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Unformatted text preview: B2- ]
must be found for different pH values. When these are plotted, a
straight line with slope of one and y-intercept of log K is obtained. The relationship between the
absorbance data at λmax and the ratio
is found from the following equation:
[HB- ] constant, K, different ratios of [B2- ] A x -A I
[HB- ] A II -A x AI = absorbance of the solution when all indicator is in the acidic form (pure HIn).
AII = absorbance of the solution when all indicator is in the basic form (pure In–).
Ax = absorbance of the solution when both some acidic and some basic form are present 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 4 Procedure
Equipment: UV-vis spectrophotometer, two cuvettes, pipette pump, three 250-mL beakers, 100-mL
graduated cylinder, 2-mL pipet, 10-mL graduated cylinder, Kimwipes, USB flash drive. Chemicals: 1.00
M acetic acid (HAc), 3.00 x 10-4 M bromocresol green (BCG), 0.200 M sodium acetate (NaAc), cuvette
containing acidic form of BCG obtained from your instructor.
Use eye protection for this experiment.
Students will work in groups as assigned by the instructor.
Part 1: Absorbance Spectrum of Bromocresol Green in Basic and Acidic Solutions
1. We will need four solutions to perform this experiment. The instructions for making the first three are
given in the text below. The fourth solution has already been prepared and will be distributed by your
instructor in a cuvette. All solutions should be prepared in a graduated cylinder since volume markings
on beakers are good to only 5%.
A. Bromocresol green in sodium acetate solution: Add 5.00 ml of 3 x 10–4 M bromocresol green
solution and 5.00 mL of 0.200 M sodium acetate (NaAc) solution to a 100-mL graduated cylinder. Dilute
to 100.0 mL with deionized water and pour quantitatively (this means all of the solution) into a clean, dry
250-mL beaker. This solution should be blue and will be referred to as the basic solution. The sodium
acetate concentration in this solution is 0.0100 M.
B. Acetic acid solution: Rinse the graduated cylinder with distilled water, add 25.0 mL of the 1.00 M...
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