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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