Lab Manual- Experiment 4

Lab Manual- Experiment 4 - Experiment 4: CHROMATOGRAPHY...

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Experiment 4: CHROMATOGRAPHY Reference: Pavia 777-800. Intr o ducti o n Separation of a mixture into its pure components is an essential part of organic chemistry. For example, a chemist may want to purify the crude extract of a medicinal plant, isolate the pure product(s) of a chemical reaction from the reaction mixture, or identify foreign compounds in a urine sample. Experiments 2, 3, 4, and 7 in this course cover basic separation and purification techniques: filtration recrystallization, distillation, chromatography, and extraction. These techniques can be distinguished by the important physical properties involved: solubility, boiling point or polarity. Essentially, the purification of a mixture takes advantage of the way any physical property varies between the components of a mixture. Background Information Chromatography is one of the most ubiquitous methods of analyzing and purifying organic compounds. Flash column chromatography separates large quantities of compounds under air pressure while TLC (thin layer chromatography) is more useful for qualitative and small-scale separations or the identification of compounds in a mixture. The fundamental principle of chromatography is the distribution equilibrium that forms when a compound is either dissolved in a mobile phase or adsorbed on a stationary phase.
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When a compound is dissolved in the mobile phase it is carried along the direction of flow, but when it is adsorbed on the stationary phase it does not move. If compound B spends more time in the mobile phase than compound A, B will move further along the direction of flow than A, and will eventually be separated in space from A. The longer the mobile phase travels, the better the separation between A and B. Stationary phases are usually very polar, while mobile phases vary widely in polarity, but are less polar than the stationary phase. The exception is reverse phase (RP) chromatography, in which a polar mobile phase, and a less polar stationary phase are used. T hin l ay er chr oma t og r a phy This technique is performed on a glass or plastic plate that is coated with a thin layer (thus the name) of dry adsorbent. Usually these plates are pre-coated with a layer of silica gel or alumina . The sample mixture is spotted on the plate near the bottom, and the plate is put in a closed beaker or jar with a small amount of the appropriate solvent or solvent mixture. Capillary action draws the solvent up the plate. When the solvent front is near the top, the plate is removed from the beaker and a separation of the sample's components may be observed (See Figure below).
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Visualizing the TLC Plate If the compounds are colored, the plate can be read easily. If the compounds are not colored then they can be visualized using an ultraviolet lamp or a chemical stain. There are a wide variety of chemical stains based on the functional groups present. The following methods of detection will be used in this laboratory: Ultraviolet Light Detection
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This note was uploaded on 10/27/2010 for the course CHEM 223 taught by Professor Hardin during the Fall '10 term at CUNY Hunter.

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Lab Manual- Experiment 4 - Experiment 4: CHROMATOGRAPHY...

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