An Inexpensive Rheostat-Controlled Oil Bath Most undergraduate organic chemistry laboratories still make use of the micro-burner t o perform distillations and reactions which require reflux. Fires resulting from ignition of extremely flammable solvents, such as benzene and toluene, are consequently a rather common occurrencein these laboratories. The use of heating mantles and oil baths in conjunction with variahle transformers, such as Variaes, have alleviated this "burning" problem for those laboratories with rather large budgets. A variable transformer with an accompanying set of heating mantles easily approaches the $100 mark in cost and, thus, is not a feasible alternative for many smaller schools. We have designed, built, and used successfully a rheostat-controlled oil bath which is shown in the accompanying photograph (Fig. 1).It can he constructed of readily available materials a t a cost of less than $15.
Figure 1. The assembled apparatus.
Figure 2. Oil bath time-temperature curves at various settings.
The oil bath consist? of a Pyrex crystallizing dish (125 X 65 mm) containing a coil of Chromel A resistance wire around the bottom. A 30 ft. length of wire ( B & S size: 24.0201) is tightly wound around a $-insteel rod to obtain a coil to fit the dish. The total resistance of the coil is aooraximatelv 50 n. The terminals of the coil are anchored in a l'h x l-in. niece of gasket material which is bolted t o a screw clamp. Regular extension cord wire fitted with banana plugs is attached to the terminals which are covered with Neoprene glue for insulation purposes. The rheostat is a600-W (120-V) single pole light dimmer control mounted with bolts inside aplasticfood storage boa (5 x 3.5 x 2.75 in.). Holes for the rheostat, banana outlets, and the 120-V line cord are easily formed in the box with hot glass tubing of the appropriate diameter..A fuse connector available from automotive stores is connected in series and is fitted with a 4-A fuse. An arbitrary scale can be drawn on adhesive labeling tape and attached t o the upper side of the rheostat. Calibration of the scale is a rather simple matter after oil has been added to the bath so that it is no more than two-thirds full. Figure 2 illustrates typical time-temperature curves obtained with the bath s t various settings on the arbitrary scale which ranged from 7R.nnitc ....... 0. to .. -. - ... -. We use heavy mineral oil in ourbaths,andany tempraturefmmambienr to 2W°C is available with thissystem. However, ronsiderrhle derumporition of the oil occurs at temperatures of 170-200°C, and flashing may occur if the temperature of mrntral oil is allowed to exceed spprorimntely 22U°C. Silicone oil which is considerably more expensive shudd be used a t these high temperatures.
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Pikeville College Pikeville. Kentucky 41501
774 / Journal of Chemical Education
L.J. Darlage S. J. King
