Introduction, Abstract, Background

                Weak acids can experience a dissociation process in which they dissociate into a conjugate base and a proton.

HA represents the acid, H+ the proton and A- the base. The dissociation constant is the equilibrium constant of a chemical reaction. It is calculated by the equation:

The dissociation constant is a measure of the strength of an acid in a solution. The purpose of this laboratory was to determine the dissociation constant of Acid-base indicators. Bromophenol blue, Bromothymol blue, Bromocresol Purple, Methyl Red, Methyl Yellow and Methyl Orange were used for this experiment. Acid-base indicators are different colors for the acid and base forms. Therefore at different pH values Acid-base indicators are different shades of colors corresponding to the concentration of the acid and base forms of the compound. Each color has a different absorption spectra.

                The dissociation constant of a species that experiences these color changes can be determined from obtaining the absorption of the colored solution using UV-visible spectroscopy. At a wavelength (λmax) in which the two forms (acid and base) have a large difference in absorption the ratio of their concentrations are equal to the following formula:

A is the absorption at λmax, AHAmax is the absorption at λmax  when the weak acid is completely in its acidic form and AAmax is the absorption at λmax when the weak acid is completely in its basic form. The dissociation constant K can be calculated using this equation and the pH value of the solution.