Phenol red, having a pH transitional range of 6.8 to 8.4 is used, because the final pH of the reaction lies between its range.
Diagram of the titration
This kind of titration is known as back titration, as the excess reactant is titrated in order to find out the number of moles of sodium hydroxide used in hydrolysis. Using the equation no. of moles = molarity * volume used (dm3), the number of moles of Sulphuric acid used can be found out and so does that of sodium hydroxide. As the number of moles of sodium hydroxide used in hydrolysis is known, the number of moles of sodium hydroxide used to react with aspirin can be calculated by subtracting the original number of moles of sodium hydroxide from the number of moles of the excess sodium hydroxide. In this way, the number of moles of aspirin can be found out due to the ratio of 2-ethanoylbenzoic acid and sodium hydroxide is 1:2. Finally, the mass of aspirin is calculated using mass = no. of moles * molar mass.
- Chemicals:
2 aspirin tablets (0.802g, brand name ASPIRIN), 25ml 1.0M sodium hydroxide, 100cm3 0.05M Sulphuric acid, 24 drops of phenol red
- Apparatus:
- Procedures:
- Two aspirin tablets were weighed and put into a conical flask.
- A few drops of distilled water were added into it and the tablets were then dissolved and crushed by a glass rod until white powers were seen.
- 25ml of 1.0M sodium hydroxide solution was pipetted and transferred into the conical flask to initiate hydrolysis.
- The solution in the conical flask was diluted with 25ml distilled water.
- Then it was warmed by heating under a luminous Bunsen flame for 15 minutes. Hydrolysis was completed.
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While waiting for the warming, phenol red was tested. Three 75 cm3 beakers were used to carry sodium hydroxide, distilled water and Sulphuric acid. Two drops of phenol red was added into each of the beakers. The colours in each beaker were observed and the beakers were left behind.
- The reaction mixture was cooled down in room temperature. Tap water was run on the outer wall of the conical flask to speed up the cooling process.
- After that, the reaction mixture was transferred to a 250ml volumetric flask. Distilled water was added to the graduation mark. The flask was stoppered and inverted for several times.
-
25ml of the diluted reaction mixture was pipetted into a conical flask using a 25cm3 pipette.
- 0.05M Sulphuric acid was added to the burette and the initial burette reading was taken.
- Three drops of phenol red were added to the conical flask. The acid was run into the flask while the flask was swirled continuously. When the purple colour had changed to yellow, no more acid was added. The final burette reading was taken.
- Procedures (ix) to (xi) were repeated for five times to obtain five consistent results.
- Results:
Mean volume of H2SO4 used = (16.25 + 16.30 + 16.30 + 16.35) / 4
= 16.30 cm3
- Calculation/interpretation:
Equations of the reaction:
CH3COOC6H4COOH(aq) + 2NaOH(aq)→HOC6H4COO-Na+(aq) + CH3COO-Na+(aq) +H2O(l)
H2SO4(aq.) + 2NaOH(aq.) → Na2SO4(aq) + 2H2O(l)
No. of moles of H2SO4 used = 0.05 * 16.30 * 0.001
= 0.000815 mol
No. of moles of NaOH left in 25cm3 diluted solution = 0.000815 * 2
= 0.00163 mol
No. of moles of NaOH left in 250cm3 diluted solution = 0.00163 * 10
= 0.0163 mol
No. of moles of NaOH used in hydrolysis = 1.0 * 0.025
= 0.025 mol
No. of moles of NaOH reacted with aspirin = 0.025 – 0.0163
= 0.0087 mol
No. of moles of aspirin = 0.0087 / 2
= 0.00435 mol
Mass of aspirin in tablets = 0.00435 * 180
= 0.783 g
Mass of aspirin in each tablet = 0.783 / 2
= 0.3715 g
= 391.5 mg
- Discussion:
In hydrolysis, the 25cm3 of distilled water added was not necessarily be accurate, because the number of moles of aspirin and sodium hydroxide is still the same in the conical flask no matter how much distilled water was added to it. The only criterion is that distilled water should be in excess compared to aspirin. This can ensure all the aspirin present was hydrolysed and some distilled water was left for evaporation during boiling. After all, the contents in the conical flask would be transferred into a volumetric flask with more distilled water for dilution, so the amount of distilled water added was not that important to be accurate.
Moreover, after warming the mixture in the conical flask, it ought to be cooled down, since the volume of the solution is different at different temperature. If the contents in the conical flask were immediately poured into a volumetric flask after warming and distilled water was then filled up to the graduation mark, after some time of cooling, the volume of the solution would increase, meaning too much distilled water was added. The solution would become more diluted, which implies that less number of moles of sodium hydroxide would be pipetted each time. The calculated number of moles of sodium hydroxide reacted with aspirin would be larger. Eventually the calculated mass of aspirin in each tablet would be heavier.
In this experiment, there were some errors that might lead to a slightly wrong answer. For instance, it was assumed that only aspirin in the tablets had reacted with sodium hydroxide and that other contents in the tablets would not be hydrolysed. However, if such contents had reacted with sodium hydroxide, a greater amount of sodium hydroxide would be used for hydrolysis, leaving less sodium hydroxide in the conical flask afterwards. As a result, less Sulphuric acid would be used to neutralize sodium hydroxide in titration, giving a greater calculated mass of aspirin than actual again.
On the other hand, since Sulphuric acid is quite hygroscopic, it would absorb water vapour from the air, diluting itself. If it is left to stand in air for too long, the volume of Sulphuric acid in the burette would become slightly larger and more diluted. So the number of moles of Sulphuric acid per cm3 would decrease and a slightly greater volume of Sulphuric acid would be required to neutralize sodium hydroxide. Then the calculated number of moles of sodium hydroxide used in hydrolysis would be smaller, and that the calculated mass of aspirin would be smaller. To eliminate this error, not only should the titration be carried out faster, the Sulphuric acid should also be kept in air as short as it could.
- Conclusion:
The mass of aspirin in each tablet was found to be 391.5 mg.