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Determination of Chemical Oxygen Demand (COD) of a Given Sample of Waste Water

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Introduction

BCH 2005 Principles of Environmental Chemistry Experiment 4 Determination of Chemical Oxygen Demand (COD) of a Given Sample of Waste Water * Introduction The chemical oxygen demand(COD) is commonly used to indirectly measure the amount of organic compounds in water. Most applications of COD determine the amount of organic pollutants found in surface water , making COD a useful measure of water quality. It is expressed in the unit of mgL-1. In this practical, COD of a given sample is measured by back titration of the FAS solution with the remaining dichromate ions after reflux. For details, the sample is refluxed in concentration sulphuric acid with excess potassium dichromate solution. After reflux, the remaining K2Cr2O7 is titrated with FAS to determine the amount of potassium dichromate solution remained. As we have known the original amount of potassium dichromate in advanced, we can find out the amount of dichromate reacted with the organic solvent during reflux. Hence, that is the COD. This method is suitable for a wide range of wastes where a large sample size is preferred. COD values of > 50 mg O2 l-1 can be determined using this method. * Objective To determine the COD of a given sample of wastewater * Procedure Please refer to the lab manual * Results 1) ...read more.

Middle

Hence, no. of mole of FAS titrant used to titrate with blank solution = 0.2502 x 25.20/ 1000 = 0.0063 mole no. of mole of K2Cr2O7 = 0.0063 / 6 = 0.0011mole Volume of water sample used = 25 ml = 0.025dm3 Weight of oxygen generated = 48,000 mg COD of my sample =?(0.0011 -0.00072 ) x 48000?/ 0..025 = 729.60 mg O2 L-1 The chemical oxygen demand of the sample is 729.60 mg O2 L-1. 3. Ag2SO4 can act as a catalyst to speed up the oxidizing reaction. However, problem arises as samples containing halide ions will form precipitates with Ag+, as the following equation: Ag+ (aq) + X- (aq) --> AgX (s) where X is halide ions. These precipitates are insoluble. As from the principle of catalysis, catalysts should not take part in the reaction, this principle would be violated if the silver precipitates are formed, which is not desirable. And, the silver sulphate may be used up in the reaction that cannot act as a catalyst. As the amount of silver sulphate decreases, it affects the rate of the reaction. Mercuric sulphate was added, as it can remove halide ions in the sample solution before refluxing instead of adding Ag2SO4. It is because HgSO4 can form complexes with the halide ions. So HgSO4 will not from precipitate with halide ion that can also suppress the oxidation of chloride ions in the COD test mixture. ...read more.

Conclusion

Can we use other reagent to oxidise the sample apart from potassium dichromate? The main point to choose the reagent is to consider if it is a strong oxidising agent or not. Thus, potassium permanganate solution can also be considered as a choice of oxidising agent. Precaution 1. Whenever adding concentrated sulphuric acid, add it little at a time slowly along the side of the flask with swirling, so that to release the heat gently and lower the temperature inside the flask. Continuous addition and swirling throughout. 2. Handle with care of the concentrated sulphuric acid. Wear gloves and safety goggles all the time. 3. Point the mouth of the flask away from any person including ourselves. 4. Ensure swirling is in a proper manner, not a very vigorous nor very gently ones. 5. Cool down the flask by water bath until cool if required. 6. Do not put the flask directly into the ice otherwise it may cause breakage of the glassware. 7. Watch the heating mantle tightly. Time is counted from the first drop condensed. Turn the power a bit lower with continuous dropping of the condensed liquid. 8. A small beaker was placed on the reflux tube to prevent foreign substances falling into it. * Conclusion The volume of FAS used to titrate with standard K2Cr2O7 and excess K2Cr2O7 in the sample solution are 10.00cm3 and 17.20cm3 respectively. The molarity of FAS titrant is 0.2502 M while the chemical oxygen demand (COD) of the sample is 729.60 mg O2 L-1. ...read more.

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