@ shanvi p In COD determination potassium dichromate (K2Cr2O7) is added in excess in an acidic medium along with addition of silver sulfate (Ag2SO4) as a catalyst, and mercuric sulfate (HgSO4)to remove interference of chlorides. Dichromate oxidizes organic and inorganic matter in the sample, and is itself reduced by the impurities from Cr+6 to Cr+3. The test is performed at 150°C under total reflux for 2 hours. After digestion, excess potassium dichromate is titrated with Mohr's salt using ferroin as indicator. The solution color changes from green to red. (Mohr's salt acts as reducing agent for excess potassium dichromate). Hope you got your answer
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