Page 96 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

Determination of Aspirin (Acetylsalicylic acid)

➢ Aspirin is a nonsteroidal anti-inflammatory drug

(NSAID) used to reduce pain, fever, and/or

inflammation, and as an antithrombotic.

➢ Aspirin is a weakly acidic compound. It can be determined by back

(residual) titration method.

NaOH + HCl → NaCl + H O
2
Principle remaining titrant
unreacted part

➢ The method depends on boiling aspirin with a known excess of standard

NaOH where aspirin is readily hydrolysed into acetic acid and salicylic acid

which react with NaOH giving sod. acetate and sod. salicylate.

➢ Then the remaining unreacted part of NaOH is back titrated against

standard HCl using ph.ph. indicator.

Boiling is necessary for the reaction between aspirin and NaOH to increase

the rate of the hydrolysis reaction.

This method requires blank determination because when NaOH is heated
Notes
and then cooled, its strength is decreased due to:

1- Interaction of NaOH with the glass of the flask.

2- Absorption of atm. CO .
2

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