Equilibrium Constant Using Spectrophotometry

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Introduction: This experiment consisted of determining the equilibrium constant of an acid base reaction by using the process of spectrophotometry as well as Beer’s law. The maximum wavelengths of three solutions were calculated in order to obtain their absorbance, which further allowed for the concentrations to be found. Through this process we were able to find the equilibrium constant between bromothymol blue in each of the different solutions. Materials and Methods: To initiate the experiment, the pH of the phosphate buffer solution was recorded, which in this case happened to be 6.83. 50 mL of this buffer solution were then obtained and twenty drops of .04% bromothymol blue solution were then added turning the overall color of the solution green. Three 50 mL beakers were then gathered and filled with 5.00mL of this green solution. Each of the beakers were then added 1mL of either 1.0 M HCl, 1.0 M NaOH, or distilled water. The beaker that was added HCl was labeled yellow, the one with NaOH was labeled blue, and the one with distilled water was labeled green. The final color of each solution corresponded with it’s label. The spectrophotometer was then left to warm up for 90 seconds before a cuvette filled with distilled water was used to calibrate it. The three solutions were then transferred into three cuvettes, which were all cleaned with lint free tissue to eliminate all fingerprints. Each of the cuvettes were then properly placed in the instrument and their spectrums were recorded. Once our spectrums were stored correctly, our file was then saved. Results and Discussion: The key assumption made for bromothymol at 616nm is that the presence of bromothymol blue in HBB form is close to zero. The presence of HBB is the one found in the yellow solution. The absorbance recorded at 616nm for the yellow solution was .006, .152 for our blue, and .469 for

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