exp3 - Anusha Vadlamudi Partner: Christen Burns TA: Lindsay...

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Anusha Vadlamudi Partner: Christen Burns TA: Lindsay Aldridge Chem 241 Lab; Section 401; Room 300 Experiment 3: Spectrophotometric Determination of the pKa of the Acid-Base Indicator Bromothymol Blue (5/22/08)
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Honor Code Pledge: I pledge that no unauthorized assistance has been given or received in the completion of the work presented in this report. Results and Calculations: Absorbance vs. Wavelength -0.2 0 0.2 0.4 0.6 0.8 1 1.2 340 440 540 640 740 Wavelength Absorbance pH 5.00 pH 10.00 Figure 1: Absorbance vs Wavelength of Bromothymol Blue solutions in pH 5 buffer and pH 10 buffer. The absorbance was collected using a spectrophotometer, the data collected using Hyper Terminal, and the graph made using MS Excel Bromothymol Blue + pH 10 buffer Bromothymol Blue + pH 5 buffer Color Blue Yellow Wavelength at the peak of the spectra 430.0 nm 610.0 nm More Intense peak Yes No Table 1: Observations of the Bromothymol Samples with pH 5 buffer and pH 10 buffer The more intense of the two belongs to the pH 10 buffer solution, and therefore the basic form (In-) is the more dominant one. The peak wavelength of the basic form (610.0 nm) was used to calculate the Absorbance of eight different buffer solutions in a spectrophotometer
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pH Absorbance 5.00 0.020 6.00 0.068 7.00 0.408 7.30 0.562 7.60 0.743 8.00 0.782 9.00 0.893 10.00 0.910 Table 2: The absorbances and their corresponding pH values of the eight buffer solutions. Absorbance vs. Buffer pH 0.000 0.200 0.400 0.600 0.800 1.000 0.00 5.00 10.00 15.00 pH of the buffer Absorbance Absorbance Figure 2: Graphical Illustration of Absorbance vs. buffer pH of the eight buffer solutions. The inflection point of this graph is where [HIn] = [In-] and pH = pKa of Bromothymol Blue. The inflection point is also where the maximum of dAbsorbance/dpH vs. Average pH (first derivative plot) occurs. The dAbsorbance/dpH values and average pH values were calculated using MS Excel. Buffer
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exp3 - Anusha Vadlamudi Partner: Christen Burns TA: Lindsay...

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