Recrystallisation of benzoic acid anddetermination by measuring its melting point
RecrystallisationTheory
An impure solid may be purified by first dissolving it in the minimumquantity of a boiling solvent. Insoluble impurities are then removed byrapid filtration of the hot mixture. The filtrate is next allowed to coolslowly. Since the solubility of the solid decreases as the temperaturefalls, crystals will form. The soluble impurities mostly remain dissolvedin the solvent as the crystal structure “re-grows” in a regular pattern.
Procedure
Boil about 100 cm3 deionised water in a 250 cm3 beaker and keep thewater just at its boiling point. Weigh out about 1 g of the benzoic acidsample in a 100 cm3 beaker.
Flute a filter paper . Place the flutedfilter paper in the funnel,and secure it by wetting it.
Prepare for hot filtration bystanding a 250 cm3 conicalflask containing about 20cm3 of boiling water on ahotplate or over a lowBunsen flame.
Sit thefunnel with fluted filterpaper in the neck of theflask.
Using a dropping pipette, add 25 cm3 of boiling water to the beakercontaining the sample, with constant stirring. Heat the mixture gentlyto keep the solution near its boiling point. Any remaining crystalsshould be dissolved by adding the minimum extra amount necessary ofboiling water.
When ready to filter, empty the conical flask, replace the funnelquickly, and stand the flask on a heat resistant mat.
Working quicklybut carefully, pour the boiling solution through the filter paper (Fig.1)in small portions. (It may be necessary to reheat the beakerperiodically.)
If any crystals form in the filter paper, add a little boilingwater to dissolve them.
Empty the filtrate into a warm 100 cm3 beaker. Heat the filtrate untiltraces of crystals begin to appear on the sides of the beaker. You nowhave the minimum amount of solvent required.
Place the beaker on aheat resistant mat and allow the filtrate to cool to room temperature.
To recover the crystals, carefully filter the recrystallised mixture, byvacuum filtration if possible. If some crystals remain in the beaker,transfer the filtered liquid back to the beaker and filter again.
Repeatthis procedure until all the crystals have been transferred to thefunnel.
Wash with 2 cm3 of ice cold deionised water and dry the crystals usingvacuum filtration for ten minutes.
Using the spatula, remove the crystals from the funnel and transferthem to a previously weighed dry clock glass. Dry the crystals byplacing the clock glass in an oven set at 80 0C, or on top of a radiatorfor 24 hours.
Melting point determinationTheory
One of the physical properties of a pure substance is its melting point,or, more correctly, its melting point range. The melting point range ofa substance is the narrow band of temperatures between thetemperature at which melting begins and the temperature at which allthe solid has liquefied. The solid state and the liquid state of thesubstance co-exist across this small temperature range.
The melting point range of an impure substance is lower and broaderthan that recorded for a pure sample of the same substance.
Comparing the melting point ranges of the benzoic acid sample beforeand after recrystallisation with the melting point range of pure (Analar)benzoic acid will therefore indicate the success of the recrystallization process in improving the sample purity. It will also indicate whether itis as pure as an Analar sample.
Melting point determination
- Grind up the benzoic acid into a fine powder using the pestle and mortar.
- Prepare a number of capillary tubes by gently breaking and sealing on end by heating in a hot Bunsen flame for a few seconds.
- Switch on the melting point equipment .
- Read the thermometer
- Dip the capillary tube in the powder and place the capillary tube in the hole next to the thermometer.
- Observe the melting point of the benzoic acid. When the compound begins to melt record its temperature. When it is completed melted record the temperature. This will give you the melting range.
- Repeat for the re-crystallised benzoic acid.
- Compare your results.
Results
Melting point of impure benzoic acid =
Melting point of purified benzoic acid =
What do your results tell you? What factors may affect the purity of your final product?
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Evaluate the accuracy of your results ensuring that you compare your results. Do you think that your results are accurate? Can you account for any differences because of your technique?
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