In COD analysis, which reagent fixes interference from halides?

In COD analysis, which reagent fixes interference from halides?

• A. Sodium sulfate
• B. Mercuric sulfate
• C. Silver nitrate
• D. Ammonium sulfate

Answer: (B)

Halide interference in COD analysis comes from halide ions reacting with the oxidant (dichromate) and skewing the result. The way to prevent this is to add a masking agent that binds halides so they can’t participate in the oxidation. Mercuric sulfate does exactly that: it forms sparingly soluble mercuric halides with chloride, bromide, and iodide, effectively sequestering the halides so they don’t react with the oxidant. This keeps the COD measurement true to the organic matter present, without the interference from halides. Sodium sulfate and ammonium sulfate don’t mask halides; they mainly affect ionic strength. Silver nitrate would also interact with halides, but it introduces complications with the COD method and isn’t the standard masking agent used, so it isn’t relied upon for this purpose.

Halide interference in COD analysis comes from halide ions reacting with the oxidant (dichromate) and skewing the result. The way to prevent this is to add a masking agent that binds halides so they can’t participate in the oxidation. Mercuric sulfate does exactly that: it forms sparingly soluble mercuric halides with chloride, bromide, and iodide, effectively sequestering the halides so they don’t react with the oxidant. This keeps the COD measurement true to the organic matter present, without the interference from halides.

Sodium sulfate and ammonium sulfate don’t mask halides; they mainly affect ionic strength. Silver nitrate would also interact with halides, but it introduces complications with the COD method and isn’t the standard masking agent used, so it isn’t relied upon for this purpose.