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Analysis of Chemical Oxygen Demand in Waste Water Samples
Analysis of Chemical Oxygen Demand in Waste Water Samples

Theory of Chemical Oxygen Demand

Using a potent oxidizing agent such as potassium dichromate or potassium permanganate, 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.

Samples of wastewater or natural waters tainted by household or industrial pollutants are frequently used 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 do not 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 combined with Ferrous ammonium sulfate, a reducing agent, reacts with excess dichromate in the titration method used to measure COD. The surplus dichromate is transformed into its trivalent form as the ferrous ammonium sulfate (FAS) is gradually added.

Chemical oxygen demand of waste water can also be analyzed by electro chemical sensor and ion selective electrode for quick and accurate results for Biological oxygen demand and Chemical oxygen demand.

CHEMICALS / REAGENTS

Sulphuric Acid 98 %(sp. gr 1.84)

Mercuric Sulphate Powdered (HgS04)

Sulphamic Acid (98%)For removal of nitrite interferences.

Potassium Dichromate Standard

Solution (0.25 N): Dissolve 12.259 g of potassium dichromate (K2Cr207) primary standard grade, previously dried at 103 oc 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 mono-hydrate together with 0.70 g of ferrous sulphate (FeS04.7H20) in 100 mL of water.

Ferrous Ammonium Sulphate (FAS) (0.25 N)

Dissolve 98 g of ferrous ammonium sulphate in purified demineralized water. Add 20 mL of Sulphuric acid (3.1) cool and dilute to I L.

To standardize, dilute 25.0 mL of 0.25 N potassium dichromate solution (K2Cr207) to about 250 mL in a 500 mL conical flask.

Add 20 mL of Sulphuric acid (3.1) and allow this solution to cool. Titrate with ferrous ammonium sulphate solution (3.3) using the phenanthroline ferrous sulphate indicator

Calculate normality of ferrous ammonium sulphate as follow;

Normality of FAS = Volume of K2Cr207 x Normality of K2Cr207 / Volume of FAS

Sulphuric Acid-Silver Sulphate Solution

Dissolve 15 g of powdered silver sulphate (Ag2S04) in 300 mL of concentrated Sulphuric Acid (3.1) and dilute to I L with concentrated Sulphuric Acid.

Potassium Acid Phthalate Standard

Solution (1000 mg/L COD): Dissolve 0.851 g of potassium acid phthalate (KC8H504) primary standard grade in purified demineralized water and dilute to 1000 mL with purified demineralized water in a volumetric flask. I mL= I mg COD 4.0

Analytical Procedure

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 reagents blank.

Note: If the COD is, more than 800 mg/L proceed with a smaller sample volume and if the COD is, less than 50 mg/L use 0.025 N potassium dichromate and Ferrous Ammonium Sulphate solutions.

Place the reflux flask in ice bath; add 0.2 g of powdered mercuric sulphate and 0.2 g of Sulphamic acid.

Add 5 mL of concentrated Sulphuric acid and few glass beads. Mix well to complete dissolution.

With flask still in the ice bath, add slowly and with swirling 25 of 0.25 N standard potassium dichrolnate (3.4).

With flask still in the ice bath, add 70 mL Sulphuric acid-silver sulphate solution slowly such that the solution temperature is maintained as low as possible, preferably below 40 oc. Mix well the content of reflux flask.

Attach the reflux flasks to the condensers and start the flow of cold water.

Apply heat to flask and reflux for 2 hrs place a small cover over the open end of each condenser to intrusion of foreign material.

Allow the flasks to cool and 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 sulphate (Ferroin) indicator and titrate both reagents blank and sample flasks with 0.25 N ferrous ammonium sulphate (FAS) solution, at end point color changed from blue-green to reddish brown.

To check the validity of test results, make standard determination using potassium acid phthalate solution . A COD of 500 mg/L should obtained on 25 mL aliquot of the standard solution diluted to 50 mL with demineralized water.

CALCULATION

COD as mg of sample = (A- B) x 0.25 N x 8x 1000 / Volume of sample

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