In COD back titration, which chemical is used to titrate the excess oxidant after digestion?

In COD back titration, which chemical is used to titrate the excess oxidant after digestion?

• A. Ferrous Ammonium Sulfate
• B. Potassium Dichromate
• C. Hydrochloric acid
• D. Sodium thiosulfate

Answer: (A)

In COD back titration, you add an excess of oxidant to the sample so all organics are oxidized. After digestion, you titrate the leftover oxidant with a reducing titrant to figure out how much oxidant was actually consumed by the organics. The reducing titrant used here is ferrous ammonium sulfate, which provides Fe2+ that reduces the remaining dichromate under acidic conditions. The reaction converts dichromate (Cr2O7^2−) to Cr3+ while Fe2+ becomes Fe3+. By measuring how much Fe2+ is used to consume the excess oxidant, you can calculate how much oxidant reacted with the organics, and thus determine COD. Key context: the oxidant added during digestion is potassium dichromate, so it’s the remaining amount that is back-titrated with ferrous ammonium sulfate. The other options don’t fit this role: hydrochloric acid helps create the acidic digestion medium but isn’t a titrant, and sodium thiosulfate is used in different redox titrations (not for back-titrating dichromate in COD).

In COD back titration, you add an excess of oxidant to the sample so all organics are oxidized. After digestion, you titrate the leftover oxidant with a reducing titrant to figure out how much oxidant was actually consumed by the organics. The reducing titrant used here is ferrous ammonium sulfate, which provides Fe2+ that reduces the remaining dichromate under acidic conditions. The reaction converts dichromate (Cr2O7^2−) to Cr3+ while Fe2+ becomes Fe3+. By measuring how much Fe2+ is used to consume the excess oxidant, you can calculate how much oxidant reacted with the organics, and thus determine COD.

Key context: the oxidant added during digestion is potassium dichromate, so it’s the remaining amount that is back-titrated with ferrous ammonium sulfate. The other options don’t fit this role: hydrochloric acid helps create the acidic digestion medium but isn’t a titrant, and sodium thiosulfate is used in different redox titrations (not for back-titrating dichromate in COD).