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DrX Whiz Niraj
Astringents: Zinc Sulphate & Potash Alum
Master the pharmacology and chemistry of Astringents. Detailed monographs on Zinc Sulphate and Potash Alum including their preparation, properties, and medical uses.
Astringents are locally applied chemical agents that cause the precipitation of proteins on the cell surface or in the interstitial spaces. This leads to the shrinking or contraction of tissues.
Mechanism of Action:
When an astringent is applied to the skin or mucous membrane, it interacts with tissue proteins and forms a protective layer (precipitate).
This layer causes:
1. Shrinkage of tissues.
2. Reduction in cell permeability (stops fluid leakage/exudation).
3. Constriction of small blood vessels (Styptic action - stops minor bleeding).
4. A hostile environment for bacteria, providing a mild antimicrobial effect.
Properties & Preparation
- Synonym: White Vitriol.
- Appearance: Colorless, transparent crystals or white crystalline powder. It is highly efflorescent in dry air (loses water of crystallization).
- Taste: Astringent and metallic taste. Very soluble in water.
- Preparation: By boiling metallic zinc or zinc oxide with dilute sulphuric acid.
Zn + H₂SO₄ → ZnSO₄ + H₂↑
Medical Uses
- Astringent: Used topically as lotions or eye drops (0.25% solution for conjunctivitis).
- Emetic: In large doses (1-2 grams), it acts as a reflex emetic to induce vomiting in cases of poisoning.
- Zinc Supplement: Used internally to treat zinc deficiency (promotes wound healing).
Chemical Formula: KAl(SO₄)₂·12H₂O OR K₂SO₄·Al₂(SO₄)₃·24H₂O
Synonyms: Alum, Phitkari, Purified Alum.
K₂SO₄ + Al₂(SO₄)₃ + 24H₂O → K₂SO₄·Al₂(SO₄)₃·24H₂O (Potash Alum Crystals)
Physical Properties
It occurs as colorless, transparent, large octahedral crystals. It has a sweetish, strongly astringent taste. Its aqueous solution is highly acidic to litmus.
Action on Heat
When heated to 200°C, it loses all its water of crystallization (24H₂O) and swells up into a white, porous mass known as Burnt Alum.
Pharmaceutical Uses
Powerful local astringent and Styptic (stops bleeding from minor cuts, used in shaving blocks). Used in water purification (coagulates mud/impurities). Used in mouthwashes for ulcers.
Clinical Alert: Toxicity Warning
Although Alum is safe for external topical use (like in aftershaves), ingestion of large quantities is toxic. It causes severe irritation of the gastrointestinal mucosa, leading to intense vomiting, purging, and potential mucosal necrosis. Astringents are meant primarily for topical application or very dilute mouthwashes (which should not be swallowed).