Chemical Oxygen Demand (COD) test chemistry: In the standard COD test, organic matter is oxidized by potassium dichromate in the presence of which acid catalyst medium?

Introduction / Context:
The COD test measures the oxygen equivalent of organic matter oxidizable by a strong chemical oxidant. The classical method uses potassium dichromate in an acidic medium with catalysts to ensure near-complete oxidation of organics in wastewater samples.

Given Data / Assumptions:

• Oxidant: potassium dichromate.
• Reflux digestion is performed.
• Acid medium and catalyst required.

Concept / Approach:

Potassium dichromate requires a strongly acidic medium—concentrated sulphuric acid—often with silver sulfate as a catalyst and mercuric sulfate to complex chlorides. Under heating, dichromate oxidizes organics; the consumed dichromate is then determined titrimetrically to compute COD.

Step-by-Step Solution:

Verification / Alternative check:

Run blanks and standards; verify chloride interference is suppressed by mercuric sulfate if present.

Why Other Options Are Wrong:

HCl and HNO₃ are not used as the standard digestion medium; citric acid is not suitable for strong oxidation conditions.

Common Pitfalls:

Ignoring chloride interference; insufficient digestion; unsafe handling of concentrated H₂SO₄ and dichromate.

Final Answer:

Sulphuric acid