VOLUME 33, NO. 12, DECEMBER, 1956
HEATING DEVICES FOR MICROCHEMICAL OPERATIONS JOHN T. STOCK and M. A. FILL Norwood Technical College, London, England
EXCLUDING organic ultimate analysis, a tiny flame usually provides ample heat for small-scale chemical operations. Although satisfactory when the duration is short, such direct heating needs careful adjustment and ample flame shielding when used for prolonged operations such as refluxing or fractional distillation. A liquid bath is often useful, but unless operating a t the boiling point the temperature needs t o be watched and, after heating in this manner, the exterior of the reaction vessel has to be dried. Two gas-heated devices incorporating sensitive and stable control have been develo~ed. Temverature stabilitv is achieved in the one case by carefh shielding and in the other by use of a comparatively large flame. The device shown in Figure 1 provides the nonlocalized heating characteristic of a liquid bath and incorporates certain features of earlier designs ( 1 4 ) . Heat is supplied by microburner A , which is merely a Bunsen burner with the flame tube removed. Chimney B is of tin plate or other thin sheet metal; particularly if reinforced with a few small rivets or bolts, the seam and bottom plate joint may be soft soldered. On assembly, the bottom plate is gripped between the burner and its base, thus rigidly snpporting the chimney. Air enters through the eight 3/la-in. diameter holes in the bottom plate, through lighting port C, and around the gas-connection opening. The butterfly valve E is mounted a little above the three rows of 3/16-in.diameter vent holes D which provide an alternative outlet for the hot gases. As rotation of the valve progressively closes the chimney the hot gases escape increasingly through the vent holes, so that the heating effect a t the top of the chimney is reduced. Rapid and sensitive heat is Over a range established by flame height.
The butterfly valve is 0.5-1 mm. smaller in diameter than the bore of the chimney and is mounted on a a/~a-in. diameter spindle F. Consisting of two thin
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JOURNAL OF CHEMICAL EDUCATION
sheet-metal discs between which are sandwiched two discs of asbestos paper and two half-discs of asbestos hoard, the valve has a total thickness of about 5 mm. Details of construction are shown enlarged a t (a) and
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A small flask or similar apparatus may be supported as shown in the detachable spring clamp G. This clamp is a "Terry clip" which is secured between nuts on the end of a length of 1/8-diameter rod, the latter being bent a t right angles as shown. Two telephonetype binding posts H, mounted vertically above one another with their cross holes in line, allow the clamp to be locked in any position. When the heater is out of use, the clamp is lowered so that the horizontal portion of the rod enters slot I in the top of the chimney. To prevent accidental fallIof apparatus into the chimney:three equally-spa~ed?/~-in. long bolts J are ar-
ranged to project inward about 25 mm. from the top of the chimney. A useful accessory for warming crystallizing tubes, etc., is a wire-gauze tray which clips onto the top of the chimney (3). Constructed mainly from the parts of a Bunsen burner and an old screw clamp, the miniature hot plate shown in Figure 2 is designed to operate with a fixed flame height. The heating of a vessel is controlled by varying the clearance between the hot plate and the vessel. After the stem of the clamp and most of the fixed jaw is sawed off, the latter is drilled and secured to the burner base by bolts K, as shown. The projecting end of the operating screw should pass freely through the central hole in the base. The moving jaw is also shortened considerably, then drilled and filed flat at the end to permit the firm attachment of burner support L and hot-plate surround M. The former, of 5/8 X '/la-in. thick brass strip, is bent as shown and carries burner N. Of approximate dimensions 3 X 2 X in., the surround is of asbestos cement sheeting. It has a central 30-mm. diameter hole, the upper side being counterbored to take an aluminum disc 0 about 33 mm. in diameter and 1.5 mm. thick. The distance between the underside of the disc and the burner jet should be about 15 mm. If set a t a "free" height of 50 mm., the flame spreads over the underside of the disc and is unaffected by normal draughts. Other versions of this apparatus are line-operated, a 60-watt soldering-iron heating element being used (4). .. LITERATURE CITED
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(1) S ~ C K J. ,T.,A N D M. A. FILL, J. CHEM. EDUC., 31,144(1954). ( 2 ) STOCK J. T.,A N D M. A. FILL,Cha. Age (London), 71, 889
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(1954). (3) Ibid., 7 2 , 847 (1955). (4) STOCKJ. T.,M.A. FILL, Laboratory Practice, 4,501 (1955).
