Exp 10' Analysis of Aspirin

1
Exp 10. Analysis of Aspirin

• 2 students

2

• Objective
• To determine the purity of aspirin by acid-base
  titrations
• To become acquainted with the concept of
  back-titration analyses

• Chemicals
• Aspirin (0.5 g 3), phenolphthalein solution,
  95 EtOH,
• Standardized 0.1 M NaOH, standardized 0.1 M HCl

• If the aspirin has been exposed to moisture for
  an extended period of time, it can decompose as
  follows

Acetic acid
Salicylic acid
Impurities!
3

• Procedures

If the weight of your aspirin is above 1.5 g, use
about 0.5 g per trial. If not exceeding 1.5 g,
divide it into three equal parts (0.3 g 0.5
g). You must record them accurately to four
decimal places with an Analytical balance..

• Put about 0.5000 g of the aspirin into 250 mL
  Erlenmeyer flask, and then add 25 mL of
  ethanol(95) and 2 drops of phenolphthalein into
  it.

? About 0.5000 g of your aspirin
? Ethanol(95) 25 mL
250 mL
? Phenolphthalein 2 drops
CAUTION!! Ethyl alcohol is flammable solvent!
4
2. Titrate the sample with standard 0.1 M NaOH to
a faint pink end point.
Record the volume of NaOH used.
Add 0.1M NaOH solution
Neutralization
5
3. To hydrolyze the aspirin, add additional NaOH
to the Erlenmeyer flask.

• Add additional NaOH
• 15 mL the volume of base required in the
  titration (2.)

4. Heat the mixture for 15 min in a bath of
boiling water to hydrolyze the aspirin.
Base-promoted hydrolysis
6
5. Cool the flask at an ice bath.
6. Back-titrate the excess base with the standard
HCl until the pink color disappears.
Add 0.1M HCl solution
Record the volume of HCl used.

• mmol reagent reacted total mmol (NaOH) mmol
  back titrated (HCl)

7. Repeat this procedure with your other two
samples.
7

• Summary

• Put 0.3 g 0.5 g of the aspirin into 250 ml
  Erlenmeyer flask, and then add 25 ml of ethanol
  (95) and 2 drops of phenolphthalein into that.

2. Titrate the sample with standard 0.1 M NaOH to
a faint pink end point.
Record
3. To hydrolyze the aspirin, additional NaOH to
the Erlenmeyer flask.
4. Heat the mixture for 15 min in a bath of
boiling water to hydrolyze the aspirin.
5. Cool the flask at an ice bath.
6. Back-titrate the excess base with the standard
HCl until the pink color disappears.
Record
7. Repeat this procedure with your other two
samples.