Page 55 - PC 101 practical notes 24-25..

MANSOURA NATIONAL UNIVERSIY
PHARM D- CLINICAL PHARMACY LEVEL I PHARM. ANAL. CHEM. I (PC 101)

Exp.: Determination of Aspirin tablets (Back titration)

P Pr ri in nc ci ip pl le e : :

(Back or Residual Titration)

known excess
Aspirin (sample) of st. NaOH Sod. acetate Sod. salicylate
(50 ml)

The method depends on boiling aspirin (acetylsalicylic acid) with a known

excess of standard alkali (NaOH) where aspirin is hydrolysed into acetic and

salicylic acids which react with sodium hydroxide giving sodium acetate and

sodium salicylate. Then the remaining unreacted part of NaOH is back titrated

against standard HCl using ph.ph. indicator.

NaOH + HCl ⎯→⎯ NaCl + H 20
remaining standard
unreacted titrant
N No ot te es s: : part

This method requires Blank Determination which is a separate determination

in which we repeat the same steps of the experiment but without the sample

and the E.P. in this case is called Blank Reading.

P Pr ro oc ce ed du ur re e : :

1) Transfer 1 Rivo tablet (equivalent to 320 mg acetylsalicylic acid) into a

clean flask.

2) Add 50 ml of 0.1 N NaOH.
3) Boil for 10 minutes inserting a funnel in the neck of the flask

to avoid evaporation of the solution.
4) Cool and add 10 drops ph.ph. indicator.

5) Titrate ≠ 0.1 N HCl.
{ Color change at E.P.: from Pink to Colorless }

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