How to Determine the Purity of an ASA Experiment

Print this articleAcetylsalicylic acid, or aspirin, is a non-steroidal, anti-inflammatory drug widely used as an analgesic and anti-coagulant. Synthesis of aspirin from salicylic acid involves a reaction with acetic anhydride in the presence of phosphoric acid which behaves as a catalyst. The major impurity in the product is salicylic acid, which precipitates along with the aspirin if the procedure is not followed correctly. Besides general tests such as melting point determination that help determine the purity of the aspirin, it is vital to test for presence of salicylic acid.

Related Searches:Difficulty:Moderately ChallengingInstructions 1

Dry about 50 mg of the aspirin you synthesized and crush it using a spatula. Fill this into a melting-point capillary tube and attach the tube to a thermometer. Suspend the thermometer-tube assembly into an oil bath and heat gradually. Record the temperature at which the aspirin in the tube melts. Compare this melting point with the literature value for the melting point of aspirin which is around 135 degrees Celsius.


Dissolve 50 mg of aspirin in 50 ml of distilled water. Dissolve 2 g of ferric nitrate in 50 ml of distilled water and add 0.5 ml of concentrated nitric acid. Take 1 ml of the aspirin solution in a test tube and add 5 ml of the ferric nitrate solution you prepared. Mix well for about 5 minutes and observe. Appearance of a violet color indicates the presence of salicylic acid.


Place the violet colored solution in a glass cell of the Spectronic 20 instrument. Set the wavelength of the instrument to 540 nm and measure the absorbance of the solution. Prepare a standard curve for the absorbance by different concentrations of salicylic acid. Compare the absorbance value you obtained with the aspirin sample to this graph to determine the concentration of the salicylic acid present in the sample.


Place 100 to 150 mg of the aspirin sample in a 125 ml Erlenmeyer flask. Mix in 15 ml of 95 percent ethanol solution; add 2 drops of phenolphthalein indicator and titrate this solution against a standard solution of sodium hydroxide from a burette. Use the titer value to calculate the molarity of the aspirin. Calculate the ratio of the observed molarity of aspirin with its theoretical molarity. Multiply this ratio with 100 to obtain the percentage purity of aspirin.

ReferencesOklahoma State University: LAB 1: Synthesis of AspirinCalifornia State University Stanislaus: The Synthesis and Characterization of Aspirin (Part 2)Santa Monica College: Aspirin SynthesisBellevue College, Science Division: Experiment 9: Titration of AspirinRead Next:

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