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CHM110H5 Lab Report 4Spectrophotometric Determination of the Dissociation Constant of an IndicatorAngela Howard – 1001085041Lab Partner –Leo JosephPRA0128 – Richard KilLab Completed On: November 8th, 2017Lab Report Due On: November 22nd, 2017
PurposeThis lab experiment focused on determining the dissociation constant of the indicator bromophenol blue by using spectrophotometric methods. To find the dissociation constant, we were provided with the formula: pKIN=pH+log[HIn]/[In-]. The goal of the in-lab experiment was to determine the values associated with each variable; throughout the lab we manipulated solutions and gathered data to find these values. We worked with both the acidic (HIn) and basic (In-) versions of bromophenol blue. We created multiple buffer derivatives of each form of solution (acidic and basic) and measured their pH and absorbance values. Using the spectrophotometer, we found the two wavelength peaks of bromophenol blue (438nm and 591nm) and tested each solution’s absorbance at those wavelengths. Additionally, we took assumptions from the Beer-Lambert law to determine the strength of light absorption at 438nm and 591nm using the molar absorptivity constant formula, ϵ=Alc, this was done for HIn and In-(Poe, 2017). From these methods, we achieved the necessary values to calculate the pKIN(s) ascan be seen in Figure 2.0, as well as the molar absorptivity concepts as seen in Figure 7.0. Bromophenol blue can exist in acidic and basic form, and it has two distinct colours in each form, yellow and blue, occurring due to the loss or gain of a proton (H+), making it an ideal indicator for determining changes in pH (LibreTexts, 2017). The blue and yellow colours occur due to changes in light absorption; by understanding the pKINvalues for bromophenol blue, we can recognize its’ importance for identifying pH endpoints and other qualitative characteristics (LibreTexts, 2017). These are useful tools for acid-base titrations and can be used for identifying solutions. Overall, the purpose of this experiment was to understand the unique qualities and
important features of bromophenol blue, as well as the positive implications of using indicators as a tool in experiments.Experimental MethodThe course lab manual was followed for this experiment with only one deviation during step 7.2 (Poe, 2017). During the final step of the experiment there was not enough solution leftover to make the B3 solution derivatives (B4, B5, and B6). This was due to my own human