Analysis of the Mass of Acetylsalicylic Acid (aspirin)

Standardization of dilute sulphuric acid

1. 1. 32g of the sodium carbonate was weighted accurately in a beaker.
2. 120cm3 of deionised water was added to the beaker containing sodium carbonate.
3. The mixture was stirred gently to let the sodium carbonate to dissolve.
4. The solution was poured into a 250cm3 volumetric flask.
5. The beaker was washed out with further small quantities of water.
6. All the washings were poured into the 250cm3 volumetric flask to ensure no solution was left on the wall of the beaker.
7. Water was added into the conical flask until the mark was made up by a wash bottle.
8. The solution was shaked well.
9. 25cm3 of sodium carbonate solution was pipetted out and ran into a conical flask.
10. Two to three drops of methyl orange indicator solution was added into the conical flask.
11. A burette was filled with the sulphuric acid of unknown concentration.
12. The flask was swirled during the addition of dilute sulphuric acid.
13. Adding of diluted sulphuric acid was stopped when the color of the solution changed from yellow to pink.
14. Step 9 to step 13 was repeated twice in order to get a more accurate result.

Determination of acetylsalicylic acid in aspirin tablets/.

1. Three tablets were weighted accurately.
2. The tablets were putted into a 250cm3.
3. 25cm3 of 1. 0M sodium hydroxide was added into the conical flask by a pipette.
4. 25cm3 of deionised water was added into the flask to dilute the solution.
5. The flask was warmed over a tripod and gauze for ten minutes.
6. The reaction mixture was cooled and transferred with washings to a 250 cm3 volumetric flask.
7. 25cm3 of the solution was titrated with the standardized sulphuric acid, using phenolphthalein as an indicator, the reaction was finished when the color of the solution change from red to colourless
8. Repeat step 7 for 2 times to get a more accurate results.

Discussions: This experiment is to find out the weight of acetylsalicylic acid (aspirin) in an aspirin tablet by the back titration method. By the titration of the NaOH (which left behind after the reaction of NaOH and acetylsalicylic acid) and H2SO4, we can know the amount of NaOH needed to react with all the acetylsalicylic acid present in the aspirin tablet. But, before this, as the molarity of the sulphuric acid used is unknown, so that we have to carry out the standardization of the sulphuric acid first.

This can be done by the titration between sodium carbonate solution of known molarity and the sulphuric acid. The reaction between acetylsalicylic acid and sodium hydroxide is a kind of alkaline hydrolysis. The reaction why we need to heat the mixture is that the reaction between them is quite slow, so we need to heat it to increase the reaction rate. In the experiment, we use phenolphthalein as the indicator, however as the alkaline hydrolysis between sodium hydroxide and aspirin will form acetic acid ad salicylic acid, which are 2 weak acids, so that rather than phenolphthalein, using phenol red indicator is much more suitable.