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Melchizedek E. Damgo 2013-51946 October 3, 2014

Exp 5 Quantitative Determination of Acetylsalicylic Acid in Aspirin Tablets by Back-Titration

ANSWERS TO QUESTIONS:

1. The use of a more dilute NaOH for the standardization of NaOH.

The dilution of NaOH was done in order to lower the number of hydroxide ions that will

react with the primary standard, KHP. A lower number of hydroxide ions reacting with KHP will

yield more accurate molarity of the NaOH.

2.

The rationale behind the dilution and aliquoting of the aspirin sample.

Diluting and using only the aliquot of the aspirin sample allows an easier titration setup.

Only a small amount of the titrant was needed to reach the endpoint. If the whole 250 mL

aspirin solution was used, it would need a very large volume of the titrant in order to reach the

endpoint.

3. The principles behind the use of back titration in the analysis of aspirin tablet.

The concept of back titration was used in order to arrive to more accurate results in the

determination of ASA in the aspirin sample. This was also done in order to prevent the immediate

hydrolization of the ester group present and the neutralization of the carboxylic acid which could

result to an overestimated amount of ASA present. The reaction between NaOH and ASA is just

gradual and will only produce water in the process.

4.

The importance of simmering the tablet in the NaOH solution for 20 minutes.

Simmering the tablet in the NaOH solution was done in order to increase the overall kinetic

energy of the system. An increase of kinetic energy would result into a faster reaction since this will

allow more collisions between molecules in the reaction. This will also break the covalent bonds

present in the aspirin sample and allow complete dissociation of the aspirin into its weak acid

components (acetic and salicylic acid) during hydrolysis.

5. Possible sources of errors and their effect on calculated parameters.

Possible sources of errors may include the following: not washing the burette with the titrant

HCl which could result to an incorrect measurement of its molarity since there may still bemolecules of water and air bubbles present in the burette; overboiling of the solution which would

decrease the NaOH present which in turn would increase the amount HCl needed for titration and

result to a higher value of the ASA obtained; and the incorrect reading of the volume of the titrant

during the titration process.

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REFERENCES

[1] Skoog, D.A., West, D.M., Holler, F.J., Crouch, S.R. Fundamentals of Analytical Chemistry (8th ed.).

Brooks/Cole, Canada. 2004.

[2] D.C. Harris. Quantitative Chemical Analysis, 7thed. W.H. Freeman and Company/ New York. 2010.

[3] Analytical Chemistry Laboratory Manual (2007 Edition). Institute of Chemistry, University of

the Philippines Diliman.