RCS1601: Separation of Mixtures by Chromatography - Chemistry Assignment Help

Assignment Task:

1. INTRODUCTION 

Chromatography is a technique frequently used to separate mixtures. The basis of the method is the variation in the ability of different compounds to be adsorbed onto solid surfaces. If a "solvent front" moves past a compound that is adsorbed onto a solid surface, the compound may be moved along the surface by the solvent. The greater the solubility of the compound in the solvent, then the faster the compound moves; the greater the attraction of the compound to the surface, then the slower the compound moves. Due to the differences in interactions between different compounds in a mixture with both the solvent and the surface, separation of the compounds can be achieved. Many different substances can be used as solid adsorbents, e.g. alumina, silica gel, filter paper, and resin beads, to name but a few. 

In thin layer chromatography (TLC), the stationary phase is a layer of very finely divided solid, usually alumina or silica, deposited on a solid support such as a glass plate or an aluminium sheet. A small drop of a solution of the sample is placed near the bottom of the plate and the plate is placed in a glass jar or "developing tank" containing a pool of suitable solvent at the bottom. This solvent then moves up the plate by capillary action carrying the spot of sample with it. The compounds most strongly adsorbed tend to remain near the bottom of the plate whilst those that are weakly adsorbed move further up the plate. At every point on the plate the compounds pass from the solvent to the solid phase and vice-versa. The compound that is most strongly adsorbed spends nearly all of its time bound to the stationary phase and will spend little of its time in the solvent (moving) phase, hence it will move up the plate slowly. Compounds that spend little time bound to the stationary phase (i.e. those that are weakly adsorbed) pass quickly up the plate. 

2. Thin Layer Chromatography 

Solutions of caffeine, aspirin and paracetamol will be available. You will also be issued with an "unknown" solution containing a mixture of two of these compounds. 

(1) Prepare a chromatographic developing jar by removing its lid and adding butyl acetate-chloroform-formic acid (6:4:1) solvent mixture to a depth of only 1 cm. Replace the lid on the jar and leave the jar to stand in a fume hood, undisturbed. 

(2) Obtain the separate solutions of caffeine, aspirin, paracetamol and your "unknown" mixture containing two of these. Also obtain a thin-layer chromatography (TLC) plate. 

(3) Draw a very feint pencil line (do not use a ball point or felt-tip pen) 2 cm from the bottom of the TLC plate. This is the "baseline". 

(4) Make four, very feint, evenly spaced, pencil marks (dots) that lie about 2 mm below the "baseline" to help you to space your samples evenly across the plate. Ensure the outermost dots are at least 0.5 cm from the edges of the plate. 

(5) Draw a very feint line that is parallel to the first line and lies exactly 0.5 cm from the top of the plate. This is the "top line". Place the prepared TLC plate aside. 

(6) Very fine capillary tubes that have drawn tips will be made available to you for spotting your chromatographic plate. Obtain four or five of these for use in the following steps. 

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