We can do this by taking advantage of the fact that phenols are acidic enough to be converted to their salts (deprotonation of the -OH group) by reaction with sodium carbonate. So, we will add sodium carbonate to the water and the tannins will be converted to phenolic anions, which are not soluble in the dichloromethane but are soluble in highly polar water. There is one practical disadvantage in converting the tannins to their salts – they become anionic surfactants. Detergents and soap are surfactants. It is the purpose of surfactants to cause materials that do not dissolve in water (like oil, grease and dichloromethane) to form an emulsion with water. We want to be able to separate the aqueous phase from the dichloromethane phase, so the last thing we want is an emulsion of the two. Consequently, as you extract the caffeine from the water into the dichloromethane do not shake the separatory funnel vigorously.
Background to the Procedure
The idea in this experiment is to extract the water soluble materials in the tea leaves into hot water. [The solubility of caffeine in water is 22 mg/ml at 25C, 180 mg/ml at 80C, and 670 mg/ml at 100C.] The hot solution is allowed to cool and the caffeine is then extracted from the water with dichloromethane (methylene chloride), which is an organic solvent that is insoluble in water. Since caffeine is more soluble in dichloromethane (140 mg/ml) than it is in water (22 mg/ml), it readily dissolves in the dichloromethane. However, the tannins are slightly soluble in the dichloromethane. But we want to separate the caffeine from the tannins by having the caffeine dissolve in the dichloromethane and the tannins remain in the water.