Step 1: Understanding the Question:
The question asks for the USP solubility range that defines a sparingly soluble substance, expressed as parts of solvent needed per part of solute.
Step 2: Key Formula or Approach:
The United States Pharmacopeia sorts substances into solubility classes based on how many parts of solvent are needed to dissolve one part of solute: very soluble (less than 1 part), freely soluble (1 to 10 parts), soluble (10 to 30 parts), sparingly soluble (30 to 100 parts), slightly soluble (100 to 1000 parts), very slightly soluble (1000 to 10,000 parts), and practically insoluble (more than 10,000 parts).
Step 3: Detailed Explanation:
This scale is set up so each class covers a wider band of solvent volume as solubility gets worse, since dissolving becomes harder in a non-linear way. A sparingly soluble drug sits in the middle of the scale: it will dissolve, but a formulator needs a noticeably large volume of solvent, or another approach such as a cosolvent or a salt form, to get it fully into solution for a liquid dosage form. This solubility class directly affects how a drug is formulated, since a sparingly soluble drug in tablet form may also show slow, incomplete dissolution in the gut and need particle size reduction or other bioavailability-boosting techniques.
Step 4: Final Answer:
A sparingly soluble substance needs 30 to 100 parts of solvent to dissolve one part of solute.