cod lab report

INTRODUCTION

Experiment Background The chemical oxygen demand (COD) determines the amount of oxygen required for chemical oxidation of organic matter using a strong chemical oxidant, such as, potassium dichromate under reflux conditions. This test is widely used to determine: a) b) c) d)

Degree of pollution in water bodies and their self-purification capacity Efficiency of treatment plants Pollution loads Provides rough idea of Biochemical oxygen demand (BOD) which can be used to determine sample volume for BOD estimation.

The limitation of the test lies in its inability to differentiate between the biologically oxidizable and biologically inert material and to find out the system rate constant of aerobic biological stabilization. Most of the organic matters are-destroyed when boiled with a mixture of potassium dichromate and sulphuric acid producing carbon dioxide and water. A sample is refluxed with a known amount of potassium dichromate in sulphuric acid medium and the excess of dichromate is titrated against ferrous ammonium sulphate. The amount of dichromate consumed is proportional to the oxygen required to oxidize the oxidizable organic matter.

Concept of Literature The Chemical Oxygen Demand (COD) test measures the oxygen equivalent consumed by organic matter in a sample during strong chemical oxidation. The strong chemical oxidation conditions are provided by the reagents used in the analysis. Potassium dichromate is used as the oxygen source with concentrated sulfuric acid added to yield a strong acid medium. Several reagents are added during the set up of the analysis to drive the oxidation reaction to completion and also to remove any possible interferences. Specifically, these reagents are mercuric sulfate, silver sulfate and sulfamic acid. Mercuric sulfate is added to remove complex chloride ions present in the sample. Without the mercuric sulfate the chloride ions would form chlorine compounds in the strong acid media used in the procedure. These chlorine compounds would oxidize the organic matter in the sample, resulting in a COD value lower than the actual value. Silver sulfate is added as a catalyst for the oxidation of short, straight chain organics and alcohols. Again, without the silver sulfate the COD of the sample would be lower than the actual value. Sulfamic acid is added to remove interferences caused by nitrite ions. Without sulfamic acid the COD of the sample would measure higher than the actual value. Even with the use of these additional reagents the oxidation of the organic matter is not always 100% complete. Volatile organics, ammonia and aromatic hydrocarbon are not oxidized to any great degree during the procedure. The advantages of the COD test as compared to the BOD test are: COD results are available much sooner. The COD test requires fewer manipulations of the sample. The COD test oxidizes a wider range of chemical compounds. It can be standardized more easily. The major disadvantage of the COD test is that the results are not directly applicable to the 5-day BOD results without correlation studies over a long period of time. The samples used for the COD analysis may be grab or composite. Preservation of the sample can be accomplished by adding sulfuric acid to depress the pH to 2 and the holding time with preservation is 7 days.

Objectives To measure the chemical oxygen demand of the given wastewater sample.

Material & Apparatus Reagent Distill water Sulphuric acid Potassium dichromate Apparatus Blander 10ml pipette Beaker Conical flask

Procedure

1. Water sample is collected from a pond. 2. Poured 10ml of the sample into three 100ml conical flask and label them as test 1,2 and 3 3. Simultaneously the distilled water is taken in three 100ml conical flask lebeled as blank 1,2 and 3. 4. Added 5ml of potassium dichromate solution in each of thesix conical flask 5. Keep the flask in water bath at 100°C (boiling temperature) for 1 hour. 6. Allowed the samples to cool for 10 miutes. 7. Added 5ml of potassium iodine in each flasks. 8. Added 10ml of sulphuric acid in each flask. 9. Titrated the contents of each flask with 0.1N Sodium thio-sulphate until the blue colour disappears completely.

Result

DILLUTION FACTOR

X1

X2

X3

SAMPLE AVERAGE

0 2 20 50

0 557 249 25

0 557 249 25

0 557 249 25

0 557 249 25

Blank Sample 1 Sample 2 Sample 3

Calculation

Average for sample 1 = 557+557+557 3 = 557 Average for sample 2 = 249+249+249 3 = 249 Average for sample 1 = 25+25+25 3 = 25

FINAL READING (mg/L) 0 557 249 25

Discussion

The COD value indicated the amount of oxygen which is needed for the oxidation of all organic substance in water in mg/l. The COD is closely related to the laboratory standard method named dichromate method. With this method the chemical oxygen demand in determine during chromic acid digestion is wastewater. After the experiment, the average of sample for 2x is 557 mg/l, sample for 20x is 249 mg/l, and 249 mg/l for sample 3.from our result we can know that the highest value is at the sample for 2x. The sample give difference result because the sample used the different volume of palm oil sample. Thus we would get the difference volume of sample shown by the result. During the experiment, there will be some error happen. When carried out the experiment and when we tittering there could be some free air bubble inside the pipette. Sometimes, we also did not make sure that the pipette is in the clean condition. We should be ensure that there is no bubble inside. For that, we can get accurate reading. The purpose of clean pipette is to avoid the chemical in pipette from mixing with our chemical. Furthermore, during our experiment, parallax error also can happen. We must ensure that we always read the pipette with the right way that s read the bottom of the meniscus. If we avoid to occur parallax error, we will getting more accurate result.

Conclusion

In environmental chemistry, the COD test is commonly used to indirectly measure the amount of organic compounds I water. Most application of COD determine the amount of organic pollutants found in surface water such as pond, lakes and rivers that making COD a useful measure of water quality. It is ex pressed in miligrams per liter (mg/l) which indicates the mass of oxygen consumed per liter of solution. Older references may express the units as parts per million (ppm).