Experiment 4.docx - Experiment 4 Spectrophotometric...

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Experiment 4: Spectrophotometric Determinationof the Dissociation Constant of an IndicatorPurpose:
The main purpose of this experiment was to determine the dissociation constant, pKIn, of bromophenol blue spectrophotometrically while applying the Beer-Lambert Law.The Beer-Lambert Law states that there is a liner relationship between the absorbance of light at a certain wavelength and the concentration of the absorbing species. The formula for the Beer-Lambert Law is expressed as A = lc where is the molar absorptivity constant, l is path length, and c is the concentration. Molar absorptivity is a constant that relates the path length and the concentration together. The equilibrium reaction of the indicator is HIn + H2O In-+ H3O+where HIn is the acid form of the indicator and Inis the basic form. In order to determine , pKIn, of bromophenol blue using the equation pKIn= pH + log [HIn]/ [In-]. Procedure:First, 5.00 mL of 0.02% bromophenol blue were added to a 100 mL volumetric flask followed by a 0.05M KHP with a pH 4.0 buffer solution. The solution was split into two clean dry breakers A and B. A 50 mL graduated cylinder was used to pour the solutions into the beakers. The pH meter (pH electrode) was used to record the pH of thesolutions. After that, the absorption spectrum was determined from 380nm to 650nm at 10 nm interval using the spectrophotometry. Then,solutions A and B were split between 3 vials each. For solutions A1, A2,and A3, different concentrations were required to be obtained. For example, vail A1 includes 8.00 mL of solution A with 2.00 mL of KHP. Vial A2 included 8mL of solution A followed by 1.60 mL of KHP and 4.00mL of HCL. Lastly vial A3 included 16 mL of solution A followed by
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