Experiment 4.docx - Experiment 4 Spectrophotometric...

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Experiment 4Spectrophotometric Determination of the Dissociation Constant of an Indicator CCHM110 – PRA102TA: Nagina Amir Experiment Performed: 04 / 11 / 2019Report Submitted: 18 / 11 / 2019
PurposeThe purpose of this experiment was to determine the dissociation constant, KIN and pKIN of the bromophenol blue indicator through the use of a spectrophotometer. Finding these values allows for the determination of how strongly the species absorb light at a particular wavelength, with respect to the Beer-Lambert Law.Experimental Method The experiment was first composed of the creation of solution A/B. To create solution A/B, a transfer of 5 mL of 0.02% of bromophenol blue stock solution and 95 mL of 0.05 M KHP was mixed into a 100 mL volumetric flask. Once the solution was mixed thoroughly, 50 mL was used to create solution A and 50 mL was used to create solution B. Before anything can proceed, the pH of solution A was measured using a pH electrode, and then recorded (the pH of solution B was the same as the pH of solution A). Then a small sample of solution A/B was inserted into acuvette and placed into the recording spectrophotometer where it was measured on a continuous scale from 380 nm to 650 nm. Then the identity of the wavelengths of the two absorption peaks were recorded: ʎ1: 440 nm and ʎ2: 590 nm. Once this section was completed solutions A1, A2, and A3 were created through the incorporation of solution A, which was followed by the chart provided in the course manual. Using the manual spectrophotometer, the absorbance of solutions A1 and A2 were measured at ʎ1 and ʎ2. With the use of the recording spectrophotometer, the absorbance of solution A3 was measured from 380 nm and 650 nm, and the resulting measurements were then recorded onto the data sheet. Then solutions A4, A5, and A6 were created by diluting solution A3, in which was followed by the chart provided in the course manual. Once again, using the manual spectrophotometer, the absorbance of each new solution was measured at ʎ1 and ʎ2, and recorded. Same procedure was done for solution B, in which solution B1, B2, and B3 were created following the charts provided by the course manual, and solutions B4, B5, and B6 were created from the dilution of B3, following the charts in the coursemanual. The absorbance of B3 were measured from 380 nm to 650 nm using the recording spectrophotometer, and solutions B1, B2, B4, B5, and B6, were measured using the manual spectrophotometer at ʎ1 and ʎ2, and then recorded. (1)
Results and Calculations 1.Plot the absorbance vs.wavelength data from step4. of the procedure in order to determine ʎ1 and ʎ2. Note that this must be done before you can do the rest of the experiment. Later, on the same graph plot the spectrum of solutions A3 and B3.Figure 1. graphed values of absorbance spectra vs. wavelength (nm) for solutions A, A3, B32.AHIn is the absorbance of solution A3 which is the absorbance of the acid form of the indicator. Ain-is the absorbance of solution B3 which is the absorbance of the base form of the indicator.

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