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



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 + H2OIn + H3O+

Where, Hln denotes protonated indicator form

InShows 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



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 + H2OIn + H3O+

Bromothymol blue is present in very acid solutions and it takes the protonated form Hln. However, Bromothymol blue is available in very basic solutions taking the unprotonated form In.The two Bromothymol species above, the protonated and the unprotonated while in the visible area, they both absorb electromagnetic radiations. At any specific wavelength the electronic radiation absorbed amount is corresponded to the concentration of molar (C) in the solution with an absorbing species using the Beer’s Law (Nemcova, 2006) . The equation is shown below.

Al = elbC


= absorbing molar that is a constant

b = the length the solution takes via electromagnetic radiation. Its unit measure is in cm. determined by the scope of used cells in the experiment.

A is the absorbance measure ‘A’ directly connected to concentration ‘c’ of the solution containing the absorbing species.

An experiment is conducted using the variables and the results used to obtain the protonated form and the unprotonated form. From the experiment two wavelengths are chosen, one should have a larger absorbance of the protonated form with a low unprotonated form while the other should be of a lower protonated absorbance form with a larger unprotonated absorbance form. The two results of wavelengths are important in the next step of the process of determining the acid dissociation constant of an acid-base indicator by visible spectrum.

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