7/26/2019 Coursework Practical Write Up

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Lanyu Fan, Lucy Henderson & Shentong Wang

Purification and identification of an impure sample using TLC and recrystallisation

The method of production of the sample was given along with an impure TLC plate,

which provided an idea of the solvents and visualisation techniques needed. The sample

is stated to contain benzoic acid, benzyl alcohol, salts, and anti-bumping granules.

UV light and a dip was used to reveal benzoic acid and benzyl alcohol were bothvisible by UV, and that only benzyl alcohol stains yellow with . Instruction was

also given to repeat the impure TLC, purify the sample, run a pure TLC and also provide

a pure sample of benzoic acid isolated from the impure mixture.

Materials:

Silica TLC plates

Pure benzoic acid

Pure benzyl alcohol

Deionised water

Ethanol

Hexane & Ethyl Acetate

Specimen tubes

Tweezers

Conical flasks

Beakers

Funnel & filter paper for hot

filtration

Hot plate

Buchner flask, funnel & filter paper

Ice bath

& UV lamp

Procedure & Rationale:

1. A small sample of the mixture was placed in a beaker and dissolved using

ethanol. Spots of the product mixture, benzoic acid, and benzyl alcohol were

placed in a line 1cm from the bottom of the TLC plate. A solvent consisting of 1:1

hexane:ethyl acetate was poured into a specimen tube to a depth of about 2mm.

The TCL plate was carefully lowered into the specimen tube with tweezers and

the lid sealed, ensuring that natural movement of the solvent was the only factor

affecting its progression. Once the solvent front had reached 1cm from the top ofthe plate and marked with a pencil line, it was first viewed under a UV light.

Then the plate was dipped in . This revealed that the product mixture did

indeed contain benzyl alcohol due to the presence of two yellow stains on the

plate.

2. Hot filtration was then carried out. The remaining mixture was dissolved in the

minimum amount of hot water required, while a flask was warmed on a hot

plate to ensure that recrystallisation did not happen during filtration. Fluted

filter paper in a funnel was used in combination with the heated flask and this

removed the anti-bumping granules. The solution was placed in an ice bath and

allowed to cool completely. Water was chosen as the solvent for hot filtration as

the solubility of benzoic acid in grams per 100is 0.34 and so if a large

amount is dissolved at a higher temperature then cooled using an ice bath, the

benzoic acid should recrystallize, leaving impurities dissolved in the solution.

Ethanol was not used as there is no useful saturation point. A Buchner funnel

was used to filter out the recrystallised compounds, which were then left to dry.

3. Next, some of the crystals were dissolved in a small quantity of ethanol and TLC

was used to compare the crystals to benzoic acid and benzyl alcohol. The same

method was used as for the impure TLC. The plate was viewed under UV light

and then dipped in . This time, there was only one yellow stain, indicating

the absence of benzyl alcohol in the crystals. It follows the crystals were pure

benzoic acid. The rest of the crystals were then placed in a labelled specimentube and submitted with the two TLC plates produced in this experiment.