Discussion
During the determination of COD, sulphanilic acid is added, because
Answer: C
The keyed answer is that the added acid removes the nitrite interference in the COD test, which is the closest of the four statements.
Nitrite is oxidised by dichromate and would add to the measured oxygen demand, so a small amount of a sulphonic acid reagent (sulfamic acid in the standard method) is added to destroy it. The keyed wording says the nitrite is oxidised to nitrate, which is the intended point: the nitrite no longer consumes dichromate. Silver sulphate, not this acid, acts as the catalyst, and mercuric sulphate, not this acid, binds the chloride, so the other statements do not apply. The acid does not keep the mixture acidic (sulphuric acid does that) and does not react with ferrous ammonium sulphate, the titrant.
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