Lab Report 4.docx - CHM110H5 Lab Report 4 Spectrophotometric Determination of the Dissociation Constant of an Indicator Angela Howard 1001085041 Lab

Lab Report 4.docx - CHM110H5 Lab Report 4...

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CHM110H5 Lab Report 4 Spectrophotometric Determination of the Dissociation Constant of an Indicator Angela Howard – 1001085041 Lab Partner –Leo Joseph PRA0128 – Richard Kil Lab Completed On: November 8 th , 2017 Lab Report Due On: November 22 nd , 2017
Purpose This 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: pK IN =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= A lc , this was done for HIn and In - (Poe, 2017). From these methods, we achieved the necessary values to calculate the pK IN (s) as can 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 pK IN values 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 Method The 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

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