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A spectrophotometer is an instrument used for measuring the amount of absorption at different
wavelengths. Many such instruments are now commercially available. Some are designed to operate in
the region of visible light, some in infra-red regions, some in ultra-violet and still others in several of
these energy bands. You will use either a Spectronic 301 Spectrophotometer or a Genesys 20
Spectrophotometer to measured absorbance in this experiment. Both instruments are designed for use in
the visible portion of the spectrum (400 to 700 nanometers, nm).
Determining the Equilibrium Constant
If the acidic form (HB–) and basic
form (B2–) of an indicator both
absorb light, then the ratio
can be determined by measuring
the absorption of light at the
wavelength at which one of the
forms absorbs a lot and the other
form absorbs a little.
Plots of the absorption of light of
various wavelengths (λ) versus
wavelength for bromocresol
green in its basic form and in its
acidic form are shown in the chart
to the right. Such graphs are
called absorption spectra. λmax: Wavelength of maximum absorbance for base form The graph shows that the basic form has maximum absorption (AII) at λmax. The absorbance of the acidic
form (AI) is small at λmax (almost zero).
If we start with a solution of the pure basic form (B2–) and add an acid to the solution (for example, acetic
acid), then some of the basic form will be converted to the acid form (HB–). Then the absorbance at λmax
will drop because the acidic form absorbs almost no light at λmax. This change may be visible in that the
solution will change from blue to yellow.
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 3 We will call an absorbance measurement with a mixture of the acidic and basic forms of the indicator Ax.
In fact, all absorbance values along the vertical line drawn at λmax in the graph are Ax values. These points
all lie between AII and AI.
Referring back to the equation, log [B2- ]
= pH + log K , we see that in order to calculate the equilibrium
[HB- ] [...
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