Experiment 14 Technical Abstract

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Determination of the Equilibrium Constant Using Spectrophotometry of Bromothymol Blue Primary Author: Chris Hernandez Introduction The Equilibrium constant is very important in many aspects of chemistry and chemists very often use spectrophotometry to find this equilibrium constant. The equilibrium constant, Ka, and the pKa of Bromothymol Blue were found using spectrophotometry. Materials and Methods The first step to find the equilibrium constant was to prepare the spectrophotometric samples. 50 mL of a sodium phosphate buffer was added to a 150 mL beaker. Approximately 20 drops of a .04% Bromothymol blue solution was then added to the beaker of the phosphate buffer. Using a clean 5 mL serological pipette, transfer 5 mL of the phosphate buffer and Bromothymol blue solution to each of three clean 150 mL beakers. Next, using the buret for HCl, add 1 mL of HCl to one of the three beakers, then label this beaker “Yellow”. Next use the buret for NaOH and add 1 mL of NaOH to another of the three beakers, and label this beaker “Blue”. Lastly add 1 mL of water using the buret for water to the last beaker, and label this beaker “Green”. Clean and dry 4 cuvettes. The next part of the experiment is to actually use the spectrophotometer to find the absorbance of the green, yellow, and blue tubes. First, make sure the spectrophotometer is connected to the computer. The first step is to calibrate the blank or the background, and to do this, a cuvette of distilled water is inserted into the spectrophotometer. Next, fill a cuvette with the “blue” solution and place it into the spectrophotometer. Click “collect” and wait until the spectrum of the solution appears. Once the spectrum appears click “stop”. Repeat the same steps with the “green” and the “yellow” solutions. All three spectras should be on the same plot. Save the data as a .cmbl file

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