Unformatted text preview: acetic acid (HAc) solution and dilute to 100.0 mL with deionized water. Pour this solution quantitatively
into a clean, dry 250-mL beaker. The concentration of the acetic acid in this solution is 0.250 M.
C. Blank Solution: Add 5 mL of 1.00 M acetic acid and 5 mL of 0.200 M sodium acetate to a 100 mL
graduated cylinder and dilute with deionize water to 100 mL.
D. Bromocresol green solution in hydrochloric acid: The instructor will supply a cuvette containing a
BCG solution that has excess acid (HCl) added to it. This solution should be yellow.
A Note About Measuring Absorbance Using the Spectrophotometer and Use of the Blank Solution.
Spectrophotometry measurements generally require that the absorbance of a blank solution be measured
in order to counteract any absorbance or scattering of light from air, the container, solvents, and solutes.
Once a “blank” has been performed, the instrument compares these signal losses from measurements of
the signal coming from the sample. So a “blank” should always be performed prior to making a
measurement at a particular wavelength. Likewise, when the wavelength setting of a spectrophotometer
is changed (for example, from 400 nm to 420 nm) the amount of light reaching the detector can change.
For that reason, it is necessary to perform a blank measurement again. If the wavelength setting isn’t
changed then a blank measurement isn’t usually needed. However, optical and electronic properties of
the spectrophotometer can “drift” over time. So if the instrument has been sitting for more than a few
minutes, it’s a good idea to re-perform a blank, even if the wavelength hasn’t been changed. When in
doubt, re-perform a blank measurement.
1. Measure the absorption spectrum of the solution containing BCG in sodium acetate (basic BCG
solution) and the BCG solution containing HCl (acidic BCG solution) beginning at 380 nm and ending at
680 nm at 20 nm intervals. Record these measurements in the Data Sheet. Department of Physical Sciences Kingsborough Community College The City University of New York Spring 2012
Experiment 5: Determination of the Equilibrium Constant for Bromocresol Green 2. Review the absorbance data and locate the greatest value of the absorbance fo...
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