Purification of Benzoic Acid by Recrystallisation. Flashcards
What are the 2 steps involved in purifying benzoic acid?
Hot filtration.
Cold Filtration
Describe the procedure for hot filtration.
Dissolve the crystals in a minimum of hot water to maximise yield.
Filter in hot apparatus to remove insoluble impurities and stop premature crystallisation.
Cool filtrate in iced water to maximise recrystallisation.
Describe the procedure for cold filtration.
Filter again to trap the pure crystals as they are the residue.
Use vacuum filtration to speed up process.
This removes any soluble impurities as they are in low concentration and do not recrystallise so they pass through in the filtrate.
Wash crystals with ice cold water to remove last traces of soluble impurities.
Dry crystals over the radiator.
How do you determine the melting point?
Place small sample on aluminium block.
Place mineral oil in hole.
Heat slowly using a bunsen burner.
Note the temperature at which the sample begins to melt.
Note the temperature at which it is all melted.
If it melts over several degrees it it impure.
If the melting point is lowered it is impure.
If it melts over 1C sharply it is pure.
Why was it important to use a minimum of hot water?
To maximise the yield.
So that crystals will form on cooling.
Why was it important to filter in a hot apparatus?
To remove insoluble impurities and stop premature crystallisation.
Why was the filtrate cooled in ice water?
To maximise recrystallisation.
Why was the solution filtered again?
To remove any soluble impurities as they are in low concentration and do not recrystallise so they pass through in the filtrate.
Why were the crystals rinsed with ice cold water?
To remove last traces of soluble impurities.
How could you have ensured that the recrystallisation was complete?
Let the solution cool fully.
How could the crystals be dried?
In a desiccator or on top of a radiator.
Give two ways in which you would expect the melting point of the impure benzoic acid to differ from that of the purified acid.
Lower melting point.
Melting point over wider range of temperatures.
State the uses of benzoic acid and its salts.
Preservative.
Disinfectant.
