VOLUME 33, NO, 10, OCTOBER, 19S6
A PRECISE, UNIVERSAL CALORIMETER W. H. SLABAUGH Oregon State College, Cornallis, Oregon
IN TEE study of stoichiometric reactions associated with solid surfaces it became necessary to design a calorimeter which would measure consecutively several heat changes. As the result of this need, the author has designed an apparatus which, although based upon generally accepted practices, is unique in that it combines several features used in modern calorimetry. The apparatus permits the measurement of the heat of solution of a solid in a liquid, followed by the measurement of the heat of interaction of this solution with various amounts of a rea~ent. For example, the heat of solution of sodium oleate in water can b e measured by breaking a sample tube of a known amount of the soap in the water contained in the calorimeter. Next, the heat of salting-out of the soap may be measured by adding measured amounts of a salt solution. The instrument has been used t o measure heats of neutralization, heats of wetting, and heats of solution. It has been operated by several different persons, some with only a minimum of training. The instrument has proved to be reliable, and reproducible results to within +0.25 per cent are regularly achieved by an experienced operator. The calorimeter is built to fit a piut Dewar flask (Figure 1). The calorimeter assembly is suspended in an air chamber of about 40 liters capacity. This chamber is in turn placed in a thermostatted air bath of one cubic meter, the larger bath being held a t *0.1O0C. The inner bath temperature fluctuates =t0.005", while both of these temperature changes are effectively lagged out in the Dewar flask. The actual temperature change in the calorimeter consists of a constant drift of no more than O.OOO1° per minute upward or downward depending upon the conditions
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T-THERMISTOR C-CALIBRATION HEATER
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JOURNAL OF CHEMICAL EDUCATION
coat of paraffin wax has proved sufficient for this purpose. A Type G-2 Mueller resistance bridge along with a Leeds and Northrup Type HS galvanometer with a one-meter scale serve t o measure the change in temperature. The galvanometer serves to measure the off-balance of the bridge. For this reason a considerably less elaborate bridge may be used. The sensitivity of this detector system permits the measurement of temperature changes of 0.00002°. An observation is made by first permitting the system to reach a steady state in which the drift of the temperature in the calorimeter is no more than O.OOO1° per minute. After a constant drift is established the O I 3 4 5 6 7 S 9 1 0 1 1 I I I 3 heat determination is made, comparing the displaceTIME IN MINUTES ment of the drift produced by the reaction to a similar displacement produced by a calibration. Each calorie under which the determination is made. The calorim- of heat released in the calorimeter causes a net scale eter is operated in a room thermostatted a t Zt0.5"C. deflection of 2.56 Zt 0.02 cm. when 200 grams of water A constantan coil made of No. 36 gage wire and are in the calorimeter. During a run the actual temhaving 41.58 ohms resistance at room temperature perature change of the calorimeter is no more than serves as a calibration heater. I n calibrating the heat 0.005", a change too small to appreciably affect the capacity of the calorimeter a potential of six volts is calibration of the instrument. Figure 2 is a n illustration of a calibration and a deplaced across the heater and the exact amperage noted. termination of a heat of wetting of a solid. I n this From the time and current which flows through the particular run, a current a t 0.145 amperes passed heater, reproducible calibrations to within +0.1 per through the heater coil for 15.05 seconds. This reprecent are obtained. sented a release of 3.14 calories. The wetting action released 8.20/8.05 times as much heat, or 3.20 calories. Determinations on additional samples can be made because the heat capacity of the calorimeter remains constant within the error of the instrument. A versatile feature of this apparatus is the provision for studying the heat changes which accompany reactions that occur in stepwise fashion. By means of the syringe pipet, operated from outside the calorimeter, small increments of reagent may be added and the heat changes accompanying each addition may be measured. For example, a polyprotic acid can be titrated calorimetrically with a base. Figure 3 shows the type of data obtained in this type of observation. The acid analyzed in Figure 3 is a colloidal electrolyte which obviously exhibits two sources of hydrogen ions. The "end points" indicated by the two abrupt decreases The change in temperature is detected with a ther- in heat of neutralization correspond exactly with mistor (No. 15A, supplied by Western Electric Co.) similar end points measured by potentiometric titrawhose temperature coefficient is . approximately five tion of the same system. From the area under these ohms per degree centigrade. This type of thermistor curves the mean heat of neutralization can be calhas been found to be considerably more stahle than a culated. For many such reactions only the mean heat thermopile of copper-constantan used in a previously of reaction is now known. Consequently, this indesigned calorimeter. The transistor must be pro- strument provides a means of studying the heat of tected from moist,nre for best performance. A thin reaction during various phases of consecutive reactions.
