Procedures
Part I – Standardization of sodium hydroxide
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25 cm3 of sodium hydroxide was transferred to a 250 cm3 volumetric flask by a pipette.
- Distilled water was added to the volumetric flask until reaching the graduation mark.
- The volumetric flask was inverted for 10 times to mix the solution well.
- The burette was filled with standard sulphuric acid.
- The initial reading on the burette was recorded.
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25 cm3 of diluted sodium hydroxide was transferred to a 250 cm3 conical flask by a pipette.
- 2-3 drops of phenol red was added to the solution inside the conical flask.
Standard sulphuric acid was run out from the burette to the solution inside the conical flask until the solution change from purple to yellow.
- The final reading on the burette was recorded.
- Titrations were repeated until the normal degree of consistency is obtained.
Part II – Analysis of aspirin tablets
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A definite number of aspirin tablets (2 or 3 tablets; not weighting more than 1.5 g) were weighted into a 250 cm3 conical flask.
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25 cm3 of 1.0M sodium hydroxide was added to the aspirin by a pipette.
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About 25 cm3 of distilled water was added to the aspirin to dilute the sodium hydroxide.
- The conical flask was warmed over a heating machine for ten minutes to complete the hydrolysis.
- The reaction mixture was cooled to room temperature.
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The reaction mixture and washing were transferred to a 250 cm3 volumetric flask.
- The reaction mixture was diluted by adding distilled water to the graduation mark of the volumetric flask.
- The volumetric flask was inverted to make sure that the solutions inside were well mixed.
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25 cm3 of the diluted reaction mixture was transferred to a 250 cm3 conical flask by a pipette.
- The burette was filled with standard sulphuric acid.
- The initial reading on the burette was recorded
- 2-3 drops of phenol red was added to the solution inside the conical flask.
- Standard sulphuric acid was run out from the burette to the solution inside the conical flask until the solution change from purple to yellow.
- The final reading on the burette was recorded.
- Titrations were repeated until the normal degree of consistency is obtained.
Data of results
Titration table of sodium hydroxide with standard 0.08572M sulphuric acid
Average volume of sulphuric acid used
= (14.8 + 14.8 + 14.8) ÷ 3
= 14.8 cm3
15.0 cm3 is rejected because this is just a trial.
Titration table of the diluted reaction mixture with standard 0.08572M sulphuric acid
Average volume of sulphuric acid used
= (12.5 + 12.6 + 12.4) ÷ 3
= 12.5cm3
12.2 cm3 is rejected because this is just a trial.
Calculation
Standardization of sodium hydroxide
H2SO4 + 2NaOH → Na2SO4 + H2O
Volume of H2SO4 used: 14.8 cm3
Volume of NaOH used: 25.0 cm3
Mole ratio of H2SO4 to NaOH = 1:2
Number of mole of H2SO4 = 0.08572M x 0.0148 dm3
= 0.001268656 mol.
Number of mole of NaOH = 0.001268656 mol. x 2
= 0.002537312 mol.
Molarity of diluted NaOH = 0.00253712 mol. ÷ 0.025 dm3
= 0.10149248M
Molarity of original NaOH =0.10149248M x 10
= 1.0149248M
Determination of amount of acetylsalicylic acid present in one aspirin tablet
Total number of mole of NaOH = 1.0149248M x 0.025 dm3 = 0.02537312 mol.
H2SO4 + 2NaOH → Na2SO4 + H2O
Volume of H2SO4 used: 12.5 cm3
Volume of NaOH used: 25.0 cm3
Mole ratio of H2SO4 to NaOH = 1:2
Number of mole of H2SO4 = 0.08572M x 0.0125 dm3
= 0.0010715 mol.
Number of mole of NaOH in 25.0 cm3 = 0.0010715 mol. x 2
= 0.002143 mol.
Molarity of NaOH = 0.002143 mol. ÷ 0.025 dm3
= 0.0857212M
Number of mole of NaOH in 250 cm3 = 0.02143
Therefore, number of mole of excess NaOH is 0.02143 mol.
Number of mole of NaOH used to hydrolyse the acetylsalicylic acid:
= 0.02537312 mol. - 0.02143 mol.
= 0.00394312 mol.
