Quantitative determination of acetylsalicyclic acid

Weigh accurately a definite number of aspirin tablets (two or three tablets weighing no more than 1.5 g) into a 250 cm3 conical flask. Initiate the hydrolysis of the aspirin by adding 25 cm3 of 1.0 M sodium hydroxide by pipette, diluting with approximately the same volume of distilled water. Warm the flask over a tripod and gauze for ten minutes to complete the hydrolysis.

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Cool the reaction mixture and transfer with washings to a 250 cm3 volumetric flask, dilute to the mark with distilled water and then ensure that the contents of the flask are well mixed by repeated shakings. c.Titrate 25 cm3 portions of the diluted reaction mixture with the standard 0.050 M sulphuric acid provided, using phenol red indicator until two or three consistent results are obtained. d. Calculate the weight of acetylsalicylic acid in each tablet and compare your results with the manufacturer’s specification. Compare your results with those given in different brands of aspirin.


1.Explain why it is necessary to warm the reaction mixture of aspirin and sodium hydroxide solution.
It is to ensure that the alkaline hydrolysis is complete.

2. Explain why phenol red (or phenolphthalein) is used as the indicator in this experiment. After the hydrolysis of aspirin, the two salts formed are alkaline. Hence, an indicator with an alkaline pH range (6.8-8.4) is suitable for the titration.

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