1.0
INTRODUCTION The chemical oxygen demand (COD) is the amount of oxygen consumed to
completely chemically oxidize the organic water constituents to inorganic end products. COD is an important, rapidly measured variable for the approximate determination of the organic matter content of water samples. Some water samples may contain substances that are difficult to oxidize. In these cases, because of incomplete oxidation under the given test methods, COD values may be a poor measure of the theoretical oxygen demand. It should also be noted that the significance of the COD value depends on the composition of the water studied. COD is used as a measurement of pollutants in natural and waste waters to assess the strength of discharged waste such as sewage and industrial effluent waters. It is normally measured in both municipal and industrial wastewater treatment plants and gives an indication of the efficiency of the treatment process. The dichromate reflux method is preferred over procedures using other oxidants because of superior oxidizing ability, applicability to a wide variety of samples, and ease of manipulation. Oxidation of most organic compounds is 95-100% the theoretical value.
2.0
OBJECTIVE
To measure and determine the organic oxidizeable matters content of water samples.
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3.0
EQUIPMENTS AND MATERIAL 1. COD Reflux System – consisting Erlenmeyer flask (250 mL or 500 mL)
Figure 3.1 2. Burette A device used in analytical chemistry for the dispensing of variable, measured amounts of a chemical solution. Figure 3.2
3. Pipette For transferring or measuring out small quantities of liquid
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Figure 3.3 4. COD Vial
Figure 3.4 3.1
Reagent
1. Standard potassium dichromate 0.01667M •
1000mL distilled water
•
4.903g potassium dichromate (K2 CR2 O7) dryfor 1 hour at 150oC
•
167mL acid sulfuric (H2 SO4)
•
33.3g mercury sulfate (Hg SO4)
2. Sulfuric acid reagent •
5.5g silver sulfate (Ag SO4)
•
1kg acid sulfuric (H2 SO4)
3.
Standard ferrous ammonium sulfate
3
•
39.2g ammonium iron II sulfate 6-Hydrate {Fe(NH4)2(SO4)2. 6H2O}
•
20mL acid sulfuric (H2 SO4)
•
1L distilled water
4. Ferroin indicator
4.0
PROCEDURE 4.1
Standard Method 5220 C
4.2
Sampling Procedure The water sample was collected by using the plastic containers.
4.3
Laboratory Procedure 4.3.1
Cod Test
1. The substances was added in COD Vail followed the sequences below: a. Placed in COD Vail 1 and 2 with: -1.5 ml Potassium Dichromate Reagent - 3.5 ml Acid Sulfuric Reagent b. COD Vail No. 1 : 2.5 ml sample COD Vail No. 2 : 2.5 ml distilled water 2. Refluxing mixture placed at COD Reactor with temperature 150oC for 1 hour.
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Figure 4.1 3. After 1 hour, cool down the condenser with distilled water. Cooling down process continued to room temperature. 4. Disconnect reflux condenser. The solution transferred to the conical flask and mixed up with 150 mL distilled water. Added with 3 drops of ferroin indicator. 5. Titrated K2Cr2O7 with Ferrous Ammonium Sulfate (FAS). Initial and final reading recorded. The titration stopped when colour changed into red brick.
Figure 4.2 4.3.2
Titration Test
1. 5mL Postassium Dichromate K2Cr2O7solution
(0.04167 M) diluted with
10mL of distilled water into conical flask 250 mL. Swirled and mixed slowly.
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2. 30mL added to the concentration acid sulfuric H2SO4 slowly and carefully. Titrated with FAS titrant using 3 drops of ferroin indicator. Immediately stopped the titration
when the colour changed into red brick. 3. Initial and final reading recorded.
5.0
DATA RESULT Table 1 Data collection for COD test Normality
Sample
of FAS (N)
volume (mL)
0.1
2.5
Volume of FAS used in the original sample, b
in the blank sample, s
(mL)
(mL)
Initial value
Final value
Initial value
Final value
7.5
8.8
8.8
10.8
Ʃ(Vi-Vf) = 1.3
Ʃ(Vi-Vf) = 2.0
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6.0
DATA ANALYSIS
1. Which volume is larger, in the blank sample or in the original sample.
Based on the result obtained, the blank sample has the larger volume which is 2.0 mL compared with original sample which has 1.3 mL of volume.
