Exp 1 Chemical Oxygen Demand



Determination of  amount of Chemical Oxygen Demand (COD) in water.


Chemical Oxygen Demand (COD)

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/Lit 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. Biochemical oxygen demand is a measure of the quantity of oxygen used by microorganisms (e.g. aerobic bacteria) in the oxidation of organic matter. Chemical Oxygen Demand


1. Open Reflux Titrimetric Method Principle01

In this method known amount of strong oxidizing agent is being added. Then reaction takes place to form CO2 and H2O. Then remaining amount of oxidizing agent is being determined by titration. The amount of oxidizing agent to be added depends upon the COD of sample which can roughly be known by knowing the source of sample. Chemical Oxygen Demand


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. Chemical Oxygen Demand

  • Erlenmeyer flask
  • Small beaker
  • Titration apparatus:
    •  25 or 50 mL burette, graduated in 0.1 mL
    • burette support02
    • 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:
    •   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 beads03
  • Fume hood

Reagents Chemical Oxygen Demand

  • Standard potassium dichromate solution, 0.25N or O.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

 Theory of Titration

The  Chemical Oxygen Demand COD  analysis,  by  the  dichromate  method,  is  more  commonly  used  to  control  Continuously monitor wastewater treatment systems, The COD of an effluent is usually higher than the BOD5 since the number of compounds that can be chemically oxidized is greater than those that can be degraded biologically, It is also common to make a correlation of BOD5 versus COD and then use the analysis of COD as a rapid means of estimating the BOD5 of a wastewater. This may be convenient since only about three hours are needed for a COD determination while a BOD5 takes at least 5 days. However, this procedure can be used only for specific situations where there is low variability in the composition of a wastewater, and the results of a system cannot be used reliably in other cases. Chemical Oxygen Demand

The method of Chemical Oxygen Demand COD which uses dichromate as oxidant is carried out by heating under total reflux a wastewater sample of known volume in an excess of potassium dichromate (K2Cr2O7) in presence of sulphuric Acid (H2SO4) for a fixed period (usually two hours) in presence of silver sulphate (Ag2SO4) as catalyst. The organic matter present is oxidized and, as a result, the dichromate ion (orange colour) is consumed and replaced by the chromic ion (green colour):
Cr2O7-2 + 14H+ + 6e       =    2Cr3+ + 7H2O

The COD is calculated by titrating the excess of dichromate or by spectrophotometrically measuring the Cr+3 ions at 606 nm. Another possibility is to measure the excess dichromate spectrophotometrically at 440 nm. Titration requires more work but is considered more precise.

The presence of silver sulphate as catalyst is needed for complete oxidation of aliphatic carbon compounds. The standard method implies cooling of the sample after the two hour digestion period, adding a few drops of indicator (ferroin) solution and titrating the excess dichromate with a solution of ferrous ammonium sulphate of known concentration, until the colour changes from brilliant green to reddish brown. The titration reaction corresponds to the oxidation of the ferrous ammonium sulphate by the dichromate:

Cr2O7-2 + 14H+ + 6Fe+2            =                 2Cr3+ + 6Fe+3 + 7H2O

The change in colour corresponds to the formation of the complex ferrous ion phenanthroline which occurs when all the dichromate ion has been reduced to Cr3+.

(Fe(C12H8N2)3)3+ + e        =            (Fe(C12H8N2)3)2+

Ferric Phenanthroline                      Ferrous Phenanthroline

(Green Blue)                                   (Reddish Brown)


A common interference factor in the COD test is the presence of chlorides. If seawater is used at some point in the processing or salt brines are used for some “curing” operations, chlorides will most probably appear in the wastewater causing interference while they are oxidized by the dichromate: Chemical Oxygen Demand

Cl + Cr2O72- + 14H+             =                    3Cl2 + 2Cr3+ + 7H2O

This interference causes erroneously high values of COD which can be prevented by the addition of mercuric sulphate (HgSO4) which reacts to form mercuric chloride and precipitates:

Hg2+ + 2Cl           =         HgCl2


1)     Place 50ml sample in 500ml refluxing flask (for samples with COD>900mg/L use a smaller sample diluted to 50ml).
2)     Add 1g HgSO4  and several glass beeds.
3)     Add slowly 5ml H2SO4 reagent while mixing to dissolve HgSO4
4)     Cool while mixing to avoid the loss of volatile materials.
5)     Add 25 ml 0.25N K2  Cr2O7  solution and mix.
6)     Attach the flask to the condenser and turn on cooling water.
7)     Add remaing H2SO4 (70ml) through open end of the condenser continue mixing while adding H2SO4.
8)     Reflux the mixture for 2 hrs and cool to room temperature, after diluting the mixture to about twice its volume with distilled water.
9)     Titrate excess of K2 Cr2O7 with Ferrous ammonium sulfate using 2,3 drops of ferrion indicator. The end point will be from blue green to reddish brown.
10) Reflux and titrate in the same manner a blank containing the reagents and the voume of the distilled water will be equal to that of sample. Chemical Oxygen Demand

Sr. #

Description of Sample

Volume of Titrant used for sample (ml)

Volume of Titrant used for Blank (ml)

COD (mg/liter)


Waster Water





Waste Water




A = mL of titrant used for Blank
B = mL of titrant used for Sample
N = normality of ferrous ammonium sulfate (FAS) = 0.25N
8000 = Equivalent Wt. of Oxygen x 1000 


The experiment was performed successfully and COD values have been determined in the above table. COD/BOD = 1.5 so we can also find BOD from here which comes out to be 174.67mg/L for 1st  sample and 120 mg/L for 2nd sample. When titrating, be certain that the burette is clean and free of air bubbles.


