Bromothymol Blue Experiment

Introduction Chemical reactions and how they behave are a keystone not only in the scientific community and in scientific education, but also in daily functions and the biology of living organisms. Understanding how chemicals can affect their reactions and the pH of the solution they are in is important in maintaining balance in the ecosystem as well as the human body and other organisms. In this experiment, acids and bases at equilibrium with an indicator will be used with a spectrophotometer to calculate the equilibrium constant and pKa for Bromothymol Blue.
Materials and Methods For part 1 of the experiment, obtain a 50 mL sample of sodium phosphate buffer solution and pour the sample into a 150 mL beaker. Take note of the…show more content…
Log onto the computer and start LoggerPro. Open the experiment file “Equilibrium”, which can be found under “Experiment Information” from the eCampus page. Under the “Experiment” heading in the toolbar, click on “Calibrate” and “Spectromoter 1”. This will turn on the light source in the spectrometer. Wait 90 seconds for the light source to warm up, then fill a cuvette with distilled water and clean the flat, clear sides on the cuvette with a Kimwipe, removing dust and fingerprints before putting the cuvette into the instrument. Orient the cuvette so that the clear sides line up with the white arrow on the top of the spectrometer. Click “Finish Calibration”, then “OK”. Next, fill a cuvette with the “yellow” sample, repeating the steps to clean the clear sides of the cuvette with a Kimwipe to remove fingerprints and dust. Place the clean cuvette containing the “yellow” sample into the spectrometer, orienting the cuvette so that the clear sides of the cuvette line up with the white arrow in the cuvette. Click the “Collect” button on the LoggerPro software to begin collecting spectrum data. After a few seconds, the spectrum should appear on the software. Click “Stop” to end the data collection. Repeat the steps used to collect the spectrum data from the “yellow” sample for the “green” and “blue” solutions. However, for these data collections,…show more content…
Using the formula [H3O+]=10-pH, we are able to use the recorded pH of the buffer solution, which is 6.79, to calculate the hydronium ion concentration.
[H3O+]green=10-6.79, so the hydronium ion concentration is 1.62x10-7 M.
Since the pH of the buffer solution if 6.79, it is slightly acidic, and thus, will have a higher hydronium ion concentration than its hydroxide ion concentration. Using the calculated hydronium concentration of the green solution, we are able to calculate the Ka of the bromothymol blue using the formula below.
Ka=[H3O+]green x (Agreen / Ablue-Agreen)*
*Absorbance at 616 nm.
The formula excludes the absorbance of the “yellow” solution because almost all of the bromothymol blue is in the HBB form in the acidic “yellow” solution, resulting in the yellow color and making its absorbance at 616 nm almost 0, or negligible. The formula uses the absorbance of the “blue” solution because its λmax is 614.9nm, which is very close to 616nm.This means that in the “blue” solution, most of the bromothymol blue is in the form BB-, displaying a blue color that absorbs the light at 616 nm, thus resulting in a higher recorded absorbance. Below are the recorded absorbance values at 616