Synthesis of Salicylic Acid and Potentiometric Determination of Its Purity and Dissociation Constant

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Synthesis of Salicylic Acid and Potentiometric Determination of its Purity and Dissociation Constant -------------------------------------------------

Abstract

The purpose of the study is to synthesize salicylic acid from the ester, methyl salicylate, and determine the acid’s dissociation constant and purity. The ester was converted to salicylic acid by base hydrolysis. The products were refluxed and recrystallized, to ensure maximum purity, and filtered, dried, and weighed. The melting point of the product was determined using a Fischer-Johns melting point apparatus. The acid then dissolved in separate beakers with 95% ethanol and water and titrated with 0.050 M NaOH, previously standardized with potassium hydrogen phthalate, through potentiometric titration. The pH after addition of base was measured and plotted against the volume of titrant added using three different plots. Results show a 61.0% yield and the melting point differed from the theoretical by a range of 3.11-6.83%. The pKa calculated was 2.865, differing from the literature value of 2.98, by 3.86%. The theoretical purity of the sample was 100.0%, which differed with the experimental one by 1.7%; the experimental purity is 101.7%. Potentiometric titration proves to be adequate in the determination of the acid dissociation constant and purity of a sample. Aside from that, the synthesis proved adequate given the high purity of the product. -------------------------------------------------

Keywords: acid dissociation constant purity melting point ester

INTRODUCTION

Potentiometric methods of analysis are based on measuring the potential of electrochemical cells without drawing much, appreciable current. For centuries, potentiometry has been used to locate the endpoint in most titration set-ups. (Skoog, et al., 2004).

Potentiometric methods offer a myriad of advantages, its main advantage being its low operational costs. Voltmeters and electrodes are, generally, far cheaper than most modern scientific instruments. Models suitable for direct potentiometry in field work, away from the laboratory, are inexpensive, compact, and easy to use. Essentially nondestructive of the sample, i.e., insertion of the electrode does not drastically change the composition of the test solution (except for the slight leakage of electrolytes from the reference electrode), electrodes are relatively free from interferences. Stable potential readings are attained fairly rapidly and voltages are easily recorded as functions of time. Finally, the wide range of analyte activities, over which some of the available indicator electrodes exhibit stable, nearly Nernstian responses, represents an important advantage (Day and Underwood, 1991).

Potentiometric titrations involve measurement of the potential of a suitable indicator electrode as a function of titrant volume. This indicator electrode is speecific to the hydronium ion, H3O+. It offers adavantages over direct potentiometry in that it is not dependent on measuring absolute values of Ecell. This is due to having the measurement based on the titrant volume that causes a rapid change in potential near the equivalence point.This makes the process relatively free from the juction potential uncertainties as this potential remains approximately constant during the titration process. Instead, the titration results depend heavily on having a titrant with accurately known concentration. The instrument merely signals the endpoint and behaves like a regular chemical indicator. Aside from that, the reference electrode potential need not be known. Most importantly, ionic strength effects are not important in the titration procedure because the result is analyte concentration, even if the electrode responds to activities.

The dissociation of a weak monoprotic acid is given by the equation

HA ↔ H+ + A– (1)

[H+][A–]
[HA]
where HA is the monoprotic...
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