In this experiment the concentration of acetyl salicylic acid (ASA) in an aspirin tablet will be determined. This can be done by dissolving a tablet in a strong base, NaOH, and titration it with a strong acid, HCl. The quantity was found to be about 75. 5% of the weight of the tablet, or about 318 mg per tablet. This value tells the pharmaceutical companies how much to state the recommended dosage as based on how much ASA is needed to feel the headache and pain resistant effects. Also it can be used to state precautions for the customers based on the known LD50 value (200 mg/kg).
INTRODUCTION 1) This experiment is used to find out if the quantity in an aspirin tablet is the same as what the pharmaceutical companies advertise. This is useful to the general public because people who use aspirin would like the amount of ASA in a tablet to be the true advertised amount. This experiment is also used to find out how much of the tablet is actually ASA and how much is filler. Most people are conscience about what they put into their body and do not wish to ingest unknown chemicals.
2) To find the concentration of ASA in aspirin a tablet is dissolved in a known concentration and volume of NaOH. This solution is then titrated with HCl to find how much excess NaOH is remaining in the flask. 3) Some advantages of the experimental method used in this experiment are that it is easy to do with equipment that is accessible and it yields results that are acceptable. The problem is that the results are not as accurate as they could be because of error due to the equipment.
It is also difficult to judge neutralization points for each standardization and the determination of the ASA content. This problem leads to errors in volumes added in each titration and therefore an accurate volume cannot be truly achieved and in turn a true percent weight cannot be achieved either.
EXPERIMENTAL 1) See page 28-30 of CHM 2354 Laboratory of Analytical Chemistry Laboratory Manual Winter 2012. Changes made: 2 trials completed for flask 1 and 3 in “Back Titration of the Excess NaOH”. 2 trails completed in “Analysis of Aspirin Powder”.
2) TABLE 1- Measurements made in preparation process Flask Number| Mass of Potassium Hydrogen Phthalate used in Standardization of NaOH (g ±0. 0001 g)| Volume of HCl used in Standardization of HCl (mL ±0. 03 mL)| Mass of Aspirin Tablet used in Preparation of Acetyl Salicylate Solution (g ±0. 0001 g)| Volume of NaOH used in Preparation of Acetyl Salicylate Solution (mL ±0. 08 mL)| Mass of Aspirin Powder used in Analysis of Aspirin Powder (g ±0. 0001 g)| Volume of NaOH used in Analysis of Aspirin Powder (mL ±0. 08mL)| 1| 0. 8936| 25| 0. 4242| 100| 0. 4199| 100| 2| 0. 8698| 25| 0.
4151| 100| —| 100| 3| 0. 8839| 25| 0. 4242| 100| —| 100| RESULTS AND DISCUSSION 1) TABLE 2- Measurements made in analyzing Flask Number| Trial Number| Volume of NaOH used in Standardization of NaOH (mL ±0. 05 mL)| Volume of NaOH used in Standardization of HCl (mL ±0. 05 mL)| Volume of HCl used in Back titration of the excess NaOH (mL ±0. 05 mL)| Volume of HCl used in Analysis of aspirin powder (mL ±0. 05 mL)| 1| 1| 44. 1| 25. 56| 15. 6| 15. 94| | 2| 42. 5| 25. 61| 15. 99| 15. 52| | 3| 43. 52| 25. 66| —| —| 2| 1| —| —| 16. 2| —| | 2| —| —| 15. 57| —|
| 3| —| —| 15. 38| —| 3| 1| —| —| 15. 91| —| | 2| —| —| 15. 6| —| 2) TABLE 3- Calculated Values For Concentration of NaOH and HCl Trial Number| Concentration of NaOH Standard (mol/L)| Uncertainty of Concentration of NaOH Standard (±mol/L)| Mean Concentration of NaOH (mol/L)| Concentration of HCl Standard (mol/L)| Uncertainty of Concentration of HCl Standard (±mol/L)| Mean Concentration of HCl (mol/L)| 1| 0. 0992| 0. 1125| 0. 0996 ±0. 1179| 0. 1019| 0. 2338| 0. 1021 ±0. 2340| 2| 0. 1002| 0. 1179| | 0. 1021| 0. 2339| | 3| 0. 0994| 0. 1143| | 0. 1023| 0.
2340| | TABLE 4- Calculated Values for Concentration of ASA in tablets (with group values, excluding major outliers) Flask Number| Trial Number| Percent Weight (%)| Uncertainty of Percent Weight (±%)| Mean Percent Weight (%)| Mean Group Percent Weight (%)| Mass in tablet (mg/tablet)| Uncertainty of Mass in tablet (±mg/tablet)| Mean Mass in tablet (mg/tablet)| Mean Group Mass in tablet (mg/tablet)| 1| 1| 76. 34| 5. 345| 74. 65 ±5. 410| 78. 13| 323. 8| 88. 83| 316. 7±88. 83| 327. 1| | 2| 72. 96| 5. 410| | | 309. 5| 85. 23| | | 2| 1| 72. 70| 5. 577| 76. 98±5. 577| | 301.
