Lab Manual- Experiment 2

Lab Manual- Experiment 2 - 8/26/2010 Experiment 2:...

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8/26/2010 Experiment 2 : CRY S T A LL I Z A T ION Reference: Pavia, Techniques 8, 9, 10 and 11 (pages 630-680) and review pages 545-579 where necessary. INTRODUCTION The technique of crystallization is one of the most valuable available for the purification of solids. The basic idea of purification is easily understood and the manipulations are straightforward. In spite of this, crystallization remains more of an art than a science. A part of the trouble arises because a good crystallization frequently requires much patience. A more serious problem is that the best solvent to use cannot be chosen by a convenient magic rule, but must be found by trial and error. Since you have little or no experience, you will have to lean on our judgment. The most fundamental property of a good solvent for crystallization of a solid is that the hot solvent must dissolve the substance readily while the cold solvent must dissolve it sparingly. This means that you would start your hunt for a good solvent by looking for one that gave borderline solubility. From here you would have to adjust the temperature range, the ratio of solid to solvent, or try combinations of solvents to find a mixture with just the right solvent properties. In crystallization of a mixture of solids one is always faced with the practical problem of knowing how much solvent to use. If one of the components is poorly soluble in the chosen solvent one could go on adding the hot solvent for a long time before the mixture is dissolved completely. If you use too much solvent the desired compound will not come out of solution when you cool it. In practice you probably will make this mistake many times and the only way to recover your compound is to concentrate the solution and see if the desired solid precipitates. Therefore, for best results, you must be able to judge whether any un-dissolved solid is the compound that you are trying to recrystallize or some poorly soluble impurity. Recrystallization using a solvent mixture (solvent pair) is very useful for purification of certain compounds. In this technique one dissolves the compound in a warm solution of the solvent component in which the compound is more soluble, and then the second solvent (in which the compound is less soluble) is added until a slight turbidity appears (drop wise). The solution is reheated (or a drop of the first solvent is added) to obtain a clear solution. The solution is cooled
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Lab Manual- Experiment 2 - 8/26/2010 Experiment 2:...

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