Chemical Education Today
Letters Heat and Temperature Conductivity of Baths Joling et al. described a convenient microscale heater in their article, “A Low-Cost and Timesaving Microscale Heater” (1). Having had the same problem several years ago, initially I adopted substantially the same solution they proposed. But soon it became apparent that the generic “sand” bath did not provide a satisfactory thermal contact in terms of both heat transfer and temperature uniformity and control. In the present case, heat transfer is quantified by thermal conductivity λ; temperature uniformity is quantified by thermal diffusivity a = λ兾(ρ CP), where ρ is the density and CP is the constant pressure specific heat capacity. λ has SI units of W m–1 K–1; a has SI units of m2 s–1 (2). My sand was slightly impure calcium carbonate (calcite); quartz sand and alumina powder were excluded from consideration because they can scratch the container to be heated, generally made of borosilicate glass. Powdered graphite (100 mesh) was better (3), but fine scales remained attached to the container. The most satisfactory powder baths were aluminum granules 10–60 mesh or 100 mesh chromium powder (both from Aldrich). Thermal conductivity and thermal diffusivity of powders depend on the intrinsic properties of the material1 but also on the average size and shape of the granules (4). Silicone oil is much better due to the absence of voids and possibility of convection, but a layer remains adherent to the reactor, thermometer, and thermoprobe, and must be removed and discarded after each use. At present I use silicone oil when fine-temperature control in time and position is important. I use aluminum or chromium powder in the remaining cases. Note 1. Thermal conductivity and thermal diffusivity of various materials: a兾(m2 s–1) Solid Material λ兾(W m–1 K–1) Aluminum 237 1.0 × 10–4 Chromium 0.937 2.9 × 10–5 Graphite 138 1.2 × 10–4 Calcite (calcium carbonate) 5.0 (average) 2.2 × 10–6 Data from Lide, D. R., Ed. Handbook of Chemistry and Physics, 78th ed.; CRC Press: Boca Raton, 1997, section 12; Perry, R. H.;
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Chilton, C. H. Chemical Engineers’ Handbook, 5th ed.; McGrawHill: New York, 1973, sections 3 and 23; http://webbook.nist.gov/ (accessed Dec 2004).
Literature Cited 1. Joling, E.; Goedhart, M. J.; van den Berg, B.; van der Spek, T. M. J. Chem. Educ. 2002, 79, 1109. 2. Mills, I.; Cvitas, T.; Homann, K.; Kallay, N.; Kuchitsu, K. Quantities, Units, and Symbols in Physical Chemistry, 2nd ed; Blackwell Scientific Publications: Oxford, 1993, p 65. 3. Particle Size Conversion Table, Aldrich 2000–2001 Catalog, T810. 4. Loncin, M., Les opérations unitaires du génie chimique; Dunod: Paris, 1961, p 631, 641. Tiny aluminum space-filling polyhedra of symmetry as high as possible are needed to reduce the probability of formation of voids: cubes, truncated octahedra, etc. would be convenient (see for instance http://mathworld. wolfram.com/Space-FillingPolyhedron.html, accessed Dec 2004); lead shot, being spherical, is not. Bruno Lunelli Dipartimento di Chimica “G. Ciamician” 2 via F. Selmi, I-40126 Bologna, Italy
[email protected] The author replies: I appreciate the suggestions made by Bruno Lunelli to improve the usefulness of the microscale heater we described. However, for use in secondary schools sand works fine. In spite of the better properties I would not recommend the use of silicone oil with relatively inexperienced 16-year-old pupils since it can be a bit messy. But that is an argument of classroom management, not of chemistry. Erik Joling AMSTEL Institute Universiteit van Amsterdam Kruislaan 404, NL-1098 SM Amsterdam The Netherlands
[email protected] Vol. 82 No. 3 March 2005
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