Astringents are chemical compounds that tend to shrink or constrict body tissues and form a protective layer on the surface. Due to their protein action, astringents reduce cell permeability, leading to decreased local edema, exudation, and inflammation. The word “astringent” derives from Latin adstringere, meaning “to bind fast.” These substances are usually applied topically to damaged skin or to the mucous membrane of the gastrointestinal tract (including the mouth).

Types of astringents include vegetable astringents (such as tannic acid and gallic acid), metallic astringents (including aluminum salts like alum and zinc salts), and miscellaneous ones like very cold water and alcohol. Astringents find indications in treating swollen, inflamed, and leaky tissues, diarrhea, dysentery, leukorrhea, minor bleeding (e.g., menorrhagia, hemorrhoids), and promoting the healing process. They also possess deodorant properties and are used in mild skin irritations caused by cuts, allergies, insect bites, or fungal infections.

Properties of astringents

  • They affect only the superficial layer.
  • They make the surface mechanically strong and decrease exudation.
  • They protect from external irritation.
  • Astringents possess local styptic (stops bleeding) and local antiseptic properties.
  • They may interfere with pain receptors, although their pain-relieving action is mild.
  • Astringents constrict blood vessels to stop bleeding.

Zinc sulphate

Description: Zinc sulfate exists in different hydrate forms, including monohydrate and heptahydrate.

Chemical Formula: ZnSO₄·xH₂O


  • Monohydrate: Contains not less than 89.0% and not more than 90.4% of ZnSO₄, corresponding to not less than 99.0% and not more than 100.5% of ZnSO₄·H₂O.
  • Heptahydrate: Contains not less than 55.6% and not more than 61.0% of ZnSO₄, corresponding to not less than 99.0% and not more than 108.7% of ZnSO₄·7H₂O.

Packaging and Storage: Preserve in tight containers.

Identification: A solution of zinc sulfate responds to tests for zinc and sulfate.

Assay: Each mL of 0.05 M edetate disodium is equivalent to 8.072 mg of ZnSO₄.

Potash alum

Chemical Formula: AlK(SO₄)₂·12H₂O

Molecular Weight: 474.39 g/mol (dodecahydrate), 258.21 g/mol (anhydrous)

potash alum
Astringents: Potash Alum source: wikimedia


Potassium Alum is a hydrated double salt composed of aluminum, potassium, sulfur, and oxygen. It forms large, colorless to white crystals and is highly soluble in water.

Identification Tests

  1. Add 1 N sodium hydroxide dropwise to a solution of Potassium Alum (1 in 20). A precipitate form that dissolves in excess reagent. No ammonia is evolved (distinguishing it from Ammonium Alum).
  2. Hold it in a nonluminous flame; a violet color appears.
  3. Add sodium bitartrate TS to a saturated solution of Potassium Alum; a white, crystalline precipitate forms within 30 minutes.
  4. A solution (1 in 20) responds to tests for Aluminum and Sulfate.
  • Loss on Drying: Potassium Alum loses between 43.0% and 46.0% of its weight upon drying.
  • Heavy Metals: The limit for heavy metals is 0.002%.
  • Residual Solvents: Meets the requirements.

Assay Method

  • Use Edetate disodium titrant.
  • Transfer about 800 mg of accurately weighed Potassium Alum to a beaker.
  • Add glacial acetic acid, water, Edetate disodium titrant, and acetic acid–ammonium acetate buffer TS.
  • Warm until complete solution, then boil gently.
  • Cool, add alcohol and dithizone TS, and titrate with 0.05 M zinc sulfate VS to a bright rose-pink color.
  • Each mL of 0.05 M Edetate disodium titrant is equivalent to 12.91 mg of AlK(SO₄)₂

Assay: Potassium Alum should contain not less than 99.0% and not more than 100.5% of AlK(SO₄)₂, calculated on the dried basis.


Astringents are chemical compounds that shrink or constrict body tissues, forming protective layers. They reduce cell permeability, alleviate inflammation, and have local antiseptic properties. Metallic astringents include alum (KAl(SO₄)₂·12H₂O) and zinc sulfate (ZnSO₄). Alum is used for minor bleeding, while zinc sulfate finds applications in various formulations.

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