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DrX Whiz Niraj
Pharmacopoeia, Impurities & Limit Tests
Comprehensive exam notes on Indian Pharmacopoeia, Sources of Impurities in drugs, and principles of Limit Tests.
The word Pharmacopoeia is derived from Greek words 'Pharmakon' (drug) and 'Poiein' (to make). It literally means "drug-making".
It is an official, legally binding book published by an authority (usually the government of a country). It contains a list of approved drugs along with their formulas, standards for quality, testing methods, and purity limits.
IP
Indian Pharmacopoeia. Published by the Indian Pharmacopoeia Commission (IPC) under the Ministry of Health & Family Welfare.
BP
British Pharmacopoeia. The official collection of standards for the UK.
USP
United States Pharmacopeia. The official pharmacopeia of the USA.
An Impurity is any unwanted foreign chemical substance present in a pharmaceutical product. Impurities can make the drug toxic, decrease its shelf-life, or change its appearance.
Common sources from where impurities enter into medicinal agents:
- 1. Raw Materials: Traces of heavy metals (Arsenic, Lead) present naturally in mined ores or plant extracts.
- 2. Reagents & Solvents: Water is the most common solvent. If tap water is used, Ca, Mg, and Cl ions can contaminate the product. Tap water must be replaced with Purified Water.
- 3. Manufacturing Process: Intermediate compounds, unreacted by-products, or physical particulate matter (dust, glass pieces).
- 4. Wear and Tear of Equipment: Iron particles or grease from stainless steel manufacturing machines.
- 5. Storage & Packaging: Alkali leaching from cheap glass containers, or plasticizers leaking from plastic bottles into liquid syrups.
Definition: Limit tests are quantitative or semi-quantitative tests designed to identify and control small quantities of impurities which are likely to be present in the substance.
Principle of Limit Tests:
Almost all limit tests are based on the comparison of Opalescence (Turbidity) or Color produced by the Test solution with that of a Standard solution of known concentration. These comparisons are done in special glass tubes called Nessler Cylinders.
| Limit Test For | Main Reagent Used | Observation (What to see?) |
|---|---|---|
| Chloride | Silver Nitrate (AgNO₃) in presence of Dil. HNO₃ | Opalescence (Due to formation of Silver Chloride AgCl) |
| Sulphate | Barium Chloride (BaCl₂) in presence of Dil. HCl | Turbidity (Due to formation of Barium Sulphate BaSO₄) |
| Iron | Thioglycolic acid in presence of Ammonia | Purple/Pink Color (Due to Ferrous thioglycolate complex) |
| Arsenic | Zinc + HCl + Mercuric Chloride Paper | Yellow Stain on paper in Gutzeit apparatus (Due to Arsine gas) |
Practical Alert: Arsenic Limit Test
Arsenic limit test is the only limit test that does not use Nessler Cylinders. It requires a special glass equipment called the Gutzeit Apparatus. Arsenic is highly toxic and even trace amounts can lead to severe poisoning, hence its limit is strictly controlled (usually 2 ppm).