Experiment 4 COD

Experiment 4: COD Objective: 1. To determine chemical oxygen demand (COD) in waste water sample.

Results:

Sample. Blank. Sample (trial 1) Sample (trial 2) Mean reading of sample

COD value (mg/L) 0 Under range Under range -

Discussion: The Chemical Oxygen Demand (COD) test is used widely to estimate the amount of organic matter in wastewater. It is a measurement of the oxygen equivalent of the materials present in the wastewater that are subject to oxidation by a strong chemical oxidant, in this case, dichromate. When wastewater contains only readily available organic bacterial food and no toxic matter, the COD test results provide a good estimate of BOD (Biochemical Oxygen Demand) values. In the reactor digestion method test, the COD procedure is greatly simplified over the Dichromate Reflux Method. Small volume of the water sample is pipette into vials containing the premeasured reagents, including catalysts and chloride compensator. The vials are incubated until digestion is complete and then cooled. The COD determination is made with the spectrophotometer. The test measures the amount of oxygen required for chemical oxidation of organic matter in the sample to carbon dioxide and water. The test procedure is to add a known quantity of standard potassium dichromate solution, sulfuric acid reagent containing silver sulfate. The mixture is refluxed for 2 hours by COD reactor. Most types of organic matter are destroyed in this boiling mixture of chromic and sulfuric acid, Organics + Cr2O72- + H+ → CO2 + H2O + 2Cr3+ After the mixture has been cooled, the chromium (III) ion in the specimen is measured by spectrophotometer. The excess chromium (III) ion show how much chromic was use by organics matter. The purpose of running a blank is to compensate for any error that may result because of the presence of extraneous organic matter in the reagents. The value of COD got from this experiment was under range. This is may be due to the COD digestion reagent vials type use was high range. Sample concentration range (mg/L) is between 20 – 1500. Hence, COD value lower than 20 mg/L cannot be detected.

Question: Dichromate reflux method. 1. What was the purpose of that blank sample of distilled water? To compensate for any error that may result because of the presence of extraneous organic matter in the reagents. 2. What will happen to COD value if HgSO4 is not added? - Chlorine will not be separated out of the sample effluent. COD will be higher than it should be. 3. Summarize the dichromate reflux method and write the chemical reaction involved. - Sample is refluxed with known amount of excess dichromate in presence of acid. HgSO4 catalyst is used to bind or complex chlorides. Remaining dichromate is titrated with FAS to determine that used for oxidizing the organic matter. 6Fe2+ + Cr2O72-

-> 6Fe3+ + 2Cr3+ + 7H2O

Ferroin indicator gives a sharp change to brown color on complete reduction of dichromate. FAS is a secondary standard, must be standardized frequently. Result expressed as mg/L COD. Reactor digestion method. 1. Why is it important to determine COD test in wastewater treatment? - Determine the amount of organic pollutants found in surface water (e.g. lakes and rivers) or wastewater, making COD a useful measure of water quality. 2. Summarize the method and write the chemical reaction involved. - Turn on the Reactor. Preheat to 150 °C. Carefully pipette 0.20 mL of sample into the vial. Cap and clean the outside of the vial. Hold the vial by the cap over a sink. Invert gently several times to mix. Place the vial in the preheated Reactor. Heat for 2 hours. Turn the reactor off. Cool down the vial. Invert the vial several times while still hot. Insert the vial into the cell holder. Close the lid. 3. A sample is collected for COD analysis and it is to be performed on the following day. Describe two ways by which the sample can be preserved. Why is it necessary to preserve the sample? - Two ways to preserved water sample are adding a little bit acid and refrigeration. Putting samples in ice and keeping them there until they are submitted to the laboratory will preserve them well enough for most tests. It is necessary to preserve the sample to retard the chemical and biological changes in the sample.

Conclusion: Chemical oxygen demand (COD) is used as a measure of oxygen requirement of sample that is susceptible to oxidation by strong chemical oxidant. The COD value for sample in this experiment was under range. Hence, we can conclude that this water sample has low organic matter. Chemical Oxygen Demand is the total measurement of all chemicals in the water that can be oxidized.

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