purification of aspirin

steps fpr recrystallisation

1. dissolve the impure compound in a minimum volume of hot solvent

2. hot filter solution through filter paper quickly

3. cool the filtered solution by inserting the beaker in ice

4. suction filtrate using a buchner flask and funnel to separate out crystals

5. wash crystalled with distilled water

6. dry the crystals between absorbant paper

why do you dissolve the compound in minimum volume of hot solvent?

• you want a solvent that when hot dissolves both the compound and soluble impurities - but when cold the compound does not dissolve well

• minimum volume is used to obtain a saturated solution and to enable crystillisation on cooling - reduce loss of produc

2. why do you hot filter the solution

to remove any insoluble impurities and heat wull prevent crystals reforming during filtration

3. cool filtered solution by inserting beaker in ice

• Crystals will reform but soluble impurities will remain in solution form because they are present in small quantities so solution is not saturated.

• Ice will increase the yield of crystals

suction filtrate with a buchner flask and funnel to separte out crystalas

The water pump connected to the Buchner flask reduces the pressure and speeds up the filtration.

5 Wash the crystals with distilled water

To remove soluble impurities

where do you lose yield in this process?

Crystals lost when filtering or washing • Some product stays in solution after recrystallisation • other side reactions occurring

suction filtration

method for purifying an organic liquid

1. put he distillate of an impure product into a separating funnel

2. wash product by adding sodium hydrogencarbonate solution, shaking and releasing the pressure from co2 produced

3. allow the layers to separate in the funnel and then run and discard the aqueous layer

4. run the organic layer into a clean, dry conical flask and add dying agent (eg. anhydrous sodium sulfate, calcium chloride) to dry the organic liquid - when dry it should appear clear

5. carefully decant the liquid into the distillation flask

6. distil to collect pure product

purpose of add sodium hydrogencarbonate

neutralise any remaining react