Question

Nitrate in milk is generally used as a marker to detect the presence of added ________.

• A. Pond or surface water
• B. Salicylic acid
• C. Skim milk powder
• D. Benzoic acid

Hint:

Remember that the nitrate test is a qualitative indicator of the source of added water.
Pure municipal tap water or distilled water may not show a positive nitrate test if their nitrate levels are below the detection limit of the diphenylamine reagent.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Adulteration of milk with water is a common fraudulent practice used to increase volume.
While adding pure distilled water can be detected by changes in freezing point or specific gravity, detecting the addition of contaminated natural water (such as pond, well, or river water) requires sensitive chemical markers.

Step 2: Detailed Explanation:
Cow's milk naturally contains negligible or zero levels of nitrates (typically less than 1 to 2 ppm).
In contrast, natural surface waters, pond water, and shallow well water often contain significant amounts of dissolved nitrates and nitrites due to agricultural runoff, fertilizer usage, and organic decomposition.
Because nitrates are stable and are not naturally present in fresh milk, their detection in a milk sample serves as a highly reliable chemical marker for the addition of pond, well, or surface water.
Standard diagnostic assays, such as the diphenylamine test or the xylenol method, are used to detect trace levels of nitrates in milk.
In the diphenylamine test:
The reagent reacts with nitrate ions in an acidic medium to produce a distinct blue-colored complex.
The presence of a blue color indicates that the milk has been diluted with natural surface or ground water containing dissolved nitrates.
This test is highly sensitive and can detect even small amounts of added surface water.

Step 3: Final Answer:
Nitrate in milk is used as a marker to detect the presence of added pond or surface water.