Aspirin in the lab

  • separation techniques   * in the reaction between salicylic acid and acetic anhydride to produce aspirin not all of the reactants will be used up   * when the reaction is completed we will be left with a mixture of substances from which we will have to separate the product   * the initial separation involves filtration this will separate the solid and liquid parts of the reaction mixture   * this will remove any leftover acetic anhydride and the ethanoic acid formed in the reaction   * the solid that remains will be the aspirin and any remaining salicylic acid
  • purification   * after separation of the product from other reactants the product must be purified and the salicylic acid removed   * this can be done using recrystallisation
  • crystallisation   * crystallisation is the process where a substance previously in solution comes out of the solution   * supersaturation is important for crystallisation   * saturated solution : a solution that contains the maximum amount of solute possible   * supersaturation the solution contains more solute than it would normally be possible (this is usually done by heating it)

re crystallisation

  • this process can be used when the product has different solubility in hot and cold solvent
  • aspirin is such a substance
  • it is very soluble in hot water
  • but almost completely insoluble in cold water
  • the solubility of other reactants is less affected by temperature and all remain soluble in cold water

method

  • heat solvent (in this case water)#

  • add product.it should all dissolve

  • cool slightly.crystals of pure product appear

  • filter. Impurities will remain in solution

  • repeat

  • allow it to dry

  • drying   * water can be removed from a mixture of liquids by adding solid anhydrous calcium chloride   * the calcium chloride is hygroscopic which means it attracts water and holds it removing it from the mixture.the remaining liquid can be separated by filtration.

  • freeze drying   * used in pharmaceutical and food industries   * helps to give product a longer shelf life   * pre treatment such as concentrating the solution of product and water   * freezing in a freeze dryer to a temperature lower than either product or waters freezing point   * usually between -50 and -80   * this allows sublimation rather than melting   * primary drying   * the pressure in the dryer is reduced and temperature raised   * under these conditions the water will sublime(turn from solid to gas without melting first)   * about 95% of the water will be removed but the process can take days   * secondary drying by raising the temperature yet again to break and interactions between the water and the product   * once the process is complete the vacuum is broken,and the pressure returns to normal   * this is usually done by introducing and inert gas such as nitrogen so that there are no reactions between the product and the atmosphere

  • spray drying   * used when large amount of liquid or wet solid need to be dried   * hot air is used to dry the material   * the liquid or wet solid is sprayed into the top of the dryer by an atomiser that produces a very fine mist   * hot air is pumped in the same direction   * as the mist is heated the water is driven off and solid falls the bottom of

  • how pure is your aspirin   * after purification has taken place an assessment must be made of the final products purity   * there are many melting techniques here are 2   * melting point determination   * chromatography

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