# Experiment 4.docx - Experiment 4 Spectrophotometric...

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Experiment 4: Spectrophotometric Determination of the DissociationConstant of an Indicator Date Performed: Nov.12.2019Date Submitted: Nov.19.2019
PurposeThe purpose of this experiment is to determine the dissociation constant of the indicator, bromophenol blue (C19H10Br4O5S), using techniques of spectrophotometric analysis. During the dissociation of BPB, a proton is lost from the hydroxyl group, to form a hydronium ion H3Oand the base form of the indicator, In-.HIn + H2O H3O+ + InThe acid form of the indicator (HIn) is yellow in colour, while the base form (In-) is blue. Since the indicator does not fully dissociate, it goes to equilibrium and allows for the calculation of KIn, using the following formula,KIn = [H3O+][In-]/[HIn].By finding the pH of the indicator in its various acid and base forms, the concentrations of each of the forms can be determined, as the Beer-Lambert Law states that the absorbance of light at a certain wavelength is directly proportional to the concentration of the form of the indicator that absorbs light at that wavelength.From the value of KIn, the pKIncan then be calculated, using the formula.¿¿¿[¿¿)[HIn]¿pK¿=pH+log¿.The molar absorptivity can also be found, according to the Beer-Lambert lawA=εlc
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where A is the absorbency, εis the molar absorptivity constant, lis the length of the medium that the light passes through (in this case it is equal to 1cm), and c is the concentration of the solution.