A Solution Calorimeter and Thermistor Bridge for Undergraduate Laboratories R. A. Bailey and J. W. Zubrick Rensselaer Polytechnic Institute Troy, NY 12181 Measurement of the enthalpies of solution reactions is a common physical chemistry experiment that can yield reasonable data from relatively simple apparatus. We describe here a solution calorimetry apparatus that we use in the Rensselaer undergraduate lahoratorv. One part is a simple, rugged calorimeter head with sample mixing system that works very well if a relatively small volume of one solution is to be mixed with a larger volume of another. The second part is an inexpensive yet sensitive thermistor bridge that provides of this kind of ex~eriment. The calorimeter head and addition system are shown in Figure 1. The dimensions given are aonromiate . . . for use with a at.~n(l.iruwidr-muuthe~lI - q Lleuar ~ iliiik us the rnlurimeter . parts are I'lvxibuly, ;rnd a L b A 131mm iidditim t ~ l b r .\I1
Figure 1. Calorimeter head and addition system
(a) Side view A. Calorimeter head; 2% in. dia. X 1% in. thick. with %in lip 6. Sleeve C. Inner tube; 1 in. dia. X 10 in. long D. Threaded plastic ring E. Test tube. 20 X 150 mm F. Plunger G. Holes. '1, in, diameter immediately above threads H. Guide slot in 8;%in. wide X 3% in. long I. Pins set into C, sliding in H; im diameter (b) Top view of calorimeter head W-sleeve B X-hole tor stirrer: % in. diameter X , Z-holes far thermistor probe and calibrating heater (laher optional)
732
Journal of Chemical Education
glas' except the outer sleeve of the addition tube, which is made of metal. This part does not come in contact with the solutions. The entire head is clamped to a stand, and the Dewar flask is raised into position beneath it. The sample addition system consists of a plastic tube C threaded a t the lower end and fitted through sleeve B that is attached to the calorimeter head A. The sleeve is slotted on two sides (HI to take pins I set into C. A test tubeE is attached to the lower end of C by means of a threaded plastic ring D. A rubber o-ring reduces the probability of cracking the test tube if the ring is overtightened. The inner tube C with the attached test tube can be set to two positions. When the pins I are engaged in the upper stops of the slots H, the test tube is immersed in the liquid in the calorimeter body, but the holes G above the threads in C are above the level of the liquid (750 ml as designed). When the tube is moved to the lower position, the holes G are immersed. Operation of the olunner F that extends above the too of C p&mits mixing o i thecalorimeter contents. In use, a solution and the plunger are placed in the test tuhe, and this is attached to tube C, which is raised to the upper position. With a stirrer in place, the Dewar flask container is raised into position. The detector can be inserted and the stirrer started. After temperature eouilibrium has been reached, the addition tube is moved to its lower position, and the plunger moved up and down several times to mix the solutions thoroughly. Disassembly should he done with care, as the addition tuhe will still be full of solution. Use of r u h b e r eloves is reeommended. A simple modification for use with caustic or toxic solutions is to eliminate the ton closure of the addition tube. or replace it with a removable k.g. threaded) stopper. ~ e f o r e disassemblv the tuhe can be raised to the rimer .. .position, the mixing plunger can be removed through the top and a larger volume ~ l u n ~inserted er to force most of the solution out of the additioniube. For stirring, we have used a polyethylene stirring paddle (available from Nalge Company) remotely driven by a stirring motor through a flexible shaft in order to keep the top of the calorimeter clear. A suitable flexible drive is available through Ace Glass Inc., but we have found the speedometer cable of a Honda motorcycle to he fully satisfactory, and cheaper. A second stirring motor chuck can be attached to the end of this to hold the stirrer. We remove the metal stiffening rod from the stirrer; the "whip" produced a t reasonable stirring speeds is not excessive. Other openings in the head accommodate an optional heater for electrical calibration and the thermistor detector. A thermometer could be used if desired. A linearized thermistor bridge for calorimetry has heen described in this Journal.' The bridge and power supply circuits that we use are shown in Figures 2 and 3. The entire electronics unit is inexpensive and easily constructed. The temperature can be recorded continuously on a 10 mv-recorder by monitoring the voltage drop produced across R g by the current flowing in the bridge.
a
' Gunn, S. R., J. CHEM.EDUC., 50, 515 (1973).
Figure 3. Bridge Power Supply Circuit BR: Full wave bridge
(e.9.. Motorola HEP 175) I.C. vdltage regulator, type 7805 0 1 - 0 7 Silicon diodes CT1000 SF, 25 V capacitor Cz 100 SF, 25 V capacitor
b Figure 2. The Thermistor Bridge Circuit Values below are lypical R, = 5600 ohms R2 = 5600 ohms R3 = 1 K ohm R, = 10 K ohm 10 turn potentiometer Rs = 2000 ohms Rib = 5780 ohms @ 25'C
Values of the hridge components can he calculated from RI = R2 = R B + R4 = R T ~where , R T is ~ the resistance of the thermistor a t the mid-point of the range of temperatures to he measured. Under these conditions, the hridge is in balance and no current will flow through Rg. A 10-turn potentiometer, Rq, can he adjusted to balance the bridge anywhere in the working temperature range; it is used as a zero control. Ra should not he less than 10%of Ra, to limit the current that can flow through the bridge. Rs should he selected to give an adequate sensitivity (AEIAT) for the application, hut very large values of R5 will give noisy signals. With the nominal values of the components given in Figure 2, we have obtained a sensitivity of 2.5 mvldegree. A range of sensitivities is possible by varying Rg. The hridge and thermistor are calibrated against a Beckman thermometer. By choosing a thermistor with a linear change in resistance with temperature over the temperature range of interest, an effectively linear hridge response over a t least a 5 degree range is readily achieved. As power supply for the hridge, the arrangement shown in Figure 3 is used. It consists of a conventional 5-volt regulated supply, with the 1.5 volts used to operate the hridge taken from a tap on a string of silicon rectifiers.
Figure 4. The Thermistor Probe A. B. C. D. E. F.
G.
Thermistor bead Heat-shrink tubing Braided wire shield Twin-conductor shielded cable Heat-shrink tubing Glass tubing to fit (8 mm) Epoxy
Thermistor detectors are widely available, but to obtain the kind of sensitivity reported here good shielding is essential (Fig. 4). The thermistor bead is carefully connected to shielded, twin conductor wire, one lead insulated with heatshrink tuhing. Belden 8739 is recommended. The shield is metal foil with a stranded wire, which is more flexible and easier to work with than wire braid. The outer jacket of this wire is notched several inches from the thermistor. Some of the stranded wire is pulled out and folded hack along the two conductor cahles. A separate length of wire braid is pulled over the thermistor so that one end starts to cover the thermistor head. The wire from the shielded cahle is soldered and dressed to this braid a t the other end. This assembly is slid into a length of 8 mm glass tuhing and sealed in place with epoxy. A