Crystallization And Extraction

Crystallization Crystallization refers to the formation of solid crystals from a homogeneous solution. It is essentially a solid-liquid separation technique and a very important one. In order for crystallization to take place a solution must be "supersaturated". Supersaturation refers to a state in which the liquid (solvent) contains more dissolved solids (solute) than can ordinarily be accommodated at that temperature. It is based on the principle that pure substance crystallizes more easily than impure substance. Therefore in an impure solution pure substance crystallizes leaving behind an impure solution. Crystallization is very inefficient method of separation and it is a very difficult to arrive at a pure substance through crystallization.

Extraction Extraction is a very common laboratory procedure used when isolating or purifying a product. Organic chemistry employs solidliquid, liquid-liquid, and acid-base extractions. The following applies to liquid-liquid extractions, which will be used in this discussion It is very common to obtain a mixture of product in any synthesis. These products are to be separated by some technique. Extraction is widely used method. A separatory funnel is used for this process and the principal of like dissolve like is followed. The organic solvent used for extraction must meet a few criteria: 1. Should readily dissolve substance to be extracted. 2. Should not react with the substance to be extracted. 3. Should not react with or be miscible with water (the usual second solvent). 4. Should have a low boiling point so it can be easily removed from the product.

Common extraction solvents are diethyl ether and methylene chloride. Place the solution to be extracted in the separatory funnel. As the organic solvent and water are not miscible with each other, the two layers (organic and aqueous layers) will be visible clearly. To remove all inorganic substances from the organic layer, shake the separatory funnel to increase the contact between these substances and the water. Allow the mixture to settle down and notice the two separate layers. To determine which layer is which, one can simply add distilled water to the funnel. Whichever layer increases in size must be the aqueous layer and the other is the organic layer. At this point the two layers can be separated into their respective beakers. Do not throw out any removed layers until the desired product has been isolated! Once the extraction process is completed, drying agents can now be used and the product can be isolated from the organic solvent.