Wastewater testing—COD: In the chemical oxygen demand (COD) test for sewage, the organic matter is oxidized by potassium dichromate (K2Cr2O7) in the presence of which reagent?

Introduction / Context:

The chemical oxygen demand (COD) test quantifies the amount of oxygen required to chemically oxidize organic compounds in wastewater. It is widely used for treatment plant monitoring because it is faster and more reproducible than biochemical oxygen demand (BOD) measurements.

Given Data / Assumptions:

• Oxidant: potassium dichromate (K2Cr2O7).
• Reflux under acidic conditions is required.
• Common catalysts and masking agents may be used (e.g., silver sulfate, mercuric sulfate) depending on chloride content.

Concept / Approach:

COD digestion is performed in strongly acidic medium so that dichromate acts as a powerful oxidant. Concentrated sulfuric acid provides the acidic environment, and silver sulfate acts as a catalyst for oxidizing certain compounds (e.g., straight-chain hydrocarbons). Mercuric sulfate is added to complex chlorides to prevent their interference.

Step-by-Step Solution:

Verification / Alternative check:

Standard methods (APHA/AWWA/WEF) specify sulfuric acid as the required acidifying medium for the dichromate COD procedure, confirming the correct choice.

Why Other Options Are Wrong:

• HNO3 and HCl are not used in the standard dichromate COD method.
• None of these is incorrect because H2SO4 is essential.
• NaOH would neutralize acid and halt oxidation.

Common Pitfalls:

• Confusing COD (chemical oxidation) with BOD (biological oxidation).
• Ignoring chloride interference mitigation steps.

Final Answer:

H2SO4 (sulfuric acid)