Experiment 5. Spectrophotometric Determination of an Equilibrium Constant

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Experiment 5. Spectrophotometric Determination
of an Equilibrium Constant
In this experiment, spectra are obtained for
different starting mixtures of I2 and mesitylene
in n-heptane. The spectra are recorded on a
Shimadzu UV-2101 spectrophotometer (below),
which is equipped with a temperature-controlled
cell compartment. The cell holder accommodates 6
sample cells and the reference cell. Light
traverses these cells right?left take care to
orient them accordingly when you place them in
the cell holder.
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It is important that you come to lab having
already planned how you will prepare the several
mixtures you will study. You will use
volumetric glassware pipettes, graduated
cylinders and volumetric flasks (six 5-mL, one
10-mL, two 25-mL). Make sure you understand
pipetting techniques. For example, the pipette
bulb is not installed on the end of the
pipette, rather is just brought into gentle
contact to achieve an adequate seal. Also,
pipettes are just allowed to drain, are not
blown empty. If no-one in your team is
acquainted with such methods, ask the TA for a
demonstration.
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Cleanliness is extremely important in this
experiment. Oil from finger-prints on the
cuvettes can give measurable absorbance. Also,
the slightest contamination by acetone in the
reaction mixes can ruin the experiment. One
reason we start recording spectra at the
long-wavelength limit of 800 nm is to check
against such effects. The absorbance in this
region should be very slight (lt 0.01 au), so if
at any time your spectra show greater absorbance
than this for l ? 800 nm, check with the
instructor. At the end of the period, make sure
that you put all remaining reagents in the waste
bottle provided. All glassware must be left
spotlessly clean and dry for the next team. (Wet
with acetone wont do, for reasons mentioned
above.)