8| 85. 00| 319. 6±92. 75| | | 2| 78. 28| 5. 471| | | 324. 9| 90. 95| | | | 3| 79. 96| 5. 442| | | 331. 9| 92. 75| | | 3| 1| 73. 66| 5. 396| 75. 00±5. 396| | 312. 4| 85. 97| 318. 1±88. 83| | | 2| 76. 34| 5. 345| | | 323. 8| 88. 83| | | TABLE 5- Calculated Values for Concentration of ASA in Powder (with group values excluding major outliers) Flask Number| Trial Number| Percent Weight (%)| Uncertainty of Percent Weight (±%)| Mean Percent Weight (%)| Mean Group Percent Weight (%)| 1| 1| 74. 15| 5. 462| 75. 98±5. 462| 78. 55| | 2| 77. 82| 5. 394| | | TABLE 6- Results for t-test.
Flask Number| Result for Percent Weight| Result for Group Percent Weight| Result for Concentration| Result for Group Concentration| 1| 3. 166| 2. 062| 1. 162| 0. 5093| 2| 1. 376| | 0. 5984| | 3| 3. 725| | 1. 204| | Powder 1| 2. 185| 1. 233| —| —| ? Target Line| – – Warning Line| — Action Line| CHART 1- Shewhart Chart for Percent Weight ? Target Line| – – Warning Line| — Action Line| CHART 2- Shewhart Chart for Concentration of ASA per tablet 3) The point of this experiment was to find the amount of ASA in an aspirin tablet. It was found that the average percent weight was 75. 75%, which correlates to 318.
3 mg/tablet for a tablet of aspirin. For aspirin powder the average percentage of ASA was 75. 98%. These values are not far from the overall mean for each group which came out to be 78. 13% ASA per tablet, 327. 1 mg/tablet and 78. 55% ASA in the powder. These differences are most likely due to systematic error in titrations. Some students may have over or under titrated some of the trials making the value of ASA change. It appears as though in my individual trials I have over-titrated the ASA solution making the amount of ASA seem lower than expected. The standard deviations for most trials are very low, only about 1-3.
Since the equipment that is being used is not the most precise these standard deviations are reasonable. These value say how far a sample is from the mean. In the beginning the standard deviations were very low but as error accumulated so did the standard deviation. Since the error grew with each piece of equipment that was used a value of 1-3 can be considered reasonable because the equipment is not very accurate. According to the t-test for a confidence level of 95% and a degree of freedom of 1 the t value is 12. 7. Since calculated t value for the flasks where only two trials were performed (Table 6, flask 1 and 3) are less than 12.
7 it can be assumed that the experimental mass and percent weight are very similar to the accepted value of 80% and 325 mg/tablet. Also for flask 2 where 3 trials were performed the t value for 2 degrees of freedom and 95% confidence is 4. 30 which is still higher than the calculated value for t. this means that all of the calculated concentrations and percent weights were similar to the accepted values. For the Shewhart Charts all points are within the limits of the action line meaning there were no systematic errors, but because most of the points are on one side of the mean they subjected to systematic errors.
This could mean that the equivalence point consistently was being over shot since most points are below the accepted value. When looking at the chart for the mass per tablet of ASA the only point subjected to systematic error was the point corresponding to 301. 8 mg/tablet. This trial was the first one performed and it is clear that the equivalence point was overshot. This point can be eliminated based on the Shewhart chart. This point is the largest outlier and changes the overall average by a large amount and can be eliminated. If eliminated the average for flask 2 becomes 328.
4 mg/tablet, which is closer to the accepted value than the old value of 319. 6 mg/tablet. CONCLUSION The objective of this experiment was to find how much acetyl salicylic acid is in an aspirin tablet. It was found that the average concentration in a tablet is actually 327. 1 mg/tablet, measured from multiple trials by several different groups. This means that there is slightly more ASA than expected for the average tablet. According to the Shewhart Chart some values that contribute to this high average can be disregarded because they have too much systematic error.
CALCULATIONS A. Calculations from MY data 1) Concentration of Potassium Hydrogen Phthalate 2) Concentration of NaOH 3) Concentration of HCl 4) Concentration of ASA in Tablets %Weight: mg/tablet: 5) Concentration of ASA in powder 6) Mean Concentration for one tablet 7) Mean Standard deviation of one tablet SPREADSHEET CALCULATIONS Individual tablets (single analysis) Mean (Cell D3): AVERAGE(B3:B5) Standard deviation (Cell E3): SQRT((((POWER(B3,2))+(POWER(B4))+(POWER(B5))-(POWER(B3+B4+B5,2))/(E5-1)))) T-tests (Cell H19): (D3-325)/(E3/(SQRT(E5))).