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Unformatted text preview: Chem 267 . Recrystallization Part 1 . (revised 7/10) Recrystallization is the most common method for purifying solid compounds. It will be used throughout this course so it is vital that you understand the principles behind it and learn to do it correctly from the start. In later experiments you will not be given detailed instructions for recrystallizations. You will simply be told to recrystallize a compound. A good recrystallization solvent is one in which the solid has a very high solubility at high temperatures and a very low solubility at low temperatures. Refer to the flow chart shown below. Note the distinction between melting and dissolving . Melting is the process in which a solid is converted to a liquid by supplying heat. Dissolving is the process that homogeneously disperses a solid into a solvent. Do the Prelab Exercise on p. 61 as part of your prelab outline. Look over the photos on the course website to help understand the handout. (1.) Solubility tests . Remember: sand baths take time to heat up so whenever a procedure calls for the use of a sand bath, the first thing to do upon entering the lab is to turn the controller on to a setting of about 35-40 % (NEVER HIGHER). Always turn the heater off before you leave for the day. You will test the solubility of four compounds - resorcinol, anthracene, benzoic acid, and 4-amino-1- naphthalenesulfonic acid, sodium salt (structures in lab text) - using water, toluene, and ligroin as the solvents. Use about 10 mg of each compound for the tests. It is not necessary to use a balance to measure out the solids accurately. A 10 mg sample of each solid in a reaction tube will be on display in the lab. Simply use an amount which approximates that. Use your plastic funnel to help transfer the solid to the tube. The solvent does not have to be measured exactly either. Use a disposable pipet and the calibration markings on p. 15 of your text or the calibrations on the reaction tube. This level of accuracy is enough for qualitative tests such as these and will be done often in certain microscale experiments, in cases where greater accuracy is not required. To heat solutions in a reaction tube, use the sand bath as the heat source and a boiling stick in the tube to promote smooth boiling. Failure to use a boiling stick will result in superheating of the solution followed by bumping of the solution out of the tube. In heating a tube with a sand bath, the temperature of the tube is controlled by adjusting the depth to which it is immersed in the sand. Use care in heating low-boiling solvents such as ligroin on the sand bath. Such solvents evaporate and bump easily. Note that ligroin and toluene are flammable. Never point the tube at yourself or at anyone else while heating....
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This note was uploaded on 11/29/2011 for the course CHEM 267 taught by Professor Samal,p during the Fall '08 term at UMass (Amherst).
- Fall '08