The its sodium salt which will migrate

The purpose of recrystallization is to purify a solid having impurities of two or more unknown components and finding out what that one unknown desired component is through multiple steps.

The first step of recrystallization is to determine the choice of the solvent. The solvent should be able to dissolve solutes in boiling point temperature than in room temperature. The second step in recrystallization is the separation of insoluble impurities from soluble impurities using dichloromethane as the solvent in the lab. The insoluble impurities (sucrose) can be filtered by a gravity filtration to remove any of the unwanted solids from the filtrate. Aspirin can be removed from the dichloromethane solution by extraction with an aqueous solution of 5% Na2CO3. The base converts aspirin to its sodium salt which will migrate from the dichloromethane layer, in which it is insoluble, to the aqueous layer, in which it is soluble.

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The unknown component will stay behind in the dichloromethane layer. After separation of the layers, aspirin can be recovered by precipitation from the aqueous layer by adding 6 M HCl followed by vacuum filtration. The unknown component can be isolated by evaporating the solvent from the dichloromethane solution. Using a filter flask attached to a trap and aspirator to evaporate the solvent from the dichloromethane solution, leaving the unknown in the flask.

Then scraping the unknown out of the flask and placing it into an Erlenmeyer flask we then added hot water to dissolve the unknown as much as possible. When the solids can no longer be dissolved from the heated solvent you can use a gravity filtration to separate out between the solvent and solute. The third step is waiting for the solvent to cool until it’s at room temperature to form crystals. If crystals do not form when cooled it is best to place it in an ice bath and scratch around the sides for crystals.

Lastly you can pour the solution and crystals in a Bushner funnel to vacuum filtrate the crystals on the filter paper. Then remove crystals from filter paper, dry, and determine the melting point. The student could have added excess amount of the hot solvent in her flask which could explain why her yield was low. In room temperature the crystals did not form because the rate of evaporation of the solvent is slow. When the student added the flask into an ice bath, crystals began to form because slow cooling of the solution leads to the formation of crystals.


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