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 rightleft. Take care to orient the cells accordingly when you place them in the 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 (< 0.01 au), so if at any time your spectra show greater absorbance than this for 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 won’t do, for reasons mentioned above.)