Activities - Benjamin-Mills

WM6An aspirin assayIn this activity you will follow the method used byhospital analysts to check the purity of aspirinsamples before they are considered fit to be used onthe wards.Requirements●●aspirin tablets (300 mg)mortar and pestle● specimen tubes (3)●●access to a balance100 cm 3 conical flask● 95% ethanol (30 cm 3 )●10 cm 3 measuring cylinder● sodium hydroxide solution, 0.1 mol dm –3 (60 cm 3 )●●burettePhenolphthalein indicatorethanolCARE Eye protectionmust be worn.HIGHLY FLAMMABLEWEAR EYEPROTECTIONWhat you do1 Grind up 1 aspirin tablet using a mortar and pestle.2 Transfer as much of the powder as possible into a specimen tube. Weigh thetube to an accuracy of 1 mg, and record the mass.3 Use a measuring cylinder to place 10 cm 3 of 95% ethanol into a 100 cm 3conical flask. Add a few drops of Phenolphthalein indicator. Then add asmuch of the powdered aspirin as you can from the specimen tube.4 Reweigh the specimen tube to the same accuracy as before and record themass.5 Swirl the conical flask carefully until all the aspirin powder has dissolved. Donot allow any of the solution to splash out of the flask.6 Titrate the solution in the flask with 0.1 mol dm –3 sodium hydroxide solutionfrom a burette (CARE Eye protection must be worn). Record the volumeneeded to produce the first tinge of pale pink colour in the indicator. Thismeasures the end-point of the titration.7 Repeat the procedure at least once more, starting with a fresh aspirin tablet.CalculationThe equation for the reaction used in the assay is:OHO – Na +COOCCH 3CO+ NaOH + H 2 OO C CH 3ONotice that 1 mol of aspirin reacts with 1 mol of sodium hydroxide. (Actually,if you were to heat the reaction mixture, a further reaction would take place andthe ester would be hydrolysed. But at room temperature this does not happen.)1 mol of aspirin has mass of 180.2 g.Concentration of sodium hydroxide solution = ........ mol dm –3O150AS LEVEL„ Salters Advanced Chemistry 2000 – see Copyright restrictions