2. If the original sample has to be diluted, how can you calculate the value of COD.
COD
8000(b s)n sample..volume
By using the formula : COD=
8000(2.0−1.3)(0.1) 2.5
COD = 224 mg/L
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7.0
DISCUSSION 1. What is the purpose of using blank sample in this experiment. Because COD measures the oxygen demand of organic compounds in a sample of water, it is important that no outside organic material be accidentally added to the sample to be measured. To control for this, a so-called blank sample is required in the determination of COD (and BOD, for that matter). A blank sample is created by adding all reagents such as acid and oxidizing agent to a volume of distilled water. COD is measured for both the water and blank samples, and the two are compared. The oxygen demand in the blank sample is subtracted from the COD for the original sample to ensure a true measurement of organic matter. 2. What is the objective of COD test and what is the different between COD and BOD test. The main objective of COD test is to determine the organic oxidizeable matters content of water samples. Differences between COD and BOD test COD test
BOD test
COD or Chemical Oxygen Demand is the Biochemical Oxygen Demand is supposed total measurement of all chemicals in the to measure the amount of food (or organic 8
water that can be oxidized.
carbons) that bacteria can oxidize.
COD is usually measured and the test is BOD usually takes 5 days. simple and easy to perform with the right equipment and can be done in 2 hours. 3. Why the COD’s value needs to be monitor for the polluted surface water such as in lakes and rivers as well as for wastewater. COD’s value needs to be monitor in order to supply data and information on the water quality such as lakes and river as well as for wastewater. Besides, to assess strength of discharged waste such as sewage and industrial effluent waters. 4. Give your opinion, if you need to compare the results of COD test to BOD and permanganate value (test COD by using potassium permanganate, KMnO4) tests. In my opinion, PV and COD are a measure of the amount of reduced compounds in a sample, which have been oxidized by a strong oxidizing agent. Although inorganic substances such as Fe2+, S2- may also be subject to oxidation, for most natural and industrial waters, the matter to be oxidized is organic in nature. Therefore these values can be used to characterize the organic load of a water. For COD determinations, the organic matter is almost completely oxidized (conversion >90%) due to the stringent oxidizing conditions (K2Cr2O7 in excess, 2 hours, 150oC, catalyst Ag). Moreover, the permanganate test is a much milder (KMnO4, 10 min., 120oC) and only the readily-oxidizable compounds will be converted. Conversion is only around 30-50% for natural waters with industrial waters conversions vary even more (10% - 80%).The BOD characterizes the biological biodegradability and is closely related to the PV, as it describes this part of the COD that is more readily oxidized. Depending on the type of water, the BOD5 (biodegradability in 5 days, expressed as ppm O 2) from 0.5 up to 3 times the PV (as ppm O2), and for most waters a value of 1.5 can be applied 5. Why the value of COD increase when the amount of pollutant increased in surface water. Because there is contain ammonia in surface water that can be organic origin, the product of decomposition plant and animal matter, or of inorganic origin, formed 9
due to chemical or biochemical reduction of nitrate and nitrite. Ammonia is a very unstable compound and easily undergoes nitrification. Ammonia is an indicator of pollution originating from soil (the excessive use of ammonarichnfertelizers) atmosphere and sewage.
6. Explain briefly the steps of COD measurement that you should follow during this experiment. 2.5mL of sample poured into test tube which contain 1.5mL of potassium
dichromate reagent and labelled as test tube 1. 2.5mL of distilled water poured into test tube which contain 3.5mL acid sulfuric
reagent and labelled as test tube 2. Both of the test tube mixed up properly and placed in COD Reactor with
temperature 150oC for 1 hour. After 1 hour, cooling down both of the test tube before pouring into conical flask.
Put the sample into conical flask and conical flask 1 and conical flask 2 labelled. Test tube 1 and test tube 2 added with distilled water and shaked two times and
poured into conical flask prepared. 3 drops of ferroin indicator added into each of the conical flask. Then, both of the solution titrated with K 2Cr2O7 with Ferrous Ammonium Sulfate
(FAS) until the colour changed into red brick. Initial and final reading recorded.
7. Briefly explain whether the value of COD obtain in this experiment are suitable to be discharged to the stream. Discharge COD limits can be different with different waste streams ( from specific industries) and COD limit values can be between 100 to 350mg/L depending on the waste stream. Based on the result obtained, 224mg/L is suitable to be discharged to the stream.
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8.0
CONCLUSION In conclusion, the objective of the experiment is achieved. The experiment was carried
out to measure and determine the organic oxidizeable matters content of water samples. Based on the experiment result obtained, the blank sample has the larger volume which is 2.0 mL compared with original sample which has 1.3 mL of volume. The COD value obtained is 224 mg/L. Discharge COD limits are different with different waste streams and COD limit values can be between 100 to 350mg/L depending on the waste stream. Based on the COD limits value, 224mg/L is suitable to be discharged to the stream.
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9.0 APPENDIX
Figure 9.1
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Figure 9.2
10.0
REFERENCES Website : http://www.environmentalleverage.com/BOD%20vs%20COD.htm Environmental Leverage, 2003.COD and BOD. http://vlab.amrita.edu/?sub=3&brch=272&sim=1413&cnt=2 vlab.amrita.edu,(2012). Chemical Oxygen Demand of Waste water.
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