Question

Discuss the physical appearance of the aqueous tea solution versus the o¬rganic solution; why is tea so dark?

Answer 1

Here the organic solvent dichloromethane is used to extract caffeine from an aqueous extract of tea leaves because caffeine is more soluble in dichloromethane (140 mg/ml) than it is in water (22 mg/ml). However, the tannins that are slightly soluble in dichloromethane can be eliminated by converting it to their salts (phenolic anions by adding sodium carbonate) (tannins are phenolic compounds of high molecular weight and being acidic in nature can be converted to salts by deprotonation of the -OH group) which remain in the water.

Tea powder is extracted from tea leaves which contain tannins, which are acidic in nature, a number of colored compounds and a small amount of unrecompensed chlorophyll and an important stimulant called Caffeine..

Caffeine, 1,3,7 - trimethylxanthine, belongs to a wide class of compounds known as alkaloids. These are plant derived compounds with complex structure containing nitrogen, and usually have roles in physiological activity. The melting point of Caffeine is 238°C.

In the case of Caffeine extraction from tea powder, the solubility of caffeine in water is 22mg/ml at 25°C, 180mg/ml at 80°C, and 670mg/ml at 100°C. Here the organic solvent Dichloromethane is used to extract caffeine from aqueous extract of tea powder because caffeine is more soluble in dichloromethane (140mg/ml) than it is in water (22mg/ml).The dichloromethane - caffeine mixture can then be separated on the basis of the different densities of dichloromethane and water because dichloromethane is much denser than water and insoluble in it. Residual water is separated from dichloromethane by drain out the dichloromethane through separating funnel, thus dichloromethane passed through the funnel while polar solvents such as water is still remains in the funnel. Water and dichloromethane is slightly soluble in each other. So, after separating the solvents, residual water will remain the organic layer. Mainly anhydrous sodium sulfite is used for the removal of water from organic layer. Anhydrous sodium sulfite is an insoluble inorganic solid which will absorb water, thus drying it.