(The ester group also reacts, so two moles of sodium hydroxide are required to react with one mole of aspirin.)
The excess alkali (which was left over from the above reaction) may then be determined by titration
HCl + NaOH NaCl H2O
The amount of alkali which is used up in the initial reaction with aspirin may be calculated, and this can be related to the quantity of aspirin.
Aspirin is a medicine commonly found in households around the world. It also is one of the least expensive and most useful drugs in the market.
A Chemist named Felix Hoffmann first synthesized aspirin, otherwise known as acetylsalicylic acid, in 1897 from salicylic acid. The name derives from the Latin word for the willow tree (Salix), from whose bark it can be obtained.
SAFETY
Hand glassware in an appropriate manner.
Wearing safety goggles and lab coats during the heating stage of the experiment
Use tongs to handle hot flasks
METHOD
Apparatus used
Conical flask
Magnetic stirrer bar
Hotplate stirrer
Standard flask
Sodium hydroxide solution (0.1mol/dm3)
Hydrochloric acid (0. l mol/dm3)
Aspirin tablet
Bromocresol Purple indicator
Deionised water
A commercial aspirin tablet was placed into a 250cm3 conical flask with a magnetic stirrer bar
Then sodium hydroxide solution (0.1mol/dm3, 60cm3) was added and the flask was placed on a hotplate stirrer.
This was then heat to over 60°C and mixed for 10 minutes.
The solution was then made up to 100cm3 in a standard flask.
Solution was split into two equal portions and 1-2 drops of Bromocresol Purple indicator was added to each portion as an indicator (Purple in alkali or yellow in acid.)
The hydrochloric acid (0. l mol/dm3) was placed into a burette and the two samples were titrated until they became a pale yellow
The level on the burette was noted at the start and at the end of the titration
RESULTS
CALCULATIONS
Volume of hydrochloric acid
3.6 cm3 + 13.4 cm3 = 27 cm3
Moles HCI = concentration X volume (dm3)
0.1 X 27 cm3 = 0.0027 Moles of HCL
1000
HCl + NaOH NaCl + H2O
1 of mole of excess Na OH is equal to 1 of mole of HCL
So the moles of excess Na OH = 0.0027
Number of moles of Na OH in the original 60cm3 of 0.1 mol/dm3 solution.
0.1 X 60 cm3 = 0.006 Moles of Na OH
1000
Number of Moles of Na OH used in reaction with aspirin = original – excess
0.006 - 0.0027 = 0.0033 moles
<==>
1 mole aspirin <==> 2 moles NaOH
0.00165 <==> 0.006 Moles
Mass aspirin = moles x formula mass of aspirin
= 0.00165 X 180 = 297 mg
DISCUSSION
In this experiment an aspirin tablet was reacted with a known excess amount of sodium hydroxide.
The number of moles of NaOH left over was found by titrating with Hydrochloric acid acid. The number of moles of NaOH required for the reaction was then calculated as the moles originally present were knows.
Using this relationship given in the equation:
1 mole aspirin is reacted by 2 moles NaOH
It was then possible to calculate the number of moles of aspirin that has been reacted. The answer was then compared to the manufactures label stating the aspirin content.
It was also found that there was 297 mg of aspirin in the aspirin tablets compared to the 300 mg stated dose on the label.
This is possibly due to experimental error.
Experimental error in the results may have been due to the use of inaccurate glassware, human error, % error in the measurements that will influence the calculations made from the data obtained, glassware not properly rinsed and dried before use, dirt and air on the analytical balance, or the acetyl salicylic acid not fully reacted.
Accuracy and precision of the measurements taken in the experiment would influence the data obtained.
If the experiment was to be repeated again it would be more accurate to use an indicator that changes colour around seven as this is where the equivalence point theoretically should be with a strong acid - strong base titration.
The use of a different indicator would give more accurate and precise results the best one would bromothymol blue as it changes colour at a pH of 6.0 and finishes at 7.6. Giving a colour change of yellow to blue.
This indicator would not produce more accurate results for the titration between the HCl and the reaction solution as the NaOH is partly neutralised and so is a more neutral solution.
So the titration is more like a strong acid – weak base titration and so the theoretical end point is going to be lower therefore an indictor like methyl red which changes colour at 4.2 – 6.3 (from red to yellow) would produce the most accurate and precise results.
CONCLUSION
The results obtained from titrimetric analysis of aspirin show that the tablet contains 297 mg of aspirin.
The manufactures label that shows the tablet to contain 300mg of aspirin.
1 of mole of excess Na OH is equal to 1 of mole of HCL
The ratio of sodium hydroxide to hydrochloric acid is 1:1; therefore, the molarity of diluted and stock sodium hydroxide solutions can be calculated by working out the number of moles of hydrochloric acid used.
The number of moles of sodium hydroxide used in the reaction can be calculated by subtracting the number of moles of sodium hydroxide taken for the reaction by the number of moles of sodium hydroxide remaining after reaction
Accuracy and precision of the measurements taken in the experiment can influence the data obtained. Experimental error in the results may have been due to the use of inaccurate glassware, calibration of measuring equipment which all a have % error.
Measurements that will influence the calculations made from the data obtained, glassware not properly rinsed and dried before use, dirt and air on the analytical balance, or the acetyl salicylic acid not fully hydrolysed.
