Determination of Equilibrium Constant, Ka, and pKa by Using Spectrophotometry
[1]∗* Jasmin Villarreal, Hannnah Phipps, Bethany Malinak
Chemistry 112, Section 534
Introduction
Solutions of bromothymol blue will be measure in highly acidic, highly basic, and neutral aqueous solutions in order to derive the absorbance at different wavelengths by using a spectrophotometer. The information derived from the spectrophotometer will also reveal the lambda max that will then be used to evaluate the equilibrium constant of the acid-base reaction of the solution. The equilibrium constant will then be used to calculate the pKa.
Materials and Methods
In order to prepare the spectrophotometric samples, 50 mL of phosphate buffer solution, with a pH of 6.76, was obtained and poured into a 150 mL beaker. Afterwards, 20 drops of a 0.04% bromothymol blue solution was added to the buffer solution, which then turned green. A 5mL serological pipet was then cleaned with the buffer solution and the rinse was discarded into the waste beaker. Three beakers were then rinsed and dried to ensure there were no previous solutions in the beaker. The same pipet that was rinsed earlier was used to transfer 5.00 mL of the green, buffered solution to each of the 3 beakers. 1.00 mL of 1.0 M HCl was added to one of the beakers by using the buret filled with HCl. This beaker was then labeled “Yellow.” With the buret filled with NaOH, the second solution was made by adding 1.00 mL of 1.0 M NaOH and then labeled “Blue.” The buret filled with water had 1.0 mL of distilled water added to the remaining beaker and then labeled “Green.”
For the second part of this experiment, the experiment file titled, “Equilibrium,” was opened in LoggerPro. To calibrate the blank cuvette, “Calibrate,” located under the “Experiment,” was selected and then followed by selecting “Spectrometer I.” There was a 90 second wait period to allow the bulb to warm up. The first cuvette was filled with distilled water and inserted in the instrument with the clear sides lined up with the white arrows on the top of the spectrophotometer. After this, “Finish Calibration,” was selected to acquire the background. A different cuvette was then filled with the yellow sample and wiped with a lint-free tissue to remove any fingerprints or water droplets. The cuvette was placed in the spectrometer with the clear sides lined up with the arrows. With the “Collect” button, the Logger Pro software collected the spectrum and the “Stop” stopped the collection. Two more cuvettes were filled, one with the blue solution and the other with the green solution. Each solution was wiped down to erase fingerprints and placed into the spectrometer. After selecting “Collect,” the option to “Store Latest Run” was selected in order to have the three different spectra on the same plot. The file was then saved as a .cmbl file and all the equipment was cleaned and put back.
Results and Discussion
When the 1.00 mL of 1.0 M HCl was added to the buffered bromothymol blue solution, the solution turned yellow. This solution is very acidic and the only solute species present is the yellow HBB. When the 1.00 mL of 1.0 M NaOH was added to the buffered bromothymol blue solution, the solution turned blue, thus showing that the solution is very basic and will only have the ion present in the measurable concentration. The last solution has acidity close to neutral. This neutrality is represented by its color green, a combination of blue and yellow. [pic 1]
After using the spectrophotometer to measure the spectra of the blue, yellow, and green solutions, the absorbance and wavelengths were derived from the plots. For , there was a lambda max of 429.3 nm at an absorbance of 0.145. For , there was a lambda max of 615.2 nm at an absorbance of 0.341. Through these plots and points of the blue and yellow solutions, the isosbestic point was derived from the graph with a wavelength of 499.9 nm and an absorbance at 0.050, as shown in Figure 1 below. [pic 2][pic 3]
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For this method, having an isosbestic point is important because at this wavelength, the reaction mixture stays constant regardless of the extent of reaction of position of the equilibrium. This point is also a good indication that there is only one reaction involved with bromothymol blue. Without this point, determining the equilibrium constant would not be possible.
In order to calculate the equilibrium constant, the for the green solution was calculated by using the equation , which resulted in . This number can then be plugged in to the following equation for along with the wavelengths derived from the data collected by the spectrometer for the A green and A blue absorbance at a wavelength of 616 nm. [pic 6][pic 7][pic 8][pic 9][pic 10]