1. Analysis of aspirin Tablet
NADA S. ALTHEYAB
PHC 428
Wednesday lab.

2. Chemical name: Acetylsalicylic acid 2- acetyloxybenzoic acid
Molecular formula: C9H8O4
Mwt: 180 Da

3. Action and use:
Salicylate; non- selective cyclo-oxygenase inhibitor; antipyretic; analgesic; anti-inflammatory

4. A. Identification of aspirin
(Alkali hydrolysis or saponification)

5. The mechanism of reaction

6. Identification Procedure: Weigh 0.5 g of the powdered tablets .
Mix the powder with 10 ml of 5M NaOH.
Heat the mixture and after boiling leave it for 2-3 min. on hot plate.
Cool it under water streaming.
Add an excess of 1M H2SO4 ( a crystalline precipitate is produced).
Add iron (III) chloride solution ( a deep violet colour is produced).

7. B. Determination of free salicylic acid in the aspirin sample
Limit test

8. 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

9. Principle

10. Limit test Nessler cylinder 2 (sample) Nessler cylinder 1 (standard)
Procedure:
Mix 3 ml of salicylic acid + 2ml of ethanol in Nessler cylinder
Diluted with distilled water to produce 50 ml
Add 1 ml by pipette of ammonium iron(III) sulfate solution
Mix and allow to stand for 1 min.
The result: (Violet colour but more intense than the sample)
Procedure:
Shake quantity of powdered tablet containing 0.20 g of aspirin + 4ml of ethanol and diluted to 100 ml with water
Filter all the solution
Transfer only 50 ml of the filtrate to a Nessler cylinder
Add 1 ml by pipette an ammonium iron(III) sulfate solution
Mix and allow to stand for 1 min.
The result: (Violet colour but less intense than the standard)
Limit test

11. The sample will be valid if the intensity of sample in Nessler cylinder 2 less than that in Nessler cylinder 1

12. C. Determination the percentage of acetyl salicylic acid in aspirin tablet
Back titration

13. 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.

14. 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.

15. 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. .

16. 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.

17. Back titration Procedure:
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)
Add 30 ml 0.5 M NaOH is added to it.
Boil gently for 10 min. On hot plate
Cool under water streaming
Add 2 drops of phenol red
Titrate the excess of alkali with 0.5M HCl
End point; pink Yellow

18. 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.

19. How to calculate the concentration of aspirin
Where the following equations describe the complete reaction of NaOH with aspirin :
C9H8O4 + 2 NaOH ---> CH3COO.Na + HOC6H4COO.Na + H2O
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 = gm of aspirin
1ml of 1/2 M NaOH = gm of aspirin so, Equivalent factor (F) = 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

20. % of aspirin in the tablet
% purity (% aspirin in tablet) =
found (X) gm / actual (0.5 gm) x 100= ……%
1ml of 1/2 M NaOH = gm of aspirin
e.p ml (30ml of exx. – ml of titrant HCl) = (X) gm of aspirin