Experiment: 7 Determination of Solubility of Aspirin in Single and Mixed Solvents

Experiment: 7

Determination of Solubility of Aspirin in Single and Mixed Solvents


Theory
Influence of Cosolvents
Frequently a solute is more soluble in a mixture of solvents rather than in a single solvent. The solvents, which are used to increase the solubility of a drug in water, are called as cosolvents. The phenomenon is known as cosolvency. Mechanism responsible for solubility enhancement through cosolvency is by reducing the interfacial tension (polarity differences) between the aqueous solution and hydrophobic solute.
The aspirin molecule is made up of a benzene ring, a carboxyl group, and an ester. It has both polar and non-polar components. Thus, the affinity of water (very polar) for this compound is not as great as the affinity of water for itself. However, the affinity of ethanol molecules (only slightly polar) for aspirin molecules is relatively similar to the affinity of ethanol molecules for each other. Thus, aspirin is more soluble in ethanol than in water.

Chemical structure of aspirin

Tools and Materials
pipette, burette, Aspirin tablets, NaOH, distilled water, ethanol and phenolphthalein indicator.
Procedure
1) Grind up aspirin tablets to a fine powder using the pestle and mortar.
2) Prepare saturated solution form aspirin in 100mL distilled water at room temperature (25 oC).