Step 1: Understanding the Concept:
The swelling index (or swelling factor) measures the volume (in mL) occupied by 1 gram of a drug after it has swollen in an aqueous liquid for a specified time. It is used for drugs containing mucilage, gums, or pectin.
Step 2: Detailed Explanation:
1. Ispaghula (Plantago ovata): The seeds and husks of ispaghula are rich in mucilage. When placed in water, they absorb large amounts of fluid and swell significantly.
2. Utility: A high swelling index (usually > 10 for seeds and > 40 for husk) indicates high-quality, pure ispaghula. Adulteration or old stock will show a much lower swelling index.
3. Mechanism: This swelling property is also why ispaghula works as a bulk-forming laxative in the human gut.
Step 3: Final Answer:
Ispaghula is the primary drug where the swelling index is the standard mandatory test for evaluating mucilage content and quality.