A Heater for Constant-Pressure Calorimetry The design of the heaters used in calorimetry depends to a certain extent on the reaction under study. For the measurement of heats of neutralization a simple heater consisting of bare nichrome wire wrapped around a Lucite wafer is often adequate. In this experiment capper leads can be used. On the ather hand, a more complex design is needed for the study of the heat of solution of potassium nitrate' since copper immersed in the solution corrodes quickly when voltage is applied to the heater. In this case as in many others it is desirable to insulate electrically the entire heater from the solution while retaining a fast response time. Daniels e t al.' recommend the use of metal-sheathed heater wire (with MgO insulation), hut the hich cost of the material alonp with the overall complexity . . of the design have led us to investieate . other approaches. Inexpensive 24-gauge bare nichrome wire was used. A 6-ft length was threaded through 5 ft of PVC heat-shrinkable t ~ b i n gDifferent .~ lengths (and for that matter different-gauge wire) may be used depending on the desired resistance. The extra wire extending past the insulation (6 in. on either side) was threaded through stainless hypodermic tubing? the outside diameter of which in.) is small enough to allow far the heat-shrink tubing to be slipped tightly over it a distance of 4 2 in. The hypodermic tuhing is now crimped in two or three places to insure solid contact with the wire. A larger diameter piece of heat-shrink tubing is now slipped over the hypodermic-wire junction and clamped a t either end using copper wire to insure liquid-tightness. Next, the heater is wound bifilarly around a suitable-size mandrel and the hypodermic posts fed through a rubber stopper or a Styrofoam plug to he used as the calorimeter cover. One disadvantage of this design is that the heater lacks rigidity, hut this can he remedied by using a third, insulating post (such as a Lucite rod) to support it. We have also found that two or three strips of 20-gauge copper wire can be corrugated (using cone-nose pliers) and pinched around each heater loop for additional support if necessary. Finally, a heat gun is used to collapse the heat-shrink tuhing around the heater wire (care should be exercised not to overheat the plastic) in order to provide better contact with the solution. For the potassium nitrate experiment we have chosen to use a very simple calorimeter design consisting of e 4 W m L beaker placed in a truncated Styrofoam jug. This simple arrangement allows for magnetic rather than mechanical stirring. Digital multimeters were used to measure the heater current and voltage, as well as the thermistor resistance. A digital timer, triggered manually simultaneously with the switching of the dc power supply was also used.
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Daniels, F.; Williams, J. W.; Bender, P.; Alherty, R.A.; Cornwell, C. D. Experimental Physical Chemistry; McGrawHill: New York. 1962. ? T h e tuhing we used has a '1,s-in. inside diameter and ir supplied by AIN Plastics. Inr.. 245 East Sendford Blvd., P.O. Box 151. MI. Vernon, h'Y 10550. Unfortunarely. the current minimum order is 2000 ft at a cost of $157. The company. howev~r.nlsosellrpolyolefin tubing wirh essentially thesamedimensionsat acost uf$26fur 1Wft (with a $50 minimumorWe recommend 'lie-in. outside diameter commercial-quality 316 stainless steel tubing, with 0.031-in. inside diameter, purchased from Tubesales, 175 Tubeway Forest Park, Atlanta, GA 30050, at a current cost of $2.75 a foot ($100 minimum order). The company also has branches in Los Angeles, New York, and Chicago. M. Sallrn Banna Vanderbilt Univerrity Nashville. TN 37235
Volume 63
Number 11 November 1986
997
