Limit Test of Iron – Complete Practical Guide for Pharmacy Students

Limit Test of Iron is one of the most important inorganic pharmaceutical chemistry practicals.

It is frequently asked in B.Pharm & D.Pharm practical exams, viva voce, and written theory papers.

This post is part of the PharmaEasyNotes – Limit Test Series, designed to help students understand, write, and score better.

📌 Aim of the Test

To determine whether the amount of iron impurity present in a given substance is within the prescribed pharmacopoeial limit.

🧪 Principle of Limit Test of Iron

Iron present in the sample is converted into thioglycolic acid complex in an alkaline medium.

🔹 Iron reacts with thioglycolic acid

🔹 Produces a pink to reddish-purple color

🔹 The intensity of color is compared with a standard iron solution

✔ If the test solution color is not more intense than the standard, the sample passes the test.

🧫 Reagents Required

Test solution

Standard iron solution

Thioglycolic acid

Ammonia solution

Distilled water

⚗️ Procedure (Step-by-Step)

Take the given test solution in a Nessler cylinder.

Add thioglycolic acid and mix well.

Add ammonia solution to make the medium alkaline.

Dilute with distilled water up to the mark.

Prepare the standard solution in the same manner using standard iron solution.

Allow both solutions to stand for 5 minutes.

Compare the color intensity of both solutions.

👀 Observation

Test solution shows a pink color.

Standard solution shows a pink color of known intensity.

✅ Result

If the color produced by the test solution is not darker than the standard,

➡️ The sample passes the limit test for iron.

❓ Viva Voce Questions (Exam Focus)

Q1. Why thioglycolic acid is used?

👉 It forms a colored complex with iron.

Q2. Why ammonia is added?

👉 To provide alkaline medium for color development.

Q3. Which color is observed in iron limit test?

👉 Pink to reddish-purple color.

Q4. Which impurities are detected by limit tests?

👉 Inorganic impurities.

📚 Why This Test Is Important for Students?

✔ Frequently asked practical

✔ Easy scoring if written correctly

✔ High viva weightage

✔ Improves concept clarity of inorganic impurities

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🔗 Related Posts (Internal Linking Tip)

👉 Limit Test of Chloride

👉 Limit Test of Sulphate

👉 Limit Test of Calcium

👉 Limit Test of Heavy Metals