QUANTITATIVE ANALYSIS OF ACETYLSALICYLIC ACID IN ASPIRIN T ABLETS BY B ACK TITRATION J.L. CORPUZ DEPARTMENT OF C HEMICAL HEMICAL E NGINEERING NGINEERING , C OLLEGE OLLEGE OF E NGINEERING NGINEERING U NIVERSITY NIVERSITY OF THE P HILIPPINES HILIPPINES , DILIMAN , QUEZON C ITY ITY , P HILIPPINES HILIPPINES ERFORMED: F EBRUARY EBRUARY 28, 2018 D ATE P ERFORMED 28, 2018 I NSTRUCTOR NSTRUCTOR S N AME : E.C. G ALLEPOSO ’
1. Discuss the use of a more dilture NaOH solution for the standardization standardization of NaOH
1M of NaOH is a high concentration to use in titration. Per drop or volume of titrant from burette, the graduations of moles reacting are sudden and high which causes a very quick end point but this experimental end point would be farther away from the true value compared to 0.05M NaOH in which over-titration would bear less impact on values upon over titration1
2. Explain the rationale behind the dilution and aliquoting of the aspirin sample
Since the titrant was diluted, the titration would be much slower. Therefore, it is necessary to dilute the analyte to achieve the end point faster in proportion to the diluted titrant with less volume of excess NaOH needed for neutralization and less volume of HCl for back-titration. The aliquots serve as back-up analytes in case of over-titration, and as reference samples2
3. Discuss the principles behind the use of back titration in the analysis of aspirin tablets
fast reaction and a slow reaction which is its rate determining step. This slow reaction is impractical for titration cases. It is therefore strategic to use excessive NaOH to the aspirin solution sol ution to optimally hasten reaction time, then heated to further hasten the hydrolysis reaction. The remaining unreacted NaOH excess is then titrated with HCl back-titration. Because the NaOH equivalence with ASA is 1:1, the reacted NaOH was therefore computed and was used to determine the amount of ASA in the aspirin sample.
Acetylsalicylic acid is a weak acid which hydrolyzes in two-step elementary reactions which involves a
4. What is the importance of simmering the tablet in NaOH for 20 minutes.
The ASA dissociated to its acetic acid and salicylic acid components. It was reacted to the excess NaOH with simmering done to hasten hydrolysis. This also hastened the neutralization of acids to completion.
5. Discuss the possible sources of errors and their effect on the calculated parameters a. Weighing and Standardization Standardization Error High molarity of NaOH in standardization will cause higher number of excess NaOH which in turn will also increase the computed %ASA. This is also the case for weighing
where a lower mass will cause a smaller denominator in percent calculation which increases %ASA, while a higher mass will lower the %ASA. b. Incomplete hydrolysis of aspirin sample Incomplete hydrolysis would produce less acid which means more unreacted or excess NaOH. These unreacted NaOH will falsely signify greater %ASA calculated. c.
Non-homogenous aliquot sample If the main solution was not mixed before taking an aliquot, the resulting %ASA might either become higher or lower. It may go higher if the aliquot taken has a more ASA mass compared to the ideal. While an aliquot that contained less mass of ASA compared to the ideal will show less %ASA in the computations.
d. Over-titration The volume of HCl titrant will exceed the true amount from over-titrating. Therefore, in the computations, it will falsely signify higher moles of NaOH excess which will increase the acquired %ASA.
References: [1] Abrash, S. A. Chem 141 Titration Lab Lecture Notes. Dr. Samuel A. Abrash. https://facultystaff.richmond.edu/~sabrash/ 141Lab/Chem_141_Titration_Lab_Lecture_No tes_F10.pdf (accessed March 07, 2018). [2] Harris, D. C. Sample Preparation. In Quantitative Chemical Analysis, 7th ed.; W. H. Freeman and Company: New York, 2007; pp 8, 645. [3] Christian, G. D.; Dasgupta, P. K.; Schug, K. A. Experiment 9 Determination of Aspirin Using Back Titration. In Analytical Chemistry, 7th ed.; John Wiley : New Jersey, 2014; pp E16-E18.
