Michael Berger, Jerry A. Bell,' and Colin Steel Brandeis University Waltham, Massochuretts 02154
Spectroscopic Determination of Thermodynamic Quantities
Measurements of the heats of vaporization are usually included in the first year physical chemistry laboratory. The isoteniscope is most often used because its use is well documented,2 and because the apparatus is simple to use and inexpensive to assemble. However there are several shortcomings to this method. The operation of the isoteniscope is usually described as "quite boring" by most students. Accuracy is determined by that of the pressure indicator, usually a mercury manometer. Hence the f 113 mm readings limit the use of the isoteniscope to liquids with high vapor pressures. Moreover, solids cannot be conveniently investigated and in any case macroscopic quantities of the material being studied are required. Below we descrihe a straightforward method which has been used successfully in our laboratory to determine thermodynamic quantities for milligram or submilligram amounts of solid or liquid materials. Essentially, a small amount of material is encapsulated in an evacuated optical cell. The cell is then enclosed in a controlled temperature oven which in turn is placed in a uv-visible spectrophotometer. Spectra are recorded a t various temperatures. Since the optical density is directly proportional to the concentration in the gas phase, thermodynamic quantities can he evaluated from these data. Apparatus The controllable constant temperature cell holder is shown in Figure 1. This box is compatible with the Perkin-Elmer 323 spectrophotometer. Other instruments, of course, would require boxes of different dimensions. We show the details of our design to indicate the necessary considerations. The box is simple to construct. I t is made of pressed asbestos hoard (for example, Johns-Manville "Transite") painted with a good quality mat black paint. Heating is achieved by coiled 26 gauge Chromel wire with a total resistance of about 9 ohms, supported in arrays a t both ends of the box, and connected to a variable autotransformer. Such an arrangement gave a temperature of 150°C for a voltage setting of 20 V. Two clamps (e.g., broom holder clips do fine) hold the 5-cm reference and sample cells in place. The four apertures of the box are insulated by cementing (epoxy) l-in. quartz squares both on the inside and outside of the box. A thermocouple attached to the sample cell is used to monitor the temperature.
Figure 1. Heated cell holder. Heater ( A ) : sample cell 1s) showing sealoff point IC) and sidearm ID): quartz window (El: thermocouDle (FI:
heater terminals (GI. Sample Preparation Before the sample is placed in the cell, its absorption spectrum should be measured. Knowledge of the extinction coefficients (even from solution data) allows the proper choice of the amount of the sample to add so that 1 Permanent address: ~epazmentofChemistry, Simmons College, Boston, Mass., 02115. ZShoemaker, D. P., and Garland, C. W., "Experiments in Physical C h e m i s t s 2nd Ed., McGraw-Hill, New York, 1967, p.
161.
Heats of Vaporization. Sublimation, and Fusion Compund
-AHuop This Work
naphthalene
11.5 (83-110)
szobenzsne benzophenone
13.6 ('I"11S) 17.0 (51-1091
@cd mole-I)---
Literature 11.8 (87-141). 14.8 (103-293) 15.1 (200-287)~ 14.7 (108-305V
-AHru& This Work
16.9 (22-69) 19.8 ( 3 0 a ) 21.5 (30-46)
(kcal mole-+Literature
--AH,., (keal mole-+This Work Literature
17.2 ( I G M ) ) = . b 17.3 (7-U)lr 17.9 (30-60)d 18.7 (1742)c
4.8
