Lab 5:
Chemical Oxygen Demand


Reading Assignment

Read pages 58-59 and 106 in Basic Laboratory Procedures for Wastewater Examination.



Introduction

The Chemical Oxygen Demand, or COD, is a measurement of the amount of material that can be oxidized (combined with oxygen) in the presence of a strong chemical oxidizing agent.  Since the COD test can be performed rapidly, it is often used as a rough approximation of the water's BOD, even though the COD test measures some additional organic matter (such as cellulose) which is not normally oxidized by biological action.  As with the BOD test, the COD test is reported as mg/L of oxygen used. 

The table below shows the normal range of COD found in various kinds of domestic wastewater.  Keep in mind that the addition of industrial waste can cause these values to vary widely. 

Source
Normal COD range, mg/L
Plant influent
300 - 700
Primary effluent
200 - 400
Trickling filter effluent
45 - 130
Activated sludge effluent
30 - 70
Advanced waste treatment effluent
5 - 15

 

Equipment

Caution:  The presence of minute traces of organic matter on the equipment will cause large errors in the test results.  So clean all equipment thoroughly before using.
  • Erlenmeyer flask
  • Small beaker
  • Titration apparatus:
    • 25 or 50 mL burette, graduated in 0.1 mL
    • burette support
    • 100 mL graduated cylinder
    • rubber-tipped stirring rod, or magnetic stirrer and stir bar
    • white porcelain evaporating dish, 4.5 inches in diameter
  • Reflux apparatus:
Diagram of a reflux apparatus
    • 500 or 250 mL erlenmeyer flasks with ground-glass 24/40 neck
    • 300-mm jacket Liebig, West, or equivalent condenser with 24/40 ground-glass joint
    • hot plate with sufficient power to produce at least 1.4 W/cm2 of heating surface
  • Blender
  • Pipets
  • Glass beads
  • Fume hood


Reagents
  • Standard potassium dichromate solution, 0.250N or 0.025N 
  • Sulfuric acid reagent containing silver sulfate catalyst 
  • Standard ferrous ammonium sulfate titrant 
  • Ferroin indicator solution 
  • Mercuric sulfate crystals 
  • Sulfamic acid 
  • Concentrated sulfuric acid
  • Distilled water


Laboratory Procedure
.
1. Standardize the ferrous ammonium sulfate. 


a. Place 10.0 mL of 0.250 N potassium dichromate solution in a 500 mL erlenmeyer flask. 

b. Dilute the potassium dichromate solution to 100 mL with distilled water. 

c. Add 30 mL of concentrated sulfuric acid.  Add the acid slowly while mixing the solution. 

d. Cool the solution to room temperature. 

e. Add 2 or 3 drops of ferroin indicator. 

f. Titrate with ferrous ammonium sulfate until the solution changes from a blue-green color to a reddish-brown color.  Record the amount of titrant used in the Data section.

g. Calculate the normality of the ferrous ammonium sulfate using the following formula.  Record the result in the Data section:

 

2. Prepare the sample water.


a. If the sample is not homogeneous, mix it in a blender. 

b. Pipet a suitable amount of the sample into a 500 mL refluxing flask. 

For samples with a COD of between 50 mg/L and 900 mg/L, use 50.0 mL of sample water.  For samples with a COD greater than 900 mg/L, use a smaller portion, diluted to 50.0 mL.  For samples with a COD less than 50 mg/L, use the alternative procedure listed at the end of this laboratory procedure.  Record the amount of sample used in the Data section. 

c. Add 1 gram of mercuric sulfate to the sample. 

d. Add several glass beads to the solution.

e. Very slowly, add 5.0 mL of sulfuric acid reagent.  Swirl the flask while adding the reagent to help dissolve the mercuric sulfate. 

f. Add 25.0 mL of the 0.250 N potassium dichromate solution and mix. 

3. Prepare a blank.

a. Pipet a volume of distilled water equal to that of the sample into a 500 mL refluxing flask. 

b. Add 1 gram of mercuric sulfate to the distilled water.

c. Add several glass beads to the solution.

d. Very slowly, add 5.0 mL of sulfuric acid reagent.  Swirl the flask while adding the reagent to help dissolve the mercuric sulfate. 

e. Add 25.0 mL of the 0.250 N potassium dichromate solution and mix. 

4. Reflux both the sample flask and the blank flask. 

a. Attach the sample flask and the blank flask to separate condensers and turn on the cooling water.

b. Add 70 mL of sulfuric acid reagent to each flask through the open end of the condenser.  Swirl the flask several times while adding the sulfuric acid reagent. 

Add reagent through the condenser.

Caution: This step in the procedure can be extremely hazardous since a violent reaction may occur.  Perform this step and the reflux step in a laboratory fume hood.  Be sure to mix the solution thoroughly before applying heat in the next step or local heating of the flask bottom may cause a blowout of the flask contents. 

c. Cover the end of the condenser with a small beaker to prevent foreign material from entering the reflux mixture.

d. Turn on the hot plate and reflux (boil) the mixture for two hours.

e. Cool the flask and condenser. 

f. Wash down the inside of the condenser with distilled water and then remove the condenser.

5. Titrate both the sample flask and the blank flask. 

a. Dilute the contents of each flask to approximately twice its volume with distilled water. 

b. Add 2 or 3 drops of ferroin indicator to each flask. 

c. Titrate each flask with ferrous ammonium sulfate until the contents change color from blue-green to reddish-brown and the color change remains for 1 minute or longer.  Record the amount of titrant used in the Data section. 

6. Calculate the COD of the sample using the following formula:



Where:

A = mL of titrant used for sample

B = mL of titrant used for blank

M = normality of ferrous ammonium sulfate


Alternative Procedure:

When your sample contains less than 50 mg/L COD, then you must change a few parts of the usual procedure.  First, in steps 1.a. and 2.f., dilute the dichromate solution to a normality of 0.0250 N.  (You will also have to change the normality in the calculation in step 1.g.)  Second, the ferrous ammonium sulfate solution used as a titrant in steps 1.f. and 5.c. will also have to be diluted to 0.025 N. 



Data


Ferrous ammonium sulfate standardization
Volume of 0.250 N potassium dichromate solution (mL)
Volume of ferrous ammonium sulfate (mL)
Normality of ferrous ammonium sulfate
10.0



.

COD calculations

Sample Source
Sample Volume (mL)
Titrant Volume (mL)
COD (mg/L)




blank
0




Sources

American Public Health Association, American Water Works Association, and Water Environment Federation.  1998.  Standard Methods for the Examination of Water and Wastewater.  American Public Health Association, Washington, D.C.