Waste Water Analysis for Chemical Oxygen Demand COD in Wastewater
Theory of Chemical Oxygen Demand COD
The method involves the use of a potent oxidizing agent, such as potassium dichromate or potassium permanganate. The chemical oxygen demand, or COD, is a measurement of the ability of water to take up oxygen during the breakdown of organic materials. The amount of organic matter in a sample can be inferred indirectly using the chemical oxygen demand (COD) method.
Sample of Wastewater for Chemical Oxygen Demand COD
Researchers frequently use samples of wastewater or natural waters tainted by household or industrial pollutants for COD determinations. It is one of the most crucial criteria for evaluating pollution in water monitoring since it can indicate the level of contamination in the water. Microbes in the receiving water will eat the organic matter. If water treatment facilities fail to lower the organic content of wastewater before it reaches natural waters.
As a result, as part of the decomposition of organic waste, these microorganisms will also consume the oxygen in the receiving water. This reduction in oxygen occurs when combined with ferrous ammonium sulfate, a reducing agent. That reacts with excess dichromate in the titration method used to measure COD. The surplus dichromate is transformed into its trivalent form as ferrous ammonium sulfate (FAS) is gradually added to the solution.
Chemical oxygen demand of wastewater can also be analyzed by electrochemical sensors and ion-selective electrodes for quick and accurate results for biological oxygen demand and chemical oxygen demand.
CHEMICALS / REAGENTS Required for Chemical Oxygen Demand COD
Sulphuric Acid 98% (sp. gr. 1.84)
Mercuric Sulphate Powdered (HgSO₄)
Sulfamic acid (98%) for the removal of nitrite interferences.
Potassium Dichromate Standard
Solution (0.25 N): Dissolve 12.259 g of potassium dichromate (K₂Cr₂O₇) primary standard grade, previously dried at 103°C for 2 hrs in purified demineralized water, and make up to 1000 mL in a volumetric flask with purified demineralized water.
Phenanthroline Ferrous Sulphate (Ferroin)
Indicator Solution: Dissolve 1.48 g of 1,10-(ortho)-phenanthroline monohydrate together with 0.70 g of ferrous sulfate (FeSO₄·7H₂O) in 100 mL of water.
Ferrous Ammonium Sulphate (FAS) (0.25 N)
Dissolve 98 g of ferrous ammonium sulfate in purified demineralized water. Add 20 mL of sulfuric acid (3.1), cool, and dilute to 1 L.
To standardize, dilute 25.0 mL of 0.25 N potassium dichromate solution (K₂Cr₂O₇) to about 250 mL in a 500 mL conical flask.
Add 20 mL of sulfuric acid (3.1) and allow this solution to cool. Titrate with ferrous ammonium sulphate solution (3.3) using the phenanthroline ferrous sulphate indicator
Calculate the normality of ferrous ammonium sulfate as follows:
Normality of FAS = Volume of K₂Cr₂O₇ x Normality of K₂Cr₂O₇ / Volume of FAS
Sulphuric Acid-Silver Sulphate Solution
Dissolve 15 g of powdered silver sulfate (Ag₂SO₄) in 300 mL of concentrated sulfuric acid (3.1) and dilute to 1 L with more concentrated sulfuric acid.
Potassium Acid Phthalate Standard
Solution (1000 mg/L COD): Dissolve 0.851 g of potassium acid phthalate (KC₈H₅O₄) primary standard grade in purified demineralized water and dilute to 1000 mL with purified demineralized water in a volumetric flask. 1 mL = 1 mg COD 4.0
Analytical Procedure of Chemical Oxygen Demand
Transfer 50 mL of sample containing < 800 mg/L COD to a 500 mL reflux flask and 50 mL of demineralized water to another reflux flask to be used as a reagent blank.
Note: If the COD is more than 800 mg/L, proceed with a smaller sample volume. If the COD is less than 50 mg/L, use 0.025 N potassium dichromate and ferrous ammonium sulfate solutions.
Place the reflux flask in an ice bath; add 0.2 g of powdered mercuric sulfate and 0.2 g of sulfamic acid.
Add 5 mL of concentrated sulfuric acid and a few glass beads. Mix well to complete dissolution.
With the flask still in the ice bath, slowly add and swirl 25 mL of 0.25 N standard potassium dichromate.
With the flask still in the ice bath, add 70 mL of the sulfuric acid-silver sulfate solution. But it should be done slowly, so that the solution temperature is maintained as low as possible, preferably below 40°C. Mix well the content of the reflux flask.
Attach the reflux flasks to the condensers and start the flow of cold water.
Digestion of Chemical Oxygen Demand COD
However, apply heat to the flask and reflux for 2 hrs. Place a small cover over the open end of each condenser to prevent the intrusion of foreign material.
Allow the flasks to cool, and then wash down the condenser with about 25 mL of demineralized water before removing the flask. Dilute the solution of flasks to about 300 mL with demineralized water and cool to room temperature.
Add 8–10 drops of phenanthroline ferrous sulfate (ferroin) indicator. Titrate both reagents in blank and sample flasks with 0.25 N ferrous ammonium sulfate (FAS) solution. Therefore, the endpoint color changed from blue-green to reddish brown.
To verify the validity of test results, make a standard determination using a potassium acid phthalate solution. A COD of 500 mg/L should be obtainable on a 25 mL aliquot of the standard solution and diluted to 50 mL with demineralized water.
CALCULATION of Chemical Oxygen Demand
COD as mg of sample = (A – B) x 0.25 N x x 1000 / Volume of sample