In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Low-Cost Temperature Control from 160 K to Ambient Temperature Using Liquid Nitrogen Evaporation
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Gustavo S. Faraudo† and Daniel E. Weibel*† INFIQC, Departamento de Físico Química, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000, Córdoba, Argentina; *
[email protected] The need for constant temperature regulation can be found in a variety of analytical, physical, biological, and organic chemistry experiments. In general, thermal control above room temperature is made by heating a liquid or a metal block via passing current through a resistance circuit controlling a heater (1–2). However, the thermal control below room temperature uses different approaches, ranging from the traditional commercial or lab-designed flow systems (3), in which fresh liquid from a reservoir flows through a cell housing, to the flow of precooled nitrogen gas though a block housing. In the latter systems, there are designs pulling liquid nitrogen through a coil (4); designs supplying cooled nitrogen gas by boiling liquid nitrogen and recooling it in a coil immersed in liquid nitrogen (5); and a combination of a flowing cooled nitrogen gas with cartridge heaters
Design of the Apparatus and Experiments The cooling device was used and optimized for two kinds of systems:
F
V
N
I
T
−
+
P L
R
Figure 1. Schematic diagram of the low-temperature photochemical reactor: (F) cold gas flow, (I) illumination region, (L) liquid nitrogen, (N) dry nitrogen chamber to avoid frosting, (P) variable power supply, (R) resistor, (T) thermocouple, and (V) vacuum. † Current address: Instituto de Química da U.F.R.J., Departamento de Físico-Química, Cidade Universitaria, Predio do CT-Bloco A, 21949-900, Rio de Janeiro, Brazil.
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to control the heating rate (6). A simple modification of these last designs directly uses the cold gas from boiling liquid nitrogen. The regulation of the current through a resistor can be done with very simple and inexpensive electronic circuits with high precision. We describe a simple and direct method for varying the temperature between 110 and 283 K that keeps the flow system without the necessity of precooling the evaporated gas from a liquid reservoir. This device can be easily used for a variety of undergraduate physical chemistry experiments that need low temperature regulation. Owing to the simple design and wide range of controlled temperatures, it is especially suitable for experiments involving gas–surface interactions, adsorption–desorption processes, heterogeneous atmospheric chemistry, thermodynamics of gases, among others.
a) A low-temperature, glass photochemical reactor. A schematic diagram of the reactor is shown in Figure 1. It consists of a cylindrically shaped (340 cm3), custom-built reactor made of glass, which has a double wall through which a stream of cold nitrogen can flow.1 This flow cools the surface to be irradiated. b) A conventional cell housing for 1-cm optical-path cuvets. In this system, 3-mm diameter cylindrical channels are made in an aluminum block to allow a stream of nitrogen gas to pass through them.
The surface to be irradiated in the photochemical reactor or the cell housing was cooled by a flow of nitrogen gas evaporated directly from a liquid nitrogen reservoir. It was often necessary to use a prechamber filled with dry nitrogen gas or a slow stream of dry nitrogen gas flowing on the irradiation window to prevent frosting or condensation of water, carbon dioxide, et cetera. The liquid nitrogen was contained in a standard commercial 1- or 3-L cryogenic Dewar. The selected temperature was achieved by controlling the current through an electrical resistor immersed in liquid nitrogen. The gas evaporated from the bottom of the liquid reservoir was used as the source of cold gas flow. The temperature regulation consisted of a proportional control of the temperature, which automatically adjusted the potential applied to the resistor according to the difference between the operator-selected temperature (reference signal) and the temperature of the reactor. Detailed control circuits similar to that used here can be found in the reference section (1–2,
Journal of Chemical Education • Vol. 80 No. 6 June 2003 • JChemEd.chem.wisc.edu
In the Laboratory
