand caused by the oxidation of ammonia into nitrate. The process of ammonia being converted into nitrate is referred to asnitrification.The following is the correct equation for the oxidation of ammonia into nitrate.

PRINCIPLEThe organic matter present in sample gets oxidized completely by potassium dichromate (K2Cr2O7) inthe presence of sulphuric acid (H2SO4), silver sulphate (AgSO4)and mercury sulphate (HgSO4)to produce CO2and H2O. The sample is refluxed with a known amount of potassium dichromate (K2Cr2O7)in the Sulphuric acid medium and the excess Potassium dichromate(K2Cr2O7)is determined by titration against ferrous ammonium sulphate, using ferroin as an indicator. The dichromate consumed by the sample is equivalent to the amount of O2 required to oxidize the organic matter

TESTING OF SAMPLETake three COD vials with stopper (two for the sample and one for the blank).Add 2.5 mL of the sample to each of the two COD vials and the remaining COD vial is for blank; to this COD vial add distilled water.Add 1.5 mL of potassium dichromate reagent digestion solution to eachofthe three COD vials. Add 3.5 mL of sulphuric acid reagent catalyst solution in the same manner Cap tubes tightly. Switch on the COD Digester and fix the temperature at 150 C and set the time at 2 hours.

Place the COD vials into a block digester at 150C and heat for two hours. The digester automatically switches off. Then remove the vials and allow it to cool to the room temperature.Meanwhile, get ready with the burette for the titration. Fill the burette with the ferrous ammonium sulphate solution, adjust to zero and fix the burette to the stand.Transfer the contents of the blank vial to conical flask. Add few drops of ferroin indicator The solution becomes bluish green in colour.Titrate it with the ferrous ammonium sulphate taken in the buretteEnd point of the titration is the appearance of the reddish brown colour. Note down the volume of ferrous ammonium sulphate solution added for the blank (A)is 14.1 mL

Transfer the contents of the sample vial to conical flask. Add few drops of ferroin indicator. The solution becomes green in colour.Titrate it with the ferrous ammonium sulphate taken in the burette.End point of the titration is the appearance of the reddish brown colour. Note down the volume of ferrous ammonium sulphate solution added forthe sample (B)is 13.2 mL.

CalculationsThe following formula is used to calculate COD:whereb =Volume of Ferrous Ammonium sulphatefor blank(A)s=Volume of Ferrous Ammonium sulphatefor Sample (B)n =Normality of Ferrous Ammonium sulphateN=Volume of Sample V , the result of the COD calculation is given in mg/L

Measurement of COD by stoichiometric reactionThe COD can also be estimated from the concentration of oxidizable compound in the sample, based on its stoichiometric reaction with oxygen to yield CO2(assume all C goes to CO2), H2O (assume all H goes to H2O), and NH3(assume all N goes to NH3), using the following formula:COD = (C/FW)(RMO)(32)Where C = Concentration of oxidizable compound in the sample,FW = Formula weight of the oxidizable compound in the sample,RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO2, water, and ammoniaFor example, if a sample has 500 wppm of phenol:C6H5OH + 7O2 6CO2+ 3H2OCOD = (500/94)(7)(32) = 1191 wppm

PRECAUTIONSThe following precautions should be observed while performing the experiment:Chlorides are quantitatively oxidized by dichromate and represent a positive interference. Mercuric sulfate is added to the digestion tubes to complex the chlorides so that it does not interfere in the determination.

Nitrites also interfere in the determination of COD and hence during the determination of samples with high concentration of nitrites, 120mg of sulphuric acid is added to the potassium dichromate solution.

FerroinpreparationFerroin sulfate may be prepared by combining phenanthroline to ferrous sulfate in water.3 phen + Fe2+ [Fe(phen)3]2+

Notes A silver compound is added as a catalyst to promote the oxidation of certain classes of organics, and a mercuric compound may be added to reduce interference fromthe oxidation of chloride ions by the dichromate. End products are carbon dioxide, water, and various states of the chromium ion

Experiment Part

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