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Experiment 15

Experiment 15. A: Isolation of Pure Aspirin From Aspirin Tablets B: TLC of a Dye Mixture. Objectives. Isolation of a pure component from a commercial tablet To become familiar with the technique of thin-layer chromatography To separate a dye mixture into its constituent components.

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Experiment 15

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  1. Experiment 15 A: Isolation of Pure Aspirin From Aspirin Tablets B: TLC of a Dye Mixture

  2. Objectives • Isolation of a pure component from a commercial tablet • To become familiar with the technique of thin-layer chromatography • To separate a dye mixture into its constituent components

  3. Introduction – 15A • Commercial pharmaceutical tablets contain a variety of components • Drug molecule • Binders • Flavours • Colours • Co-administered drug molecules

  4. Experimental • Weigh one tablet of aspirin • Grind and transfer to a 50 ml flask • Add 20 ml of acetone and heat • Filter off the insoluble binding material • Evaporate to dryness • Record the weight of crude aspirin

  5. Experimental • Crude aspirin is recrystallized from water • Dissolve in 3-4 ml of boiling water • Allow to cool and crystallize • Isolate the crystals • Allow to dry, get weight and mp

  6. Introduction – 15B • Thin Layer Chromatography • Common separation technique • Separation based on polarity • Qualitative analysis • Used to test purity, reaction completion, decomposition

  7. Introduction • TLC plate – sheet of metal covered in a thin layer of silica • Silica is polar – polar compounds have a greater affinity for silica • Solvent allowed to travel up the plate • Compounds with a higher affinity for the solvent will travel further up the plate

  8. Introduction • Choice of solvent is important • Affects distance travelled by spots • Affects separation of spots • Common solvent systems include mixtures of Ethyl Acetate/Hexane and Dichloromethane/Methanol • Large scale – purification by column chromatography

  9. Rf values • Rf value of a spot is the distance travelled by the spot over the distance travelled by the solvent front • Rf depends on solvent system

  10. Thin Layer Chromatography

  11. Viewing TLC plates • Spots not always visible – can be colourless • Can be viewed under UV light • Developed using iodine to stain spots • Other dyes/stains – vanillin, ninhydrin

  12. Viewing plates

  13. Experimental • Use a capillary spotter to spot the dye mixture on a TLC plate • Insert slide into a jar for development • Solvent level must be below the spot level – otherwise spots dissolve • Allow solvent front to rise close to top, remove plate, mark the solvent front

  14. Experimental • Using pencil, circle all the spots • Calculate the Rf values for each colour spot in the dye mixture • Develop the TLC plate in both toluene and ether – solvent affects the distance travelled and separation • Keep toluene and ether in the fumehood at all times

  15. Report • Record the distance travelled and Rf value for each colour component in both the solvents used • Comment on the relative solubilities of the colour components in the two solvents used • Brief discussion on TLC

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