rp10a - prep of organic solid and a test of its purity

prep of aspirin, exact masses/vols likely unnecessary but for completion's sake

why does the process of recrystallisation purify the aspirin?

• aspirin is highly soluble in hot water and less soluble in cold water

• when heated in the hot water (solvent), aspirin and impurities dissolve in the solvent

• cooling the solution - cold water - causes the aspirin to crystallise out of the solution as it is less soluble

why is a small volume of ethanol used to dissolve the crude aspirin?

so as much crystallises out as possible as it cools

why is the conical flask cooled slowly when allowing the crystals to form?

to ensure the max amount of crystals form

why does scratching the side of the conical flask help the crystals form?

provides cracks/crevices which act as ‘nucleation sites’, facilitating crystal formation

why must you leave the solid aspirin to dry?

to ensure that mass is accurate and not affected by water present on it

name 7 hazardous materials present in the preparation and purification of aspirin and explain why they are hazardous:

• salicylic acid (2-hydrobenzenecarboxylic acid) - corrosive, irritant

• ethanol - flammable

• aspirin - irritant

• concentrated sulfuric acid - corrosive

• ethanoic anhydride - irritant, flammable

• glassware (Büchner flask/beaker etc.) - broken glassware could be sharp and cut skin

• hot/boiling water - could burn skin if spilled

how can we use control measures to decrease the risks presented by salicylic acid?

• wear safety goggles and lab coat to prevent irritation to eyes/skin

• take care not to inhale/swallow

how can we use control measures to decrease the risks presented by ethanol?

keep away from naked flame

how can we use control measures to decrease the risks presented by aspirin?

• wear safety goggles and lab coat to prevent irritation to eyes/skin

• take care not to inhale/swallow

how can we use control measures to decrease the risks presented by concentrated sulfuric acid?

wear safety goggles and lab coat to prevent irritation to eyes/skin

how can we use control measures to decrease the risks presented by ethanoic anhydride?

• wear safety goggles and lab coat to prevent irritation to eyes/skin

• keep away from naked flame

how can we use control measures to reduce the risks presented by glassware?

keep glassware away from edge of table

how can we use control measures to reduce the risks presented by hot/boiling water?

• keep beaker/kettle away from edge of table

• take care when using kettle and preparing water bath

name the 3 stages of the preparation and purification of aspirin:

1. preparation

2. purification by recrystallisation

3. testing the purity

give the method for stage 1 - preparation:

1. weigh out approx 6.00 g of salicylic acid directly into a 100 cm³ conical flask

2. record mass of salicylic acid used

3. using a 10 cm³ measuring cylinder, add 10 cm³ of ethanoic anhydride to the flask and swirl the contents

4. add 5 drops of concentrated sulfuric acid to the flask and swirl the mixture in the flask for a few minutes to ensure thorough mixing

5. warm the flask for 20 mins in a 400 cm³ beaker of hot water at approx 60^oC - temp should not be allowed to rise above 65^oC

6. allow the flask to cool and pour its contents into 75 cm³ of water in a beaker, stirring well to precipitate the solid

7. filter off the aspirin under reduced pressure, avoiding skin contact

8. collect the crude aspirin on a double thickness of filter paper and allow to dry

give the method for stage 2 - recrystallisation:

1. using a 25 cm³ measuring cylinder, measure out 15 cm³ of ethanol into a boiling tube

2. prepare a beaker half filled with hot water at a temperature of approximately 75^oC - temp should not be allowed to go above 78^oC as this is ethanol’s bpt

3. use a spatula to add the crude aspirin to the boiling tube and place the tube in the beaker of hot water

4. stir the contents of the boiling tube until all of the aspirin dissolves into the ethanol

5. pour the hot solution containing dissolved aspirin into approx 40 cm³ of water in a 100 cm³ conical flask

6. if a solid separates at this stage, gently warm the contents of the flask in the water bath until solution is complete - avoid prolonged heating as this will decompose the aspirin

7. allow conical flask to cool slowly e.g. ice bath and white needles of aspirin should separate

8. facilitate the formation of crystals by scratching the insides of the flask with a glass stirring rod and continue cooling in ice bath

9. filter off purified solid under reduced pressure and allow to dry on filter paper

10. record mass of dry purified solid

give the method for stage 3 - testing the purity:

1. powder a sample of the organic solid by crushing it gently with a spatula onto the surface of some filter paper

2. fill a melting point apparatus provided and mount the melting point tubes ready for taking a measurement

3. heat the apparatus gently and observe the temp at which the solid collapses into a liquid - likely range of 100-200^oC

4. allow melting point apparatus to cool

5. compare data book value with your own

what is the data book mpt for aspirin?

138-140^oC

what is the data book mpt for salicylic acid?

158-160^oC