Transcript LAB. 2 Aspirin.pptx

Analysis of aspirin Tablet
NADA S. ALTHEYAB
PHC 428
Wednesday lab.
Chemical name:
Acetylsalicylic acid
2- acetyloxybenzoic acid
Molecular formula: C9H8O4
Mwt: 180 Da
Action and use:
• Salicylate; non- selective cyclo-oxygenase
inhibitor; antipyretic; analgesic; anti-inflammatory
A. Identification of aspirin
(Alkali hydrolysis or saponification)
The mechanism of reaction
Procedure:
Identification
1) Weigh 0.5 g of the powdered tablets .
2) Mix the powder with 10 ml of 5M NaOH.
3) Heat the mixture and after boiling leave it for 2-3 min. on hot
plate.
4) Cool it under water streaming.
5) Add an excess of 1M H2SO4 ( a crystalline precipitate is
produced).
6) Add iron (III) chloride solution ( a deep violet colour is
produced).
B. Determination of free salicylic acid in
the aspirin sample
Limit test
Limit test
• Acetylsalicylic acid; it an active ingredient of aspirin
• It can be degraded over time due to bad storage or
manufacturing, afforded salicylic acid
• The amount of degradation can be measured through
Limit test
Principle
Nessler cylinder 1 (standard)
Nessler cylinder 2 (sample)
Procedure:
•
• Procedure:
1. Mix 3 ml of salicylic acid + 2ml 1. Shake quantity of powdered tablet
of ethanol in Nessler cylinder
2.
Diluted with distilled water to
produce 50 ml
3.Add 1 ml by pipette of
ammonium iron(III) sulfate
solution
4.Mix and allow to stand for 1 min.
The result: (Violet colour but
more intense than the sample)
containing 0.20 g of aspirin + 4ml of
ethanol and diluted to 100 ml with
water
2.Filter all the solution
3.Transfer only 50 ml of the filtrate to a
Nessler cylinder
4.Add 1 ml by pipette an ammonium
iron(III) sulfate solution
5.Mix and allow to stand for 1 min.
The result: (Violet colour but less intense
than the standard)
The sample will be valid if the intensity
of sample in Nessler cylinder 2 less
than that in Nessler cylinder 1
C. Determination the percentage of
acetyl salicylic acid in aspirin tablet
Back titration
Concept of back titration
Back titration is a titration done in reverse; instead of titrating the original
sample, a known excess of standard reagent is added to the solution, and the
excess is titrated.
Back titrating should only be used when made necessary.
When it is often used ?
The solution being titrated is either too weak or
too slow to give a reaction.
How to choose proper Indicator
• The purpose of the indicator is to show when enough standard
solution has been added to fully react with the unknown
concentration.
• Indicators must only be added to the solution of unknown
concentration when no visible reaction will occur.
• Depending on the solution being titrated, the indicator is
chosen to match the PH of strong solution.
The pH of the equivalence point can be estimated using the following
rules:
 A strong acid will react with a strong base to form a neutral thus
equivalence point is pH = 7 so to get an end point to occur after this
means either Bromothymol blue or Methyl red would be a good
choice.
 A strong acid will react with a weak base to form an acidic
equivalence point just below pH of 5, since the acid is strong, so to
get an end point to occur after this means Methyl orange would be a
good choice. .
 A weak acid will react with a strong base to form a basic equivalence
point is about pH of 9, since the base is strong, so to get an end point
to occur after this means Phenolphthalein would be a good choice. .
The pH of the equivalence point can be estimated using
the following rules: ( cont.)
 weak acids are not often titrated against weak bases
because the colour change shown with the indicator is often
quick, and therefore very difficult for the observer to see the
change of colour.
Back titration
Procedure:
1.
2.
3.
4.
5.
Take 10 tablets of aspirin 300mg and weight it using sensitive balance .
Grind the tablets using mortar and pestle
weigh 0.5 g of aspirin.
Using pipette WITH PUMP add 30ml of 0.5M NaOH in 250 ml conical flask
An accurate weight of powder equivalent to 0.5 g was taken (should be
calculated)
6. Add 30 ml 0.5 M NaOH is added to it.
7. Boil gently for 10 min. On hot plate
8. Cool under water streaming
9. Add 2 drops of phenol red
10.Titrate the excess of alkali with 0.5M HCl
11.End point; pink.
Yellow
How much the weight should be taken from the grind
tablets that equivalents to 0.5gm aspirin.
• 10 tablets x 300mg = 3000 mg =3 gm
(3.6 g) weigh you get it
from the balance
• We need to take weight equivalent to (0. 5g)
(X gm)
X means the weight should be taken from the grind tablets which equivalents to 0.5gm
aspirin.
How to calculate the concentration of aspirin
Where the following equations describe the complete reaction of NaOH
with aspirin :
1. C9H8O4 + 2 NaOH ---> CH3COO.Na + HOC6H4COO.Na + H2O
2. NaOH (exx.) + HCl ----> NaCl + H2O
So, Titrant
Sample
2 moles of NaOH
= 1 mole of aspirin
moles of NaOH
= 180/2 gm of aspirin
1000 ml x 1M of NaOH
= 90 gm of aspirin
1 ml of 1M NaOH
= 0.090 gm of aspirin
1ml of 1/2 M NaOH
= 0.045 gm of aspirin
so, Equivalent factor (F) = 0.045 gm/ml
e.p (30ml of exx. – ml of titrant HCl) = (X) g
Conc. of aspirin = E.P ( ml of NaOH in excess x f" - ml of HCL titrant consumed x
f") X F X f
% of aspirin in the tablet
% purity (% aspirin in tablet) =
found (X) gm / actual (0.5 gm) x 100= ……%
 1ml of 1/2 M NaOH
= 0.045 gm of aspirin
 e.p ml (30ml of exx. – ml of titrant HCl) = (X) gm of aspirin