Experiment 5

Experiment 5 - Experiment 4 Buffers and Indicators Abstract...

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Experiment 4: Buffers and Indicators Abstract This experiment involved determining the pK HIn of the indicator bromocresol green by measuring its percent transmittance, converted to absorbance, in various solutions of different pH values. The first part of this experiment involved finding the wavelengths of maximum absorbance for both a solution of high [HIn] and one of high [In - ] which were determined to be 445 nm and 605 nm, respectively. All solutions had absorbance measured at these two wavelength which would in turn create a trendline from which pK HIn can be determined. Measuring at 445 nm and 605 nm, it was determined that bromocresol green had pK HIn values of 4.75 ± 0.73 and 5.05 ± 0.84, respectively. Introduction The purpose of this experiment is to find the p K a (p K In ), of the indicator (changes color at certain pH, a weak acid or weak base) bromocresol green. This acid-base indicator is an organic acid which dissociates with the following reactions: HIn + H 2 O ↔H 3 O + + In - Using the Henderson-Hasselbalch equation, the equilibrium expression for this reaction can be written as: - + HIn In log HIn pH pK Which can be converted to: - In log - HIn HIn pH pK This is equivalent to the line form of y = mx + b where y is log([In - ]/[HIn]), x is pH, m (the slope of the line) is equal to one, and b (how much the line is shifted vertically) is pK HIn . When this is log([In - ]/[HIn]) is plotted against pH, the following trend is obtained as shown in Figure 1. 1
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Figure 1: Sample graph of log([In - ]/[HIn]): plotting log([In - ]/[HIn]) versus pH of the bromocresol green indicator will give the following linear trend. 1 When y=0 or log([In - ]/[HIn])=0, pH=p K HIn which makes sense because when log([In - ]/[HIn])=0, [In - ] /[HIn]=0, which means that the indicator is 50% disassociated. This concept will be used in this experiment in order to find the p K HIn . But instead of finding the concentrations of the HIn and In - at certain pH’s, only the ratio needs to be established. This ratio does not only represent the ratio of the concentrations between acid and base, but it also represents the ratio of the absorbance between acid and base since absorbance is proportional to concentration using the equation A=ξ bc (A= absorbance; ξ= molar absorbtivity (cm -1 M -2 ); b= cell length (cm); and c = concentration of component in solution): : base acid Absorbance log - Absorbance HIn pH pK Because of this, one can measure the absorbance values of solutions of various concentrations of bromocresol green with a spectrophotometer. This can then be plotted against their respective pH values (measured with pH meter) to get a linear trend and the y intercept will equal the p K HIn . To be able to eliminate any error due to poor readings of too diluted solutions, the wavelength of
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Experiment 5 - Experiment 4 Buffers and Indicators Abstract...

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