Question

Which one of the following are the commonly used oxidants in COD (Chemical Oxygen Demand) assays?

• A. K\(_2\)Cr\(_2\)O\(_7\) + HNO\(_3\)
• B. K\(_2\)Cr\(_2\)O\(_7\) + HCL
• C. K\(_2\)Cr\(_2\)O\(_7\) + H\(_2\)SO\(_4\)
• D. KMnO\(_4\) + HCL

Hint:

For water quality tests, remember the key reagents: COD uses potassium dichromate and sulfuric acid. Knowing that strong acids are needed and that chloride interferes with the test can help you choose the correct combination.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Chemical Oxygen Demand (COD) is a measure of the amount of oxygen required to chemically oxidize both organic and inorganic compounds in water. The test uses a strong chemical oxidant in an acidic medium.

Step 3: Detailed Explanation:
The standard method for COD analysis involves refluxing a water sample with a known excess of a strong oxidizing agent in a highly acidic solution.
\[\begin{array}{rl} \bullet & \text{Oxidant: The most commonly used strong oxidant is potassium dichromate (K\(_2\)Cr\(_2\)O\(_7\)). Potassium permanganate (KMnO\(_4\)) can also be used but is less effective for many organic compounds.} \\ \bullet & \text{Acid: The reaction is carried out in the presence of concentrated sulfuric acid (H\(_2\)SO\(_4\)). Sulfuric acid provides the necessary acidic environment and also helps to digest complex organic materials. Hydrochloric acid (HCl) is generally avoided because chloride ions can interfere with the oxidation reaction.} \\ \end{array}\] Therefore, the standard combination of reagents for a COD assay is potassium dichromate and sulfuric acid.

Step 4: Final Answer:
The commonly used oxidants in COD assays are K\(_2\)Cr\(_2\)O\(_7\) + H\(_2\)SO\(_4\).