Ishita_GandhiExperiment3

Ishita_GandhiExperiment3 - Experiment 3 Spectrophotometric...

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Experiment 3: Spectrophotometric Determination of the pKa of the Acid-Base Indicator Bromothymol Blue Name: Ishita Gandhi Date: September 17, 2009 Lab Partners: Kim Nguyen and Michael Brady TA: Tyler Keith Davis Chem 241L Section 421 Room 404 Honor Pledge Introduction : There were two objectives of this lab, the first being the determination of the acid dissociation constant for bromothymol blue. Bromothymol blue appears in two forms, an
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acidic, protonated form (HIn) and a basic, deprotonated form (In-). The protonated form appears yellow while the deprotonated form appears blue; this difference in appearance is accounted for by the difference in chemical structure. This difference in chemical structure is due to electronic delocalization when the protonated form loses a proton (H+) and rearranges to delocalize the negative charge. The second objective was to observe the absorbance of the acidic and basic forms of bromothymol blue in both aqueous and organic solvents. Bromothymol blue has chemical properties that make it appear different colors in a solution of different pH values. Because of this, bromothymol blue is often used as an acid-base indicator, or to directly observe the change in pH of acid - base equilibrium. Therefore the importance of the determination of the pKa is to be able to use it in calculations in weak acid-base equilibria and titrations. Determining whether the acidic or basic form is more soluble in an aqueous or organic solvent is important in deciding which solvent would be most useful for the form an experimenter is working with. Beer’s Law along and a spectrophotometer were used in order to obtain the dissociation constant of bromothymol blue. Beer’s Law relates absorbance to concentration of a solution and states that they are proportional variables. The absorbencies of different pH levels of bromothymol blue at a certain wavelength can be measured using a spectrophotometer to find the point at which the concentration of acidic and basic forms of bromothymol blue are equivalent. According to the Henderson-Hasselbach equation , when the concentration of an acid and base are equivalent, the pH is equivalent to the pKa. Therefore using simple data analysis of various absorbencies of bromothymol blue, the acid dissociation constant (pKa and Ka) can be determined. To determine whether the acidic or basic form is more soluble in dichloromethane or water, simple separation techniques can be used. Adding an aqueous solvent to an immiscible solvent will result in a solution with two separate layers. The bromothymol blue is the aqueous solution while dichloromethane can be used as an organic solvent. Two separate extractions of the aqueous layer can be prepared using both pH 5.00 and pH 10.00 bromothymol blue solutions. The
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absorbency of each can be found using the spectrophotometer and the molar absorptivity can be found using Beer’s Law. Then, the absorbancies and molar absorptivities from the original sample (before extraction) may be compared with the
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Ishita_GandhiExperiment3 - Experiment 3 Spectrophotometric...

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