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Unformatted text preview: r the basic solution.
Note what wavelength this absorbance corresponds to. Now measure the absorbance of the basic BCG
solution and the acidic BCG solution at 5 nm intervals from 20 nm below to 20 nm above this
wavelength. For example, if the wavelength of maximum absorbance occurs at 520 nm, make
measurements at 5 nm intervals from 500 nm to 540 nm. However, don’t duplicate readings—in this case
don’t take readings at 500 nm, 520 nm, and 540 nm.
3. Pour the BCG in sodium acetate solution in the cuvette back into the 250-mL beaker, taking care not to
lose any of the solution.
4. Return the cuvette containing the acidic form of BCG to your instructor.
5. From the data taken for the basic and acidic forms, decide which wavelength is suitable for measuring
Ax values and set the spectrophotometer at this wavelength. The suitable wavelength, λmax, is the
wavelength in which the basic form has a maximum absorbance and the acidic form has a very small
Part 2: Effect of pH on Absorbance of Bromocresol Green
1. Add precisely 2.00 mL of the 0.250 M acetic acid solution to the 250-mL beaker of BCG solution
using a 2 mL pipet and a pipette pump. Mix the solution with a glass stirring rod and measure the
absorbance at λmax. Pour the solution in the cuvette back into the 250-mL beaker. Don’t throw the
solution in the cuvette out.
2. Add another 2.00 mL of the 0.250 M acetic acid solution, mix well, and measure the absorbance at
λmax. Again, pour the solution in the cuvette back into the beaker.
3. Repeat this procedure with a third and fourth 2.00 mL increment of the 0.250 M acetic acid solution.
There should now be a total of 8.00 mL of 0.250 M acetic acid solution added to the BCG solution.
4. Rinse all solutions down the drain and clean any glassware used in the experiment before performing
any calculations. Department of Physical Sciences Kingsborough Community College The City University of New York Spring 2012
Experiment 6: Determination of the Equilibrium Constant for Bromocresol Green 6 Data Sheet
Name Partner Partner Absorbance Data at 20 nm Intervals
form) Absorbance Data at 5 nm Intervals
(basic form) Volum...
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