36 - A P CHEMISTRY Lab 1-2 Thin Layer Chromatography...

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A P CHEMISTRY Lab 1-2 Thin Layer Chromatography Pre-Lab Questions - (Must be completed before lab work can begin.) 1. In preparing a TLC slide or filter paper for chromatography, a baseline is drawn for positioning the spots in pencil. Why is ink never used for drawing the baseline? 2. The indicator dyes used in this experiment are also used in acid/base titration analyses because they change color at particular values of pH. Use a handbook of chemical data to find the colors of each of these dyes under low and high pH ranges. methyl red, xylenol orange, bromocresol green INTRODUCTION - General Introduction to Chromatography The word chromatography means color-writing. The name was chosen at the beginning of this century when the method was first used to separate colored components from plant leaves. Chromatography in its various forms is perhaps the most important known method of chemical analysis of mixtures. Paper and thin-layer chromatography are simple techniques that can be used to separate mixtures into the individual components of the mixture. The methods are very similar in operation and principle, differing primarily in the medium used for the analysis. Paper chromatography uses ordinary filter paper, which consists primarily of the polymeric carbohydrate cellulose, as the medium upon which the mixture to be separated is applied. Thin-layer chromatography (universally abbreviated as TLC) uses a thin coating of aluminum oxide (alumina) or silicagel on a glass microscope slide or plastic sheet to which the mixture to be resolved is applied. A single drop or spot of the unknown mixture to be analyzed is applied about half an inch from the end of a strip of filter paper or a TLC slide. The filter paper or TLC slide is then placed in a shallow layer of solvent or solvent mixture in a jar or beaker. Since filter paper or the coating of the TLC slide is permeable to liquids, the solvent begins rising by capillary action. As the solvent rises to the level at which the spot of mixture was applied, various effects can occur, depending on the constituents of the spot. Those components of the spot that are completely soluble in the solvent will be swept along with the solvent front as it continues to rise. Those components that are not at all soluble in the solvent will be left behind at the original location of the spot. Most components of the unknown spot mixture will take an intermediate approach as the solvent front passes. Components in the spot that are somewhat soluble in the solvent will be swept along by the solvent front, but to different extents, reflecting their specific solubilities. By this means, the original spot of mixture is spread out into a series of spots or band, with each spot representing one single component of the original mixture. The separation of a mixture by chromatography is not solely a function of the solubility of the
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36 - A P CHEMISTRY Lab 1-2 Thin Layer Chromatography...

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