It consists of the amount of oxygen required for the organic matter to be oxidized by a chemical oxidant (usually potassium dichromate in acid medium). The sample is digested for 2 h at 150°C. COD is indirectly measured through Cr(VI) or Cr(III) contents by titration or UV–vis spectrophotometry.
What type of titration is used in COD test?
Titrimetric Method of COD
In the titration method for determining COD, the excess dichromate is reacted with a reducing agent, ferrous ammonium sulfate. As the ferrous ammonium sulfate (FAS) is added slowly, the excess dichromate is converted into its trivalent form.
Which method is used for COD?
The traditional chemical oxygen demand (COD) analysis method is the wet chemistry method. This involves a two hour digestion at high heat under acidic conditions in which potassium dichromate acts as the oxidant for any organic material present in a water sample.
What are the two methods of determining the COD value of a water sample?
There are two methods available for COD determination namely open reflux and closed reflux.
Is used for titration in COD experiment?
For all samples the COD can be calculated by titrating the excess dichromate with ferrous ammonium sulfate (FAS), using a ferroin Page 2 indicator. The titration procedure is similar to the standard spectrophotometric method with respect to sample treatment and digestion.
How COD can be determined?
A typical determination of the COD is to introduce an excess of the dichromate ion. After the reaction is completed the excess amount of dichromate is measured by titration with ferrous ammonium sulfate. COD is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution.
Why do we use reflux method for determining COD?
It is used to indicate organic matter contamination and it helps in knowing overall organic load to the receiving body. There are two methods for COD determination. The first method: open reflux method is suitable for a wide range of wastes where large volume of sample is required (for samples with COD= 50 mg O2/L).
How do you analyze COD in water?
COD testing typically involves digestion of the water sample in a sealed vial with potassium dichromate and sulfuric acid at 150C for 2 hours. Vials are read in a spectrophotometer to determine results. Of the two tests, COD analysis is more commonly performed as it takes less time and is more replicable.
What is COD and How Is It measured?
COD is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution. It is commonly expressed in mass of oxygen consumed over volume of solution, which in SI units is milligrams per liter (mg/L). A COD test can be used to easily quantify the amount of organics in water.
How do you test for BOD and COD in water?
Biochemical oxygen demand test procedure
- The water shouldn’t have chlorine. If chlorine is present in the water, a dichlorination chemical like sodium sulfate can be applied.
- The pH of the water sample must fall in the 6.5 -7.5 range.
- The water should have a microbiological population.
How do you do the COD test?
Test for Chemical Oxygen Demand:
- Take 10 ml of sample into a round bottom reflex flask.
- Add some glass beads to prevent the solution from bumping into the flask while heating.
- Add 1 ml of Mercury sulfate (HgSO4) solution to the flask and mix by swirling the flask.
- Add 5 ml of Potassium dichromate (K2Cr2O7) solution.
What is closed reflux titrimetric method?
5220 C. Closed Reflux, Titrimetric Method
Most types of organic matter are oxidized by a boiling mixture of chromic and sulfuric acids. A sample is refluxed in strongly acid solution with a known excess of potassium dichromate (K2Cr2O7).
How does Winkler method work?
The Winkler method is done by noting a color change when titrating a fresh water sample.
Dissolved oxygen analysis can be used to determine:
- the health or cleanliness of a lake or stream,
- the amount and type of biomass a freshwater system can support,
- the amount of decomposition occurring in the lake or stream.
Which is the better method to determine oxygen demand in sewage water BOD or COD?
BOD measures the amount of oxygen required by the aerobic organisms to decompose organic matter and COD measures the oxygen required to decompose organic and inorganic constituents present in the wastewater by chemical reaction. Hence, the value of COD is greater than BOD.
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