Choice of cuvettes - Use a 1 cm glass cuvette with a round teflon stopper and make sure to fasten the stopper well. It is important that the corrosive iodine vapor does not escape into the spectrometer. As a blank, you need to find another cuvette that is as similar as possible. Make sure there are no finger prints on the cuvettes. Measure the absorbance of the two cuvettes when they are empty to compare the absorbance. The absorbance of the iodine vapor is so small that impurities or difference in the glass would make a significant error.
Operation of the spectrometer - Use wavescan with adjustments set to 1 nm, medium and "with ref." to scan the baseline from 400 nm to 750 nm. The absorbance of the empty sample and reference cuvettes should not exceed 0.002 at 520 nm. If a similar enough cuvettes are not found, then the absorbance in a region outside the range where iodine absorbs needs to be recorded and used to correct the measurements. Use 700 nm for that purpose. Make sure to include the spectrum recorded for the baseline with your report. Report both the bath temperature and your estimated temperature at the sample.
Attach the wheel with the cuvette holders to the hoses from the heat bath. The water from the heat bath gets pumped through the weel to adjust the temperature of the samples. Keep an eye on the hoses. If they come loose, water will flow through the instrument and cause damage. Because of heat flow to the surrounding air, it is likely that the temperature in the sample is lower than the temperature of the bath. Think of a way to estimate the actual temperature at the sample, especially when the bath temperature is high.
The measurement of the absorbance should be carried out at 520 nm (and also 700 nm if needed to correct for differences in the cuvettes) as a function of temperature. Put the blank cuvetted in slot 1, the cuvetted with the iodine in slot 2 and the cuvetted with the thermocouple in slot 5. To set the wavelength of the spectrometer choose "Basic/Absorbance", then "wave", enter the wavelength, and then select "enter". Then press "1" and "Set Ref" to get the background. The screen then shows the measured absorbance in slot 2, the iodine sample.
After the temperature of the heater has been increased (by setting MODE S, entering the temperature, and then selecting "enter"), it is very important to wait long enough until the iodine vapor has reached thermal equilibrium before the absorbance is measured. To monitor the approach to equilibrium, keep an eye on both the absorbance. It is best to measure a new baseline for each temperature before the absorbance is recorded. Check to see if a iodine crystal has formed on the wall of the cuvette that lands in the beam of the spectrometer. If so, it needs to be removed.
To monitor the outcome of the measurements, you should make a Clausius-Claperon graph, ln P vs. 1/T, as you go along, i.e. after recording the absorbance at a new temperature.
The goal is to compare the thermodynamic description of sublimation and the statistical mechanical treatment (which is based in basic results from quantum mechanics). The experiment brings together the three main disciplines of physical chemistry: Thermodynamics, Statistical Mechanics and Quantum Mechanics.
Follow the description of calculations and discussion (except the last paragraph) in NGS. Use Matlab to do the calculations and include a printout of the Matlab program with your report, but you also have to show detailed calculations for one example case in your report.