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Exp 10. Analysis of
Aspirin
2 students
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bjectivedetermine the purity of aspirin by acid-base
titrations
become acquainted with the concept of back-titration
analyses
Chemicalsspirin (0.5 g 3), phenolphthalein solution, 95%
EtOH,tandardized 0.1 M NaOH, standardized 0.1 M HCl
If the aspirin has been exposed to moisture for an
extendedperiod of time, it can decompose as follows :
Salicylic acid Acetic acid
Impurities!
O
CO2H
CCH3
O
H2O
OH
CO2H
CH3COOH
H+ catalyst
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1.Put about 0.5000 g of the aspirin into 250 mL
Erlenmeyer flask, and then add 25 mL ofethanol(95%) and 2 drops
of phenolphthaleininto it.
250 mL
About 0.5000 g of your aspirin
Ethanol(95%) 25 mL
Phenolphthalein 2 drops
Procedures
CAUTION!! Ethyl alcohol is flammable solvent
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..
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2.Titrate the sample with standard 0.1 M NaOH to afaint pink end
point.
Record thevolume of NaOHused.
Neutralization
Add0.1M NaOHsolution
O
CO2H
CCH3
O
OH-
O
CO2-
CCH3
O
H2O
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Add additional NaOH
[15 mL + the volume of base required inthe titration (2.)]
3.To hydrolyze the aspirin, add additional NaOH tothe Erlenmeyer
flask.
4. Heat the mixture for 15 min in a bath of boilingwater to
hydrolyze the aspirin.
Base-promotedhydrolysis
O
CO2-
CCH3
O
OH-
OH
CO2-
CH3CO2-
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. Cool the flask at an ice bath.
6. Back-titrate the excess base with the standard HCluntil the
pink color disappears.
epeat this procedure with your other two samples.
mmol reagent reacted= total mmol (NaOH) mmol back titrated
(HCl)
Add
0.1M HCl solution
Record the
volume of HClused.
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Summary
1. Put 0.3 g 0.5 g of the aspirin into 250 ml Erlenmeyer flask,
andthen add 25 ml of ethanol (95%) and 2 drops of
phenolphthaleininto that.
e the sample with standard 0.1 M NaOH to a faint pink end
point.
drolyze the aspirin, additional NaOH to the Erlenmeyer
flask.
t the mixture for 15 min in a bath of boiling water to hydrolyze
the aspirin.
. Cool the flask at an ice bath.
ck-titrate the excess base with the standard HCl until the pink
colorpears.
epeat this procedure with your other two samples.
Record
Record
