Limit Test of Ammonium (NH₄⁺)(Inorganic Pharmaceutical Chemistry – Semester I)

 Why This Practical Matters?

After Chloride, Iron, Heavy Metals, Arsenic, Nitrate, Lead, and Boron, the Limit Test of Ammonium is an important impurity test asked in:

✔ Practical exams

✔ Viva questions

✔ Theory MCQs

✔ Competitive pharmacy exams

Understanding this test also helps students link chemical reactions with visual observation, which examiners love.

🎯 Aim of the Practical

To determine the amount of ammonium impurity present in a given pharmaceutical substance and ensure it is within the prescribed pharmacopoeial limit.

🧠 Principle (Easy to Remember)

Ammonium salts, when treated with alkaline potassium mercuric iodide (Nessler’s reagent), produce a yellow to brown colored complex.

👉 Intensity of color ∝ amount of ammonium present

The color produced by the test solution is compared with the standard solution.

⚗️ Reagents Required

Nessler’s reagent

Ammonium standard solution

Alkaline medium (usually sodium hydroxide)

Distilled water

🧪 Apparatus

Nessler cylinders

Measuring cylinders

Pipette

Test tubes

🧬 Chemical Reaction (Exam-Friendly)

Ammonium ions react with Nessler’s reagent to form a brownish colored mercuric amido-iodide complex.

📌 Write this line clearly in exams to score full marks.

🧾 Procedure (Step-by-Step)

Take the test solution in a Nessler cylinder.

Take the standard ammonium solution in another Nessler cylinder.

Add Nessler’s reagent to both.

Make up the volume with distilled water.

Allow the solutions to stand for a fixed time.

Compare the color intensity of both solutions.

👀 Observation

Test solution color ≤ Standard solution color

👉 Sample passes the limit test

Test solution color > Standard

👉 Sample fails the limit test

✅ Conclusion

The given sample complies / does not comply with the limit test of ammonium as per pharmacopoeial standards.

📚 Viva Voce – Important Questions

Q1. Why is ammonium impurity harmful?

→ It affects drug purity and stability.

Q2. Name the reagent used.

→ Nessler’s reagent.

Q3. What is the observation?

→ Yellow to brown coloration.

Q4. Where is this test used?

→ In pharmaceutical raw materials and chemicals.

🔗 Previous Practical Posts (Internal Linking for Blog Growth)

Limit Test of Chloride

Limit Test of Iron

Limit Test of Heavy Metals

Limit Test of Arsenic

Limit Test of Nitrate

Limit Test of Boron

👉 Linking these increases page views and SEO strength.

💬 Student Engagement Box

📢 Question for Students:

Why do you think visual color comparison is still preferred in limit tests instead of instruments?

📝 Comment your answer below 👇