Outline the principal steps of the COD (dichromate) method.

• A. Digest sample with potassium dichromate in sulfuric acid under heat in sealed vessels, then measure remaining dichromate color spectrophotometrically after cooling.
• B. Titrate sample with sodium thiosulfate directly.
• C. Use UV digestion without acid.
• D. Measure COD by pH meter reading.

Answer: (C)

The dichromate COD method relies on oxidizing the sample’s organics with potassium dichromate in a strong acidic medium and then determining how much oxidant was consumed. This shows how much organic matter is present because more organics being oxidized means more dichromate is used up, giving a higher COD.

The typical steps start with acidifying the sample and adding an excess of potassium dichromate, usually with a catalyst to handle possible interferents (such as chloride, which can skew results). The mixture is then heated in sealed digestion vessels to a reflux temperature for a defined time to ensure complete oxidation of the organics. After digestion, the tubes are cooled and diluted to the prescribed volume. The amount of dichromate remaining is then measured to back-calculate how much was consumed. In the standard procedure this measurement is done by titrating the remaining dichromate with a standard solution of ferrous ammonium sulfate using an indicator (ferroin) to determine the endpoint. Some modern variations use photometric or spectrophotometric measurement of the residual dichromate instead of titration, but the underlying principle remains: complete digestion with dichromate in acid under heat, followed by quantifying the remaining oxidant to compute COD. This method does not use UV digestion, nor is it determined by a pH meter reading, and it’s not simply a direct titration of the sample with a reducing agent like sodium thiosulfate.