This method describes how to determine the chemical oxygen demand in wastewater through oxidation with potassium dichromate.
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The chemical oxygen demand (COD) is determined through a reaction with strong oxidizing agents, in which the consumption of an oxidizing titrant solution serves as a measure of the content of oxidizable impurities present in the sample. Almost all water-soluble substances and many undissolved organic substances are oxidized in the reaction. A volumetric standard solution of potassium dichromate is mixed with a strong sulfuric acid solution; silver sulfate is utilized as a catalyst, and mercury(II) sulfate serves as a masking agent for chlorides.
Potassium dichromate is added to the acidified sample as an oxidizing agent along with silver sulfate as a catalyst; mercury sulfate is also added to prevent the formation of elemental chlorine from chlorides. After oxidation of the organic substances in the sample (the dichromate ion is reduced to the chromium(III) ion in an acidic solution), the chromate required to achieve this is determined through reverse titration of the excess potassium dichromate with iron(II) solution (adjusted) against ferroin as an indicator [1].
Cr2O72- + 6 e- + 14 H3O+ → 2 Cr3+ + 21 H2O
Cr2O72- + 6 Fe2+ + 14 H3O+ → 2 Cr3+ + 6 Fe3+ + 21 H2O