In standard colorimetric testing of sewage, which reagent combination is used to develop a matching color specifically for detecting nitrites (NO₂⁻)?

Introduction / Context:Colorimetric tests for nitrogen species in wastewater are routine in environmental engineering labs. Nitrite detection relies on a diazotization–coupling reaction commonly known as the Griess reaction.

Given Data / Assumptions:

• Analyte: nitrite (NO₂⁻) in sewage.
• We are choosing the reagent system that develops the diagnostic pink–red azo dye.

Concept / Approach:The Griess reaction converts nitrite to a diazonium salt (often via sulfanilic acid in acidic medium), which then couples with an aromatic amine such as α-naphthylamine to produce a colored azo dye. The question’s phrasing points to the acid plus naphthylamine system used for nitrite color development.

Step-by-Step Solution:Identify the species: nitrite, not nitrate.Associate the correct reaction: Griess reaction reagents (acid medium + aromatic amine like naphthylamine).Select the option corresponding to this chemistry.

Verification / Alternative check:Laboratory practice distinguishes nitrite (Griess) versus nitrate (phenol disulphonic acid after reduction) to avoid cross-interference.

Why Other Options Are Wrong:

• Potassium permanganate is an oxidant, not the standard color reagent for nitrite.
• Phenol disulphonic acid with KOH is used for nitrate determination.
• None of these is wrong because a correct reagent system is listed.
• Orthotolidine relates to chlorine residual testing.

Common Pitfalls:Confusing nitrite and nitrate methods. Ensure the correct analyte–reagent pairing to avoid erroneous results.

Final Answer:Sulphuric acid with naphthylamine (Griess reaction reagents).