An EFFICIENT, INEXPENSIVE LEONARD C. KREIDER
HOT PLATE
The Ohio State University, Columbus, Ohio
Directias, with diagram, are given for the construction of a simple hot plate that can be built from readily available materials at a cost of approximately f i f y cents. Its sole source of heat i s a 100-mtt electric lamp. I t furnishes a safe, convenient, and cheap source of heat where tempera-
tures from 30' to 70°C.are required. It heats rapiddy and i s capable of maintaining a firedelermined temperature within reasonably narrow limits i n the above range. A useful air heating attachment i s also described. Brief directions for some of its special uses are included.
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cles. The free flame as a heat source is difficult to regulate to a desired temperature, is subject to drafts, presents a certain fire hazard if left unattended, and is practically out of the question if low-boiling organic
HE CHEMIST is often confronted with the need of a steady source of heat at temperatures between thirty and eighty degrees Centigrade. The conventional ways of obtaining this are beset with obsta-
solvents are to be heated in open vessels. When used with the air bath or the water bath, most of the undesirable features of the free flame are diminished but they are not entirely removed. It is often suggested that the use of the steam bath is more satisfactory, but in practice it is difficult to regulate it even approximately to any temperature below 90°C. and be certain that it will remain constant. Then, too, many laboratories are not supplied with steam lines, and their installation and operation is expensive. The ordinary three-heat hot plate is quite satisfactory, but the initial cost is five dollars or more and it costs about five cents an hour to operate one. The author believes that the hot plate described in this paper eliminates all the disadvantages mentioned above, and that, in addition, it presents positive advantages as a convenient, safe, and economical source of low-temperature heat. Its construction is simple. It can be made from new materials for less than fifty cents, including the cost of the light bulb, and can be operated at the low cost of about one cent an hour, as the sole source of heat is a 100-watt lamp. For the construction of this hot plate an ordinary porcelain light socket is mounted on the center of a square piece of wood (15 X 15 X 2 cm.). A convenient length of electric cord (5 to 10 feet) is then attached to make connection with the power line. A tin can of suitable size (10 cm. in diameter and 12 cm. high.is satisfactory) is next selected, the top is removed, and a hole 4 cm. in diameter is cut in the center of the bottom. By means of three heavy tin strips (2 X 9 cm.) soldered to the sides of the can near the base a tripod is formed that will hold the can rigidly in place over the center of the light socket with the bottom of the can 6 cm. above the wooden base. A flat piece of heavy wire gauze, with a diameter 2 cm. greater than that of the top of the can, has radial slits cut to a depth of 1cm. from the outside edge a t short intervals around the circumference. The flaps thus formed are then bent, a t right angles to fit snugly over the outside of the open top of the can. To conserve heat, the outside of the can is lbrapped with asbestos paper which is fastened in place by binding with copper wire. The bright inside walls of the can serve to reflect and intensify the heat furnished by the light bulb. Because of the hole in the bottom, the can acts much like a chimney, and the major part of the heat is made available a t the top of the can. The rapidity with which this hot plate heats is demonstrated by the fact that 50 cc. of ether in a beaker can be brought from a room temperature of 20' to the boilmg point in 11/2minutes, a like amount of acetone in 4 minutes, and ethyl alcohol in 8 minutes. Water, because of its high beat of vaporization, cannot be boiled on this hot plate. However, a large beaker half full of water can be maintained a t a temperature of 65-70'. which suggests the use of the hot plate for digesting precipitates in quantitative analysis. If a lower temperature is desired, it may readily be obtained bv ~ l a c i n zthe vessel to be heated on a riuz-stand a t somi hei& above the top of the hot plate. For ex-
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ample, a water bath half filled with water can be maintained a t 40° if placed 5 cm. above the top of the hot plate. This temperature is very desirable when removing solvents from organic preparations in wacuo. A very convenient attachment for this hot plate is also pictured in the diagram. It is essentially a device for heating air and is very useful where compressed air is available. It consists of three turns of 6-mm. copper tubing coiled around the light bulb as in the diagram. In use, the compressed air should b6 dried of moisture and traces of oil by bubbling it through concentrated HzSO, before leading it through the heating coil. This hot, dry air, when gently blown on the surface of a solution, will cause very rapid evaporation. If the solution to be evaporated is inorganic, the containing vessel may be placed directly on the hot plate and the hot air played on the solution. However, if a solution containing organic matter is to be evaporated, it is safer to set the containing vessel beside the hot plate and use only the current of heated air, or charring may result when nearly all the solvent has been removed. This convenient source of hot, dry air is also useful when it is necessary to dry small pieces of glassware completely in a short time. Other uses for the hot plate will suggest themselves to the observant worker. For example, it is a safe source of heat for the distillation of small portions of ether, methyl or ethyl alcohol, acetone, or chloroform. It could also be used to reflux solutions of these solvents, or as the heat source for a continuous extraction in a Soxhlet apparatus. It will furnish a source of safe heat to speed the solution of a substance in flammable organic solvents. The hot plate may also be used as a spotlight to illuminate a darker corner of the laboratory when exact observations must be made. such as the determination of a melting point.
