Detection of free (ammoniacal) ammonia in sewage: Which reagent/method is used to detect free ammonia in wastewater?

Introduction / Context:
Rapid qualitative and quantitative tests for nitrogen species (ammonia, nitrite, nitrate) are fundamental in wastewater analysis. Free (ammoniacal) ammonia is commonly detected colorimetrically using specific reagents.

Given Data / Assumptions:

• We are testing for free ammonia (NH3/NH4+), not nitrates or nitrites.
• Standard colorimetric methods are to be considered.

Concept / Approach:
Nesslerization uses Nessler's reagent (alkaline potassium mercuric iodide). In the presence of ammonium ions, a yellow to brown coloration (or precipitate at high concentrations) develops, enabling detection and measurement of ammonia in water and wastewater.

Step-by-Step Solution:
1) Identify the target analyte: free ammonia.2) Recall the classical method: Nessler's reagent test.3) Recognize the expected response: yellow to brown color proportional to ammonia concentration.4) Select the option that matches this method.

Verification / Alternative check:
Analytical chemistry references list Nesslerization as a standard approach for free ammonia detection; alternative methods (e.g., ion-selective electrodes, phenate method) also exist but are not among the options provided.

Why Other Options Are Wrong:
Phenol-disulphonic acid: Primarily for nitrate, not free ammonia.Boiling: Not a reliable specific test for ammonia in complex matrices like sewage.Potassium permanganate: An oxidant used in various demand tests; not a specific ammonia indicator.Ammonium molybdate: Used mainly for phosphate (phosphomolybdate complex), not ammonia.

Common Pitfalls:

• Confusing tests for nitrates, nitrites, and ammonia.
• Assuming any color change with a strong oxidant indicates ammonia.

Final Answer:
Nessler's reagent (alkaline potassium mercuric iodide) producing a yellow to brown coloration