• 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