CH3.COOC6H4.COOH + 2NaOH → CH3.COONa + HO.C6H4.COONa + H2O
Mole ratio of CH3.COOC6H4.COOH to NaOH = 1:2
Number of mole of NaOH = 0.00394312 mol.
Number of mole of CH3.COOC6H4.COOH = 0.00394312 mol. ÷ 2
= 0.00197156mol.
Number of mole of CH3.COOC6H4.COOH in each aspirin tablet:
= 0.00197156 mol. ÷ 3
= 0.0006571867 mol.
Molar mass of CH3.COOC6H4.COOH:
= 12 + 1x3 + 12 + 16 + 16 + 12x6 + 1x4 + 12 + 16 + 16 + 1
= 180
Mass of acetylsalicylic acid in each aspirin tablet:
= 0.0006571867 mol. x 180
= 0.1182936 g
As the mass of each aspirin tablet is 0.18333g,
the percentage by mass of acetylsalicylic acid present in each aspirin tablet:
= (0.1182936 ÷ 0.18333) x 100%
= 64.5%
Conclusion
The percentage by mass of the active ingredient (acetylsalicylic acid) present in each aspirin tablet is 64.5%.
Discussion
The percentage by mass of acetylsalicylic acid in each aspirin tablet we found is much greater than the value that the producer claimed. Here are some possible reasons.
Acetic acid and salicylic acid in the resulting solution may neutralize some of the sodium hydroxide in the solution. This may make the result greater.
Some ingredients in the aspirin tablet are microcrystalline cellulose, croscamellose sodium, titanium dioxide, hydroxyl-methylcellulose, talc, iron oxide, polyvinyl acetate phthalate.
Microcrystalline is an used in the formulation of . It can be used as a due to its excellent compression properties, a disintegrant, in order to increase the biological availability of a medicine and as a to aid in the tableting procedure. But it may not react with the sodium hydroxide. Croscarmellose sodium is an in medical formulations. It is highly absorbent and insoluble. Croscarmellose sodium is the sodium salt of a cross-linked, partly O-(carboxymethylated) cellulose. But it may not react with the sodium hydroxide.
Titanium dioxide acts as a pigment to provide whiteness and to tablets and may react with sodium hydroxide.
Talc is a composed of . Talc is not soluble in water, but it is slightly soluble in dilute . In medicine talc is used as a agent to prevent recurrent pneumothorax. Talc may react with sodium hydroxide to form metal hydroxide.
Study Questions
- Why is ‘back titration’ instead of ‘direct titration’ used in this experiment?
We cannot titrate standard sodium hydroxide with the aspirin tablets directly because aspirin tablets are insoluble in water and we cannot take an aqueous solution of it. And also the reaction between the aspirin tablets and sodium hydroxide is slow as the acetylsalicylic acid in aspirin tablet is a weak acid, the reaction between acetylsalicylic acid and sodium hydroxide is slow and the end point is difficult to identify. Oppositely, the reaction rate of reaction between the sodium hydroxide which is excess in the hydrolysis of aspirin and sulphuric acid is much faster. Therefore the end point is much easier to identify.
- If phenol red is not available, what other indicator would you choose?
Methyl orange and phenolphthalein can also be used in this titration. As this titration is a strong acid-strong base titration. When we study the titration curve of this titration (fig. a), we can see that the vertical portion of the curve is large. So almost all the indicators are suitable for detecting the end point of this titration.
(fig. a)
- Why must the heated mixture be cooled before transferred to volumetric flask?
Volume of liquid will expend under high temperature. The volume of liquid inside the volumetric flask is exactly 250 cm3 when the liquid reach the graduation mark only when the liquid is in the room temperature. If we do not cool down the heated mixture before transferred to volumetric flask, then the volume of solution inside the volumetric flask is less than 250 cm3 when the solution reached the graduation mark.
- Why do people consider aspirin as an equivalence to dibasic acid?
The basicity of an acid is the number of ionizable hydrogen atom in each acid molecule. As the acetylsalicylic acid can be hydrolysed by sodium hydroxide into two week acids, which one mole of acetylsalicylic acid required two moles of sodium hydroxide to hydrolyse it, therefore aspirin as an equivalence to dibasic acid.
End of Report
