Inorganic Chemistry lacture - 4 “Limit test: Principles, Examples & Easy Explanation”

 

🧠 Introduction

Limit tests are semi-quantitative tests used to check whether the amount of impurity in a substance is within permissible limits or not.

👉 These tests are very important for drug safety and quality control.

⚙️ What is the Principle of Limit Test?

👉 Based on comparison method
👉 Test solution is compared with a standard solution

✔ If test ≤ standard → Pass
✔ If test > standard → Fail

🧪 Limit Test for Chloride

🔬 Principle:

Chloride ions react with silver nitrate (AgNO₃) to form silver chloride (AgCl) (white turbidity).

👉 Turbidity of test solution is compared with standard.

🧪 Limit Test for Sulphate

🔬 Principle:

Sulphate ions react with barium chloride (BaCl₂) to form barium sulphate (BaSO₄) (white turbidity).

👉 Compared with standard solution.

🧪 Limit Test for Iron

🔬 Principle:

Iron reacts with thioglycolic acid → forms purple colored complex.

👉 Color intensity is compared with standard.

🧪 Limit Test for Arsenic

🔬 Principle:

Arsenic → converted into arsine gas (AsH₃)
👉 Reacts with mercuric chloride paper → yellow stain

👉 Compare stain intensity with standard.

🧪 Limit Test for Lead

🔬 Principle:

Lead reacts with dithizone → forms red colored complex.

👉 Compared with standard.

🧪 Limit Test for Heavy Metals

🔬 Principle:

Heavy metals react with hydrogen sulfide (H₂S) → form black/brown sulfide precipitate.

👉 Color compared with standard lead solution.

🔄 Modified Limit Test

Chloride (Modified)

• Uses controlled pH & reagents
• Gives more accurate turbidity comparison

Sulphate (Modified)

• Uses stabilizers (like alcohol/gum)
• Ensures uniform precipitation

👉 Improves accuracy & reproducibility

🎯 Conclusion

👉 Limit tests ensure that impurities are within safe limits
👉 Essential for drug quality, safety & regulatory compliance