🧪 Limit Test of Calcium(Inorganic Pharmaceutical Chemistry – Practical Guide)

Calcium is an essential element, but excess calcium as an impurity in pharmaceutical substances can affect drug stability, solubility, and quality.

That’s why the Limit Test of Calcium is performed to ensure that calcium content remains within pharmacopoeial limits.

📌 Objective of Limit Test of Calcium

To detect and control the amount of calcium impurity present in pharmaceutical substances by comparing the test solution with a standard solution.

🧠 Principle

Calcium reacts with ammonium oxalate in an alkaline medium to form a white precipitate of calcium oxalate.

👉 The turbidity produced in the test solution should not be more intense than that of the standard solution.

🧫 Chemical Reaction

White precipitate of Calcium Oxalate is formed.

⚗️ Apparatus Required

Nessler cylinders

Measuring cylinder

Beakers

Glass rod

🧪 Reagents Required

Ammonium oxalate solution

Dilute ammonia solution

Calcium standard solution

Distilled water

🧪 Procedure

🔹 Preparation of Test Solution

Take the given sample solution in a Nessler cylinder.

Add ammonium oxalate solution.

Add dilute ammonia solution to make the medium alkaline.

Dilute with distilled water up to the mark.

Allow the solution to stand for 5 minutes.

🔹 Preparation of Standard Solution

Take a known quantity of calcium standard solution in another Nessler cylinder.

Add the same reagents in the same order and quantity.

Make the volume equal with distilled water.

Allow to stand for 5 minutes.

👁️ Observation

Compare the turbidity of the test solution with the standard solution.

📌 Inference

✔ If turbidity of test solution ≤ standard → Sample passes the test

❌ If turbidity of test solution > standard → Sample fails the test

🎯 Importance of Limit Test of Calcium

✔ Ensures pharmaceutical purity

✔ Prevents drug incompatibility

✔ Essential for quality control laboratories

✔ Frequently asked in practical exams & viva

📝 Viva Voce Questions

Q1. Why ammonium oxalate is used?

👉 To precipitate calcium as calcium oxalate.

Q2. Why alkaline medium is required?

👉 It favors complete precipitation of calcium oxalate.

Q3. What is the white precipitate formed?

👉 Calcium oxalate.

Q4. Is this a qualitative or quantitative test?

👉 Semi-quantitative test.

Q5. What happens if calcium exceeds limit?

👉 Drug may fail quality standards.

🧠 Common Student Mistakes

❌ Improper alkalinity

❌ Unequal volumes in Nessler cylinders

❌ Not allowing sufficient standing time

🔗 Related Limit Test Series

👉 Limit Test of Iron

👉 Limit Test of Heavy Metals

👉 Limit Test of Arsenic

👉 Limit Test of Nitrate

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💬 Student Engagement

🧪 Quick Question:

Which limit test do you find easiest?

⬇ Comment below

A) Calcium

B) Iron

C) Arsenic

D) Heavy Metals

🚀 Final Tip for Exams

✍️ Always write:

Reaction

Comparison statement

Inference line