Aspirin, acetylsalicylic acid is both an organic ester and an organic acid. It is used extensively in medicine as an analgesic, pain releiver and as a fever-reducing drug. It is generally prepared by

Question

Aspirin, acetylsalicylic acid is both an organic ester and an organic acid. It is used extensively in medicine as an analgesic, pain  releiver and as a fever-reducing drug. It is generally prepared by reaction of  salicylic acid with acetic anhydride according to the following  reaction.

[VISUAL]

The amount of acetylsalicylic acid can be determined by titrating with a strong base such as sodium hydroxide.

CH3CO2C6H4CO2H(aq)  +  OH-(aq)  →  CH3COOC6H4COO-(aq)  +  H2O(l)

However, the ester of  acetylsalicylic acid is easily hydrolyzed, and thus, during a normal titration  with a strong base the alkaline conditions break it down leading to errors in  analysis. Thus, a back titration method is applied, in which in the first step,  all the acid present in solution is completely hydrolyzed by excess strong base  such as NaOH. The aspirin/NaOH acid-base reaction consumes one mole of hydroxide per mole of aspirin. The slow aspirin/NaOH hydrolysis reaction also consumes one mole of hydroxide per mole of aspirin, Thus, the number of moles of NaOH added  should be more than twice that of aspirin. Then, the amount of excess hydroxide  is determined by titration with standard acid solution.

In this experiment,  acetylsalicylic acid will be prepared. The total amount of acid present will be  determined by using a back titration method.

Chemicals and reagents
*   Salicylic acid, CH3CO2C6H4CO2H
*  Acetic anhydride, CH3C2O3CH3
*  Phosphoric  acid, H3PO4 or sulfuric acid, H2SO4, concentrated
*  Ethanol, C2H5OH
*  Sodium hydroxide, NaOH solution, 0.50 mol dm-3
*  Hydrochloric acid, HCl  0.30 mol  dm-3
*  Phenolphthalein indicator

Apparatus and glassware
*  Beakers, 100  cm3,
*  Erlenmeyer, 250 cm3 (2)
*  Pipettes, 5 cm3 and 10 cm3
*  Graduated  cylinder, 50 cm3
*  Burette, 50 cm3
*  Stirring rod
*  Watch glass
*   Buchner funnel
*  Filter paper
*  Vacuum filtration flask
*  Melting point  capillary tube
*  Thermometer, 110 °C
*  Melting point apparatus
*  Washing  bottle

A. Synthesis of Aspirin, acetylsalicylic acid
1.  Place accurately  weighed 3.00 g of salicylic acid  in a 100 cm3 Erlenmeyer flask.
2.  Add  6.0  cm3 of acetic anhydride and 4 to 8 drops phosphoric acid to the flask and swirl  to mix everything thoroughly.
3.  Heat the solution to about 80 – 100°C by  placing the flask in hot water for about 15 minutes.
4.  Add  2 cm3 of cold  water dropwise until the decomposition of acetic anhydride is completed and then 40 cm3 of water and cool the solution in ice bath. If crystals do not appear,  scratch the walls of the flask with a stirring rod to induce  crystallization.
5.  Weigh the filter paper that will be used in filtration.  Filter the solid by suction filtration through a Buchner funnel and wash the  crystals with a few milliliters of ice cold water at about –5 °C.
6.  For  recrystallization, transfer by dissolving the crystals into a beaker and add 10  cm3 ethanol and then add 25 cm3 warm water.
7. Cover the beaker with a watch  glass and once crystallization has started place the beaker in an ice bath to  complete the recrystallization.
8. Apply suction filtration as described in  step 5.
9. Place the filter paper with the product onto a watch glass and dry  in oven at 100 °C for about 1 h and weigh the product.
10.  Determine the  melting point (135 °C) to verify  purity.

Determination of amount of  acetylsalicylic acid
1.  Dissolve 0.5 g of aspirin in 15 cm3 of ethanol in a  250 cm3 Erlenmeyer flask.
2.   Add 20 cm3 of 0.50 mol dm-3 NaOH solution.
3.   In order to speed up the hydrolysis reaction, heat the sample in a water bath  about 15 min after addition of two or three boiling chips to the flask swirling  the flask occasionally. Caution: Avoid boiling, because the sample may  decompose.
4.  Cool the sample to room temperature and add  2 – 4 drops of  phenolphthalein indicator to the flask. The color of the solution should be  faint pink. If the solution is colorless add 5 cm3 of 0.50 mol dm-3 NaOH  solution and repeat the steps 3 and 4.
5.  Record the total volume of 0.50 mol  dm-3 NaOH solution added.
6. Titrate the excess base in the solution with 0.30  mol dm-3 HCl solution until the pink color just disappears and the solution  becomes cloudy.
7.  Record the volume of 0.30 mol dm-3 HCl solution added.
8.  Repeat the titration two more times using two new samples.

Accepted Answer

34.1: Theoretical amount of aspirin: n(salicylic acid) = 3.00 g / 138.12 g mol-1 = 0.0217 mol n(aspirin) = 0.0217 mol m(aspirin) = 0.0217 mol × 180.2 g mol-1 = 3.906 g Experimental: Mass of the dried product (aspirin) obtained experimentally = 3.03 g n(aspirin) = 3.03 g / 180.2 g mol-1 = 0.01682 mol Yield of aspirin = 3.03 g / 3.91g * 100 = 77 % The yield is low due to the solubility of aspirin in cold water. 34.2: n(acetylsalicylic acid) theoretical = 1.00 g / 180.0 g mol-1 = 5.55 mmol 40 cm3 0.50 mol dm-3 NaOH = 20 mmol In the titration of 1.00 g samples with HCl solution (0.30 mol dm-3), an average value of 27.0 cm3 is found: 27 cm3 × 0.30 mol dm-3 = 8.10 mmol HCl n(NaOH used by acetylsalicylic acid) = 20.0 – 8.10 = 11.9 mmol 2 NaOH ⇔ 1.0 mol of acetylsalicylic acid n (acetylsalicylic acid) = 11.9 / 2 = 5.95 mmol In order to remove all the acetic acid produced during reaction, the recrystallization process is repeated and the sample is washed with excess water. Thus, the amount of sample is reduced from 1.50 g to 1.05 g. In the titration of the recrystallized 1.00 g samples with 0.30 mol dm-3 HCl, an average value of 28.9 cm3 is found: 28.9 cm3 × 0.30 mol dm-3 HCl = 8.67 mmol n(NaOH used by acetylsalicylic acid) = 20.0 - 8.67 = 11.3 mmol 2 NaOH ⇔ 1.0 mol acetylsalicylic acid n(acetylsalicylic acid) = 11.3/2 = 5.67 mmol 34.3: Melting point of the sample is 132 °C indicating that the aspirin sample is impure. The amount of acetylsalicylic acid found in (b) is more than the theoretical amount which indicates that the method does count not only the acetylsalicylic acid but also the unreacted salicylic acid and byproduct acetic acid. Thus, it is not possible to make a statement on purity of the aspirin sample.

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