Determination of the Acid Dissociation Constant of an Acid-Base Indicator by Visible Spectrum

Determination of the Acid Dissociation Constant of an Acid-Base Indicator by Visible Spectrum

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Introduction

Bromothymol blue is a widely used indicator in the acid-base titrations. Similar to numerous acid-base indicators, this indicator (bromothymol blue) is by itself a weak acid and thus dissociates as shown below in a general form as:

HIn + H2O[pic]In- + H3O+

Where, Hln denotes protonated indicator form

In- Shows the unprotonated form

Weak acids can undergo dissociation procedure whereby they dissociate into a proton and conjugate base. The measure of the vigor of an acid in a solution is referred to as the dissociation constant. An acid-base indicator is the base form and the color of the acid. This means that at various pH figures acid-base indicators are unlike shadows of colors subsequent to the concentration of the base forms and acid of the mix. Therefore, different absorption spectra have different colors. UV-visible spectroscopy can be used to establish a dissociation constant of a given type by getting the colored solution obtained (Murova, 2009). The experiment determines the pKa for Bromothymol blue that is essential in the determination of an indicator to be used. A spectrophotometric procedure is used because of the straight understanding of the protonated and the unprotonated. The information derived from the spectrophotometric measurements is useful in the computation of the two acidic and basic forms.

This laboratory report aims at presenting information on the determination of the acid dissociation constant of an acid-base indicator by a visible spectrometry by using Bromothymol

Blue.

Establish the Visible Spectrum of Protonated and Unprotonated Indicator forms

Since Bromothymol blue is weal acid in solutions that are aqueous the reaction below occurs

HIn + H2O[pic]In- + H3O+

Bromothymol blue is...