© Ultrafast Systems LLC /Vernier Software & Technology
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Procedure
Obtain and wear goggles. Prepare 100 mL of 20% water in ethanol using de-ionized or distilled
water. It may be convenient to use 190 proof ethanol, which is 5% in water, and add the requisite volume
of water to bring it to 20%. Note that the exact percentage of water is not important, but the water must be
pure. Add small quantities of solid CR to the water-alcohol mixture until the color is a light red. Again the
amount of dye is not important; you need to have just enough to generate a quantity of the cis-
photoproduct to provide an absorbance that has a good signal-to-noise ratio in the Vernier Flash
Photolysis Spectrometer. A useful check is to transfer some of the solution to the 10 mm x 10 mm cuvette
and place it in the sample position with the 600 nm band pass filter inserted in front of the detector. If the
I
0
reading on the lower-left of the Vernier Flash Photolysis Spectrometer Software is about 20% lower
than the value without the cuvette in the sample position (or with an empty cuvette), the solution is
adequate for the purpose.
Remove 50 mL of the prepared dye solution and add sufficient 0.1 M NaOH solution in water to
bring it to 2 mM in NaOH (the amount of water you add here is not significant in the total).
Now you have two equivalent solutions of dye in the solvent mixture, one of which is 2 mM in OH
-
ions.
By mixing appropriate amounts of the two solutions, prepare 5 mL amounts that range from 0 to
0.1 mM in OH
-
ions. These solutions all contain the same CR concentration in the same water/ethanol
mixture.
Starting with the 0 mM sample, transfer about 3 mL to a clean, dry cuvette. Place the cuvette in
the sample position and confirm that Averages is set to 1. Then, click the Start button. You should see a
time profile similar to that in Figure 10, where the y-axis is Amplitude in mV and the x-axis is in ms.
