Limit Test of Nitrate – Principle, Procedure, Reaction, Observation Table & Viva Questions

📌 Introduction

The Limit Test of Nitrate is performed to ensure that nitrate impurities present in inorganic substances are within acceptable limits. Excess nitrate is harmful and may alter chemical properties of drugs.

This test is important in Pharmaceutical Inorganic Chemistry (B.Pharm – 1st Year).

🎯 Purpose (Aim)

To perform the Limit Test of Nitrate as per IP and compare the amount of nitrate impurity in the sample with the standard solution.

🧪 Principle

Nitrate reacts with brucine (an alkaloid) in presence of sulphuric acid, producing a yellow to reddish brown color.

• If sample contains nitrate impurity → stronger color form

• If nitrate is within limit → sample color ≤ standard color

👉 Color comparison = Result of limit test

🧫 Reagents Required

Reagent	Purpose
Brucine Solution	Color development
Concentrated Sulphuric Acid	Reaction medium
Sample Solution	Test solution
Standard Nitrate Solution	Comparison
Distilled Water	Dilution

🧪 Apparatus Used

• Nessler Cylinders (2)

• Pipettes

• Test Tubes

• Measuring Cylinder

• Glass Rod

📝 Procedure

1️⃣ Preparation of Sample Solution

• Take required amount of the substance to be tested.

• Dissolve in 10 ml distilled water.

2️⃣ Preparation of Standard Nitrate Solution

• Use standard potassium nitrate solution (IP).

• Dilute to same volume (10 ml).

3️⃣ Color Development

👉 In Sample Cylinder:

• Add Brucine solution (2 ml)

• Add Concentrated H₂SO₄ (gently along the wall)

👉 In Standard Cylinder:

• Repeat the same steps with the standard solution.

4️⃣ Mix properly and keep both cylinders undisturbed for 10 minutes.

🔍 Observation

Cylinder	Color Developed	Result
Sample	Light yellow / Dark yellow	Compared with Standard
Standard	Light yellow	Reference color

📌 Interpretation of Result

✔ Pass:

Sample color ≤ Standard color

❌ Fail:

Sample color > Standard color

🧬 Chemical Reaction

Nitrate reacts with brucine + sulfuric acid:

NO₃⁻ + Brucine + H₂SO₄ → Yellow / Reddish Brown Color Complex

📘 Result

The given substance passes / fails the Limit Test of Nitrate based on color comparison with the standard.

(You will choose Pass/Fail depending on observation.)

📚 Viva Questions (High Scoring)

1. What is the principle of Limit Test of Nitrate?

It is based on color formation between nitrate and brucine-sulphuric acid mixture.

2. What is the role of brucine?

It acts as a color-producing reagent.

3. Why is sulfuric acid added?

It provides acidic medium and enhances color formation.

4. What is compared in limit tests?

Color intensity of sample vs standard.

5. Nitrate impurity maximum limit?

As specified in Indian Pharmacopoeia (IP)

💬 Question for Students:

“What reagent is responsible for the yellow color in the Limit Test of Nitrate?”