Solubility
When a solute is dissolved in a solvent, two types of interactions are possible. One is the intramolecular force of attraction between the solute molecules and the other is the intermolecular force of attraction between the solute and solvent molecules. When a solute dissolves, the intramolecular force or cohesive force must be overcome by the force of attraction between the solute and solvent molecules, that is, adhesive force. This involves breaking the solute–solute forces and the solvent–solvent forces to achieve the solute–solvent forces of attraction. The solubility of a drug in a particular solvent indicates the maximum concentration to which a solution may be prepared with that drug and that solvent. The expression of solubility according to Indian Pharmacopoeia is given in Table 6.1.
Table 6.1 Expression of Solubility According to Indian Pharmacopoeia
| Descriptive Phrase | Approximate Quantities of Solvent by Volume (ml) for 1 Part of Solute by Weight (1 g) |
|---|---|
| Very soluble | less than 1 part |
| Freely soluble | from 1 to 10 parts |
| Soluble | from 10 to 30 parts |
| Sparingly soluble | from 30 to 100 parts |
| Slightly soluble | from 100 to 1000 parts |
| Very slightly soluble | from 1000 to 10,000 parts |
| Practically insoluble | more than 10,000 parts |
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