Homework 1 solutions - Liquid Chromatography Revised LIQUID...

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Liquid Chromatography Revised: 9/11/08 1 LIQUID CHROMATOGRAPHY: Separating Mixtures Based on Polarity Differences OBJECTIVES A mixture of three compounds will be separated using a column of silica gel and solvents of varying polarity. After solvent evaporation, the percent mass each component of the mixture will be calculated. SAFETY PRECAUTIONS Safety goggles and aprons must be worn at all times. All work must be done in a fume hood! Diethyl ether, hexane and acetone are EXTREMELY flammable; in fact, diethyl ether has the highest possible flammability rating. Silica gel is a severe respiratory hazard that permanently damages the lungs if inhaled; transfer it in the hood only and immediately clean up any spills. INTRODUCTION Chromatography is a widely used technique to separate mixtures of two or more compounds based on differences in polarity. The word “chromatography” literally means “color writing” and refers to the first time this technique was used: to separate brightly colored pigments of leaves. Liquid chromatography consists of a stationary phase and a mobile phase. The stationary phase, as suggested by its name, does not move; rather, chemical compounds travel through it at different rates depending on their polarity. The two most common stationary phases are extremely fine particles (3.5 x 10 -5 m to 7.5 x 10 -5 m) of silica (SiO 2 ) or alumina (Al 2 O 3 ). The small particle size produces a large surface area that strongly adsorbs other molecules via intermolecular forces such as hydrogen-bonding and dipole-dipole interactions. Remember,
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Liquid Chromatography Revised: 9/11/08 2 adsorption is an attraction to a surface whereas absorption means to take in a fluid internally. The more polar a compound is, the more strongly it adheres to the stationary phase and the more slowly it moves through the stationary phase. The mobile phase consists of liquid solvents (or eluants) of different polarities that move the compounds to be separated across the stationary phase. The more polar the mobile phase, the faster all compounds run through the stationary phase . Here is an example of some solvents that are commonly used in chromatography, and their relative ability to displace compounds adsorbed onto the stationary phase: C H H H OH > C C O C H H H H H H > C H Cl H Cl C H H H > C H H O C H H C H H H > C H C H C H H C C H H C H H H H H H H H H O H > water methanol acetone dichloromethane diethyl ether hexane move solutes fastest move solutes slowest In fact, water and methanol are so polar that they dissolve silica and alumina and therefore can only be used in small concentrations with these stationary phases. Nowadays, chromatography can be used to separate colorless compounds as well. Of course, colorless compounds are not visible as they travel through the stationary phase, however there exist numerous methods to visualize them. One such technique is absorption of ultraviolet (UV) light, which is found in the 200-400 nm region of the electromagnetic spectrum. The minimum
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Homework 1 solutions - Liquid Chromatography Revised LIQUID...

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