# LAB 4- Spectrometric Determination of the Dissociation Constant of an Indicator.docx

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Experiment 4: Spectrometric Determination of the Dissociation Constant of an IndicatorSection 0113Ying LimPreformed: November 6, 2018Submitted: November 13, 2018
Purpose The purpose of this experiment is to use a spectrophotometer to determine the dissociation constant, KIN, and the pKIN of the indicator bromophenol blue which is yellow in its acidic form (HIn) and blue in its basic form (In-).1The pKINof the different solutions of different levels of acidity and basicness of bromophenol blue can be determined using equation 11:1.pKIN= pH + logAA¿¿AHINA¿where pH is the potential for hydrogen, A is the absorbance of the solutions that contain both forms of the indicator, AIN-is the absorbance of solution B3, the basic form of the indicator, and AHINis the absorbance of solution A3, the acidic form of the indicator1The Beer-Lambert law equation can be used to determined the molar absorptivity constant after graphing the absorption vs. concentration and using its slope and given length to solve for it using the following equation1:2.A=lcWhere A is the absorbance of light at a certain wavelength , is the molar absorptivity constant in mol-1cm-1, c is the concentration of the form of the bromophenol blue that absorbs the light that certain wavelength in mol/L and l is the length of the cuvette that the sample was put in, in centimeters, cm.13.c = nWhere c is the concentration of the indicator in molar, M, n is the moles of solution in mol, and vis the volume of solution in litres, L.
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Experimental MethodTo begin, 5.00mL of 0.02% bromophenol blue stock solution and 0.05M of potassium hydrogen phthalate was put into a 100mL volumetric flask.1Half of the solution was then put into a beaker labelled A and the rest into another beaker labelled B. The pH was then recorded. The absorption spectrum of solution A was recorded with the use of a spectrophotometer. Using the device and the data collected the two absorption peaks λ1and λ2 are noted. Using the Asolution, A1, A2 and A3 are measured into three different vials accordingly to the table on page 69 of the course manual.1These solutions are then blanked and the absorbance’s for each are recorded at λ1and λ2