1.0
method is used, the amount of Cr3+ produced is determined. The COD
TITLE
reagent also contains silver and mercury ions. Silver is a catalyst, and Determination of Chemical Oxygen Demand (COD) In Water and
mercury is used to complex chloride interferences. Test results for the 3 to
Wastewater
150 mg/L range are measured at 420 nm. Test results for the 20 to 1, 5000 and the 200 to 15,000 mg/L range are measured at 620 nm.
2.0
OBJECTIVE
To determine the amount of organic pollutant in water and wastewater 3.0
INTRODUCTION 4.0
MATERIALS & METHODOLOGY
Chemical oxygen demand (COD) is used as a measure of oxygen requirement of a sample that is susceptible to oxidation by strong chemical
COD Digestion Reactor
oxidant. The dichromate reflux method is preferred over procedures using
Spectrophotometer, HACH DR/2400 @ DR/2800
other oxidants (e.g. potassium permanganate) because of its superior
COD Digestion Reagent Vial LR @ HR
oxidizing ability, applicability to a wide variety of samples and ease of
COD rack
manipulation. Oxidation of most organic compounds is 95-100% of the
Volumetric pipette, 2 mL
theoretical value.
Paper towel/Tissue
The mg/L COD results are defined as the mg of O 2 consumed per liter of sample under conditions of this procedure. In this procedure, the
1.
Homogenize 100 mL of sample for 30 seconds in a blender.
sample is heated for two hours with a strong oxidizing agent, potassium
*For samples containing large amounts of solids, increase the
dichromate. Oxidizable organic compounds react, reducing the dichromate
homogenization time.
ion (Cr2O72-) to green chromic ion (Cr3+). When the 3-150 mg/L + 6
2.
For the 200-15,000 mg/L range or to improve accuracy and
colorimetric or titrimetric method is used, the amount of Cr remaining is
reproducibility of the other ranges, pour the homogenized sample
determined. When the 20-1,500 mg/L or 200-15,000 mg/L colorimetric
into a 250-mL beaker and gently stir with a magnetic stir plate.
*If the sample does not contain suspended solids, omit step 1
13.
and step 2. 3. 4.
one to remove fingerprints or other marks.
Turn on the COD Reactor. Preheat to 150°C. Place the safety
14.
Install the 16-mm adapter. Place the blank into the adapter.
shield in front of the reactor.
15.
Touch Zero. The display will show: 0 mg/L COD.
Remove the caps from two COD Digestion Reagent Vials.
16.
When the timer beeps, place the sample vial into the adapter.
*Be sure to use vials for the appropriate range. 5.
Touch Read. Results will appear in mg/L COD.
Hold one vial at a 45-degree angle. Use a clean volumetric pipette to add 2.00 mL of sample to the vial. This is the prepared sample.
6.
Clean the outside of the vials with a damp towel followed by a dry
5.0
RESULTS & DISCUSSIONS
Hold a second vial at a 45-degree angle. Use a clean volumetric pipette to add 2.00 mL de-ionized water to the vial. This is the
Sample (different sources)
COD, mg/L
blank. 7.
Cap the vials tightly. Rinse them with de-ionized water and wipe with a clean paper towel.
8.
Hold the vials by the cap over a sink. Invert gently several times to mix. Place the vials in the preheated COD Reactor. *The sample vials will become very hot during mixing.
9.
Heat the vials for two hours.
10.
Turn the reactor off. Wait about 20 minutes for the vials to cool to
120°C or less. 11. 12.
Your discussion may include the following questions:
1. Explain the differences of COD values found in the different sources of sample.
2. State the importance to determine COD in water and wastewater,
Invert each vial several times while still warm. Place the vials into
and interpret your test results based on Environment Quality Act
a rack and cool to room temperature.
1974 Standard A or B, Sewage & Industrial Effluent.
Touch Hach Programs. Select program 430 COD LR (Low Range) or 435 COD HR (High Range/High Range Plus). Touch Start.
3. Write the chemical reactions involved in the digestion process in determining COD.
4.
A sample is collected for COD analysis and it is to be performed on the following day. Describe two ways which the sample can be preserved. Why it is necessary to preserve the sample?
5. Briefly explain a chemical treatment to reduce COD. 6.0
REFERENCES
Andrew, D. E., Lenore, S. C., Eugene, W. R. & Arnold, E. G. 2005. Standard Methods For The Examination of Water And Wastewater. 21st Edition. USA. Howard, S. P., Donald, R. R. & George, T. 1985. Environmental Engineering. McGraw-Hill. Singapore. Kasmawati, M. & Lee, K. K. 2007. Methods of Analysis for Water and Wastewater. UiTM. Shah Alam.
