Solvent extraction
You have learned that the nature of the solvent affects solubility as much as the nature of the solute does. This technique is that fact put to work.
Some solutes dissolve in large amounts in one solvent and in very small amounts in another. Take iodine as an example. When solid iodine is put into water it dissolves only very slightly and gives a pale yellow solution. But a much larger amount of iodine dissolves in a solvent such as carbon tetrachloride or cyclohexane.
What happens when the two are shaken together
Figure to be added
The observation
When carbon tetrachloride is added to an aqueous iodine solution the two do not mix but separate into layers. If that mixture is put into a separating funnel, shaken hard and left for a little while, the iodine travels into the carbon tetrachloride layer and turns it purple, while the yellow colour of the aqueous solution grows fainter.
What has happened is that the iodine has been extracted into the carbon tetrachloride layer, in which it has the greater solubility. The remarkable part is that a small volume of carbon tetrachloride is enough to extract the iodine from a large volume of aqueous solution. The layers are then separated and the carbon tetrachloride evaporated, and the solid iodine is recovered.
The method, stated
Solvent extraction
Taking a solution of a substance that is only slightly soluble in one solvent, and bringing it into contact with a second solvent in which that substance has a higher solubility and which does not mix with the first, so as to transfer the substance into the second solvent, is called solvent extraction.
Two conditions, both necessary
The second solvent must dissolve the substance better than the first, or there is no reason for it to move. And the two solvents must not mix with each other, or there are no separate layers to pour apart.
Where it is used
The medicinal components of some plants are present inside the plant only in trace amounts. Using solvents such as ethanol, more concentrated medicinal solutions are prepared. Solvent extraction is used in making tharala sara and arishta, and — as the last skill of this chapter shows — in getting essential oils out of plant material.