For detecting nitrates in sewage in the laboratory, which reagent combination is used to develop the characteristic yellow colour that is later intensified with alkali?

Introduction / Context:
Nitrogen species monitoring (ammonia, nitrite, nitrate) is important for wastewater process control and environmental compliance. A classical colorimetric method exists for nitrate determination in sewage.

Given Data / Assumptions:

• Bench-top chemical testing without sophisticated instrumentation.
• Interferences are minimal or accounted for by standard procedure.

Concept / Approach:
The phenol-disulphonic acid (PDA) method converts nitrates in a dried residue to a nitro-derivative that produces a yellow colour. After neutralization with a strong alkali such as potassium hydroxide (KOH), the colour intensity becomes proportional to nitrate concentration and is read against standards.

Step-by-Step Solution:
Evaporate a measured sample to dryness.Add phenol-disulphonic acid reagent to react with nitrates (nitration step).Neutralize with KOH; a yellow colour develops.Compare the colour intensity with calibration standards to quantify nitrate.

Verification / Alternative check:
This classical method appears in many standard laboratory manuals as a reliable colorimetric technique for nitrate in the presence of minimal nitrite interference.

Why Other Options Are Wrong:

• Potassium permanganate: Not a specific colour reagent for nitrate.
• Sulphuric acid and naphthylamine: Associated with nitrite (Griess reaction), not nitrate directly.
• Ammonium molybdate with stannous chloride: Used in the phosphate (molybdenum blue) method.

Common Pitfalls:
Confusing nitrite and nitrate tests; failing to correct for colour/turbidity; not accounting for chloride interference which may require prior treatment.

Final Answer:
Phenol-disulphonic acid followed by potassium hydroxide