2I.Introduction/PurposeSpectrophotometry is a technique which measures the amount of light absorbed by a chemicalsubstance. The absorbance is determined by measuring the intensity of the light before and after it passes through the chemical medium (1). The spectrophotometer was first invented and used by Arnold J. Beckman and his colleagues at National Technologies Laboratories in 1940. The spectrophotometer was the company’s greatest discovery and has since been used as one of the most effective methods of quantitative analysis in various fields such as chemistry, physics, biochemistry, and clinical medicine (2).The purpose of this investigation is to determine the dissociation constant, KIN, for the indicator bromophenol blue (C19H10Br4O5S) (3) through spectrophotometric analysis. When bromophenol blue dissociates in water, it loses a proton from a hydroxyl group forming quantities of the base form of the indicator (represented as In-) as well as hydronium, H3O+, ions1.Since the dissolved indicator does not dissociate entirely, it reaches an equilibrium which results in quantities of the acid form of the indicator (represented as HIn) being present in the solution as well. The acid form of the indicator is yellow in appearance and the base form of the indicator is blue in appearance. Measurement of the pH as well as spectrophotometric analysis of the solution in both its acid and base forms can allow for the determination of the concentrations of both the acid and base forms of the indicator at various equilibrium points. This is because the concentration of a certainchemical is directly proportional to the amount of light the chemical absorbs at a given wavelength according to the Beer-Lambert law2. The absorption of the acid and base forms will be converted to concentrations3and used with the pH to determine an average pKINvalue (using 1 The balanced chemical equation for the dissociation of the indicator bromophenol blue is represented by equation i.2 The Beer-Lambert law equation is represented by equation ii.3 The equation used to convert between absorbency and concentration is represented by equationiii.