Exp 4 Lab Report

Exp 4 Lab Report - Determination of the pKa of bromocresol...

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Determination of the pK a of bromocresol green using the method of spectrophotometry and the Henderson-Hasselbach equation Jubilee Ngo, Nora Roberts TA: Christine Huang Section: A00 Chemistry 100A at University of California, San Diego 9500 Gilman Drive, La Jolla, California 92093 20 July 2006 Abstract The pK a (or pK HIn ) of bromocresol green (bcg), an acid-base indicator, denoted by HIn, is determined using spectrophotometry and a modified version of the Henderson-Hasselbach equation, log ([In - ]/[HIn]) = pH – pK HIn . First, 8 solutions were made from various volumes of bcg, 0.1 M HCl, 0.1 M CH 3 COOH, 0.2 NaCH 3 COO, and ~1.5 M NaOH. Absorbances of two solutions (one with the highest pH [#1] and another with the lowest pH [#8]) were measured using at different wavelengths. The wavelengths at which their absorbance values are highest were recorded, which was 440 nm for HIn and 615 nm for In - . Then, buffered solutions with pH values 0.3-0.5 away from either side of each of the 8 solutions were made and their absorbance values were measured at both of the maximum wavelengths. From the plots of absorbance versus pH for each wavelength, the absorbances of HIn (A a ) and In - (A b ) were determined, which then allowed for the calculation of the ratio of [In - ] to [HIn] at each wavelength (R1 is the ratio at 440 nm and R2 at 615 nm). The plots of log(R1) versus pH and log(R2) versus pH resulted in trendlines that are in the form of the equation above and allowed for the calculation of pK HIn . Thus, the pK HIn value at 440 nm is 5.5029 and 4.9482 at 615 nm. The average value of the pK HIn is 5.23 ± 0.29 and 4.68 ± 0.89 from the class data. Result The plot of absorbance versus wavelength for both HIn and In - is shown below in Figure 1. Figure 1. Absorbance spectrum of HIn and In - Absorption vs. Wavelength 0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 0.450 0.500 350 445 465 500 580 600 620 Wavelength/lambda (nm) Absorbance for HI 0.000 0.200 0.400 0.600 0.800 1.000 1.200 Absorbance In Absorption spectrum of HIn Absorption spectrum of In-
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Based on this figure, the λ max of HIn is 440 nm and the λ max of In - is 615 nm. (See Table 4 in the Appendix for the data). The plot of absorbance versus pH of HIn is shown in the following figure. Figure 2. Absorbance of HIn (at λ max of 440 nm) versus pH for all solutions Absorbance vs. pH 0.000 0.100 0.200 0.300 0.400 0.500 0.00 2.00 4.00 6.00 8.00 10.00 12.00 pH Absorbance Based on the plot above, the absorbance of HIn (A a ) is determined to be 0.433 while the absorbance of In - (A b ) is 0.076. (See Table 5 in the Appendix for the data). From these absorbance values, the ratio R1 was calculated, where R1 = (A a - A)/(A – A b ), and the following plot of log (R1) versus pH in Figure 3 resulted. Figure 3. Log (R1) (at λ
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Exp 4 Lab Report - Determination of the pKa of bromocresol...

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