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Unformatted text preview: partially dissociates to give B–2:
HB– (aq) + H2O (l) H3O+ (aq) + B2– (aq)
Writing the dissociation without water (shorthand notation), we have
HB – ( aq ) H + (aq) + B2- (aq) K= basicform (blue) acidicform (yellow) [H + ][B2- ]
[HB- ] HB– is the acidic form and is yellow in solution. B2– is the basic form and is blue in solution.
Taking logarithms of the above equation gives
log K = log [H + ]+ log log y [B2- ]
[HB- ] [B2- ]
= pH + log K
[HB- ] =mx + b where we have rearranged and noted that pH = –log [H+].
versus pH should yield a straight line with a slope of 1 (m
= 1) and an intercept equal to log K, where K is a concentration equilibrium constant. So our strategy
will be to measure the ratio log
as a function of pH and use this data to determine the equilibrium
constant for the dissociation of bromocresol green. This is a linear equation. A plot of log 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 2 Absorbance and Spectrophotometry
Solutions that possess colors absorb visible light energy of specific wavelengths. Recall that a red
solution appears red because it absorbs much of the blue-green part of the spectrum (complementary
colors). Measurements of the amount of light absorbed by a substance at each wavelength (color) can be
graphed giving an “absorption curve.” The shape of this curve depends almost entirely on the electronic
structure of the substance and is almost unique for each substance. Thus the curve serves as an aid to
identification and, with the aid of modern theory, a clue to the structure of a substance.
At a given wavelength the amount of light absorbed by a solute is proportional to its molar concentration,
thus providing a widely used method of concentration analysis. The Beer-Lambert Law states that A =
εlc, where A = absorbance, ε = a constant characteristic of the absorbing molecule, l = path length, c =
concentration. In our case, ε and l are each constant (known absorbing substance and a path length
determined by the width of the cuvette). Thus the absorbance is proportional to the concentr...
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