8–9). In addition, the above signal difference was compared with the output signal from a potential wave generator, which continually fixed the application of power to the resistor. This procedure allowed a rapid decrease of temperature from 298 K to the desired temperature, obtaining the stabilized temperature within a few minutes. It is worth mentioning that the present design can be operated with only a common 0– 25 V variable power supply by selecting a resistance suitable to achieve the desired final temperature. The power used to boil the liquid nitrogen was between 0 and 100 W, and the thermocouple used was an iron–constantan type referenced to the melting point of ice. With the described experimental setup, the temperature was varied between 160 and 285 K with a stability of about ± 1.1 K or smaller. The stability in the temperature control (Table 1) was measured for the photochemical reactor, which would represent the maximum expected change in temperature primarily as a result of the large cold area (geometric area ~200 cm2) to be maintained at constant temperature. The minimum temperature reached in the photochemical reactor was 110 K. The maximum consumption of liquid nitrogen measured in the photochemical reactor was less than 0.60 liters per hour
Table 1. Temperature Stability Measured in the Photochemical Reactor T/K
∆T
230
± 0.5
224
± 0.5
203
± 0.9
189
± 1.0
180
± 1.1
173
± 1.1
160
± 1.1
at temperatures lower than 160 K and about 0.4 liters per hour between 160 and 190 K. The consumption decreased considerably above 200 K; for example, at 240 K it was less than 0.2 liters per hour. A linear dependence of the temperature with a flow of cold nitrogen gas was measured between 285 and 240 K for the cell housing for 1-cm optical-path cuvets (Figure 2). This linear dependence could also be observed, approximately, at temperatures lower than 240 K. Finally, we would like to mention that a system similar to that shown in Figure 1 was used successfully by final-year research students of physical chemistry at this university to study the dependence of the vapor pressure on the temperature of several important stratospheric gases. The pressures of O3, Cl2, and HCl were measured between 273 and 120 K. Conclusions The temperature controller described provides a low-cost improvement over previous designs. It can be easily adapted to different cell housings maintaining the utility of a flow system. The flow of cold nitrogen gas evaporated from a boiling reservoir of liquid nitrogen can be used to regulate the temperature in gas, liquid, or solid samples at atmospheric pressures and in vacuum environments, allowing both the student and the professor to set up several physical chemistry experiments at low cost. Hazards Nitrogen is an inert substance with a boiling point of ᎑195.8 ⬚C (77.35 K). It is colorless, odorless, nontoxic, and nonflammable. The cold gas can be more dense than air, leading to a tendency for nitrogen accumulation to occur in confined spaces, particularly at or below ground level. Although relatively safe in terms of toxicity and risk of explosion, liquid nitrogen, in common with all cryogens, presents its own specific hazards. These are: cold burns, frostbite, and hypothermia from the intense cold; over pressurization from the large volume expansion of the liquid as it evaporates; fire from condensation of oxygen creating locally oxygen enriched atmospheres; and asphyxiation in oxygen deficient atmospheres.
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Acknowledgments Temperature / K
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The authors thank the Consejo de Investigaciones Científicas y Tecnológicas de la Provincia de Córdoba (CONICOR) and the Secretaría de Ciencia y Técnica de la Universidad Nacional de Córdoba (SECyT-UNC) for financial support. We also thank Esteban Mondino for the construction of the automatic temperature controller.
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Material
Notes for the instructor are available in this issue of JCE Online.
240
230 4
6
8
10
12
14
Nitrogen Gas Flow / (L/min) Figure 2. Flow of nitrogen evaporated from boiling liquid nitrogen as a function of the temperature in the cell housing for 1-cm optical-path cuvets.
Note 1. A similar system has been successfully used to study the heterogeneous chemistry and photochemistry of stratospheric reservoir species in relation to the ozone depletion problem in the Antarctica (7). ClONO2 adsorbed on ice and HCl-doped ice crystals were studied at 181 and 190 K.
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In the Laboratory
Literature Cited 1. Sheulin, C. G.; Coppersmith, W.; Fish, C.; Block, S.; Vellema, W. J. Chem. Educ. 1997, 74, 958–960. 2. Mercer, G. D. J. Chem. Educ. 1992, 69, 568–569. 3. Flash, P. J. Chem. Educ. 1994, 71, A66. 4. Wood, T. A. J. Chem. Educ. 1967, 44, 423. 5. Wardman, P. J. Phys. E: Sci. Instrum. 1972, 5, 17.
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6. Gorman, A. A.; Hamblet, I.; Lambert, C.; Spencer, B.; Standen; M. C. J. Am. Chem. Soc. 1988, 110, 8053–8059. 7. Faraudo, G.; Weibel, D. E. Progress in Reaction Kinetics and Mechanism 2001, 26, 179–199. 8. Alzabet, H. R.; Barbero, J. A. J. Chem. Educ. 1987, 64, 380– 381. 9. Badger, R. C. J. Chem. Educ. 1978, 11, 747–748.
Journal of Chemical Education • Vol. 80 No. 6 June 2003 • JChemEd.chem.wisc.edu
