© Ultrafast Systems LLC /Vernier Software & Technology
29
Figure 13:
Kinetic profile of Congo red in 20% water in ethanol
In Figure 14, you can see that the flash at t = 0 causes a vertical (on these time scales) drop in
the mV reading; it drops from ~1.275 V prior to the flash to ~1.245 V after. Subsequently, the value
increases over hundreds of milliseconds. The flash induces the changes outlined earlier, and very quickly,
at shorter times than software can follow, the ground state of the cis-form is generated. The ground state
of the cis-form absorbs light passing through the 600 nm filter, and the detector registers a drop in
transmission. Then, the cis-form returns to the trans-form over many milliseconds, and it is the rate of this
process that you are required to extract from the data set.
Repeat data collection with the averages set to 10 (or more) and save the resulting data.
Do the same procedure for the other samples you have prepared, and save their averaged data
sets. As you go through the series, you see that the recovery rate increases; to account for this, you will
need to adjust the time window setting and the time scale in order to obtain curves that use as much of
the time window as possible.
The samples that you have examined have been stored as comma-separated values (CSV) files.
Import the data into Logger
Pro
to evaluate the rate constants for the cis-trans conversion of CR as a
function of [OH
-
]. You should find that the decays follow an exponential rate law with a constant that is a
linear function of [OH
-
] and the slope of the line (or the best fit of the regression) is the bimolecular rate
constant for the catalytic process.
