🧪 Limit Test of Iron | Pharmaceutical Inorganic Chemistry Practical

🎯 Aim

To perform the Limit Test of Iron (Fe³⁺ ions) as per Indian Pharmacopoeia (IP) standards.

🔬 Principle

Iron reacts with thioglycolic acid in an alkaline medium to form a purple-colored complex.

The intensity of this color is compared with that of a standard iron solution using Nessler cylinders.

Reaction:

Fe3++Thioglycolic acid+NH3  ⟶  Purple ComplexFe^{3+} + Thioglycolic \, acid + NH_3 \; \longrightarrow \; Purple \, Complex

Fe3++Thioglycolicacid+NH3​⟶PurpleComplex

🧰 Requirements

• Sample solution (suspected iron impurity)

• Standard Iron solution (20 ppm Fe³⁺)

• Thioglycolic acid

• Ammonia solution

• Distilled water

• Nessler cylinders

⚗️ Procedure

1. Test Solution

   • Dissolve given sample in distilled water.

   • Add 2 mL thioglycolic acid.

   • Make alkaline with dilute ammonia.

   • Dilute to 50 mL with distilled water.

2. Standard Solution

   • Prepare a solution of 20 ppm Fe³⁺.

   • Add thioglycolic acid + ammonia.

   • Dilute to 50 mL with distilled water.

3. Place both in Nessler cylinders and compare.

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📊 Observation Table

S. No.	Solution	Color Produced	Intensity Compared with Standard	Inference
1	Test Solution	Light Purple	Lighter than Standard	Pass
2	Standard (20 ppm)	Purple	Reference	–

✅ Result

The given sample passes the Limit Test of Iron if the test solution’s color is not darker than the standard solution.

📎 Related Practical

👉 Already checked: Limit Test of Chloride – Click here to Read

🎤 Viva Questions (with Answers)

Q1. What is the principle of Limit Test of Iron?

➡️ Iron reacts with thioglycolic acid in alkaline medium to form purple color, compared with standard.

Q2. Why is ammonia used in the test?

➡️ To make the solution alkaline which is necessary for color development.

Q3. What is the role of thioglycolic acid?

➡️ It acts as a reducing agent and forms a purple-colored complex with iron ions.

Q4. What is the permissible limit of iron in pharmaceuticals?

➡️ As per IP, not more than 20 ppm in most substances.

Q5. Which apparatus is used for color comparison?

➡️ Nessler cylinder.

🚀 What’s Next?

Stay tuned for upcoming practicals in this Limit Test Series:

• Limit Test of Sulphates

• Limit Test of Heavy Metals

• Limit Test of Arsenic

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