1)    Compare BOD and COD.

Chemical oxygen demand (COD) is a measure of the capacity of water to consume oxygen during the decomposition of organic matter and the oxidation of inorganic chemicals such as ammonia and nitrite. COD measurements are commonly made on samples of waste waters or of natural waters contaminated by domestic or industrial wastes. Chemical oxygen demand is measured as a standardized laboratory assay in which a closed water sample is incubated with a strong chemical oxidant under specific conditions of temperature and for a particular period of time. A commonly used oxidant in COD assays is potassium dichromate (K2Cr2O7) which is used in combination with boiling sulfuric acid (H2SO4). Because this chemical oxidant is not specific to oxygen-consuming chemicals that are organic or inorganic, both of these sources of oxygen demand are measured in a COD assay.

Chemical oxygen demand is related to biochemical oxygen demand (BOD), another standard test for assaying the oxygen-demanding strength of waste waters. However, biochemical oxygen demand only measures the amount of oxygen consumed by microbial oxidation and is most relevant to waters rich in organic matter. It is important to understand that COD and BOD do not necessarily measure the same types of oxygen consumption. For example, COD does not measure the oxygen-consuming potential associated with certain dissolved organic compounds such as acetate. However, acetate can be metabolized by microorganisms and would therefore be detected in an assay of BOD. In contrast, the oxygen-consuming potential of cellulose is not measured during a short-term BOD assay, but it is measured during a COD test.


2)    Why COD values are always higher then BOD values?

Biochemical oxygen demand (BOD) is a measure of the amount of oxygen that bacteria will consume while decomposing organic matter under aerobic conditions or biochemical oxygen demand only measures the amount of oxygen consumed by microbial oxidation and is most relevant to waters rich in organic matter Whereas Chemical oxygen demand (COD) does not differentiate between biologically available and inert organic matter, and it is a measure of the total quantity of oxygen required to oxidize all organic material into carbon dioxide and water. COD values are always greater than BOD values, but COD measurements can be made in a few hours while BOD measurements take five days.


A Chemical Oxygen Demand COD test measures all organic carbon with the exception of certain aromatics (benzene, toluene, phenol, etc.) which are not completely oxidized in the reaction. COD is a chemically chelated/thermal oxidation reaction, and therefore, other reduced substances such as sulfides, sulfites, and ferrous iron will also be oxidized and reported as COD. Moreover COD measures some additional organic matter such as cellulose, whereas BOD or Biological Oxygen Demand is supposed to measure the amount of food (or organic carbons) that bacteria can oxidize. So COD values are always higher than the BOD values.

3)    Write the applications of COD data to environmental Engineering?    

Chemical Oxygen Demand COD test is a measure of the relative oxygen-depletion effect of a waste contaminant. It has been widely adopted as a measure of pollution effect. To determine the amount of pollution in a water stream to try to control and limit the amount of chemicals that can pollute the lakes and rivers if left in a final effluent or discharge stream. Some Municipalities want to measure the amount of chemicals in the incoming stream in order to asses surcharges as a way of measuring how much additional treatment their plant will have to do in order to get the incoming water clean. It is used in Process Control in Influent/effluent for removal efficiencies.

Chemical Oxygen Demand COD is extensively used in analysis of industrial waste. It is particularly valuable in surveys designed to determine the losses of sewage. Results are obtained within short time and control measures can be taken on the same day. It is very useful in finding out the toxic condition and presence of biologically resistant organic substance.Chemical oxygen demand is a vital test for assessing the quality of effluents and waste waters prior to discharge. The Chemical Oxygen Demand (COD) test predicts the oxygen requirement of the effluent and is used for monitoring and control of discharges, and for assessing treatment plant performance. The impact of an effluent or waste water discharge on the receiving water is predicted by its oxygen demand. This is because the removal of oxygen from the natural water reduces its ability to sustain aquatic life. The COD test is therefore performed as routine in laboratories of water utilities and industrial companies. Chemical Oxygen Demand

4)    Write the NEQS for COD.

National Environmental Quality Standards for Municipal and Liquid Industrial Effluents (mg/L, Unless Otherwise Defined)

Sr. No


Existing Standards

Revised Standards

Into Inland Water

Into Sewage Treatment

Into Sea


Chemical Oxygen Demand (COD)1

150 mg/l




5)    What would be the role of Ag2SO4  in COD determination?

Silver Sulphate catalyses the reaction and also assists in the oxidation of the nitrogen compounds. The secondary catalyst, Silver Sulfate (AgSO4) assists oxidization of straight-chain hydrocarbons such diesel fuel and motor oil. The reaction between chloride and silver sulfate creates silver chloride (AgCl). The reduction of silver sulfate correspondingly reduces the activity needed to  oxidize straight chain hydrocarbons, a negative interferent one would think. Chemical Oxygen Demand

8 Replies to “Exp 1 Chemical Oxygen Demand”

  1. what is the mathematical formula by which BOD can be converted into COD or vice versa??

  2. What are steps that can be taken to reduce oxygen loss while conducting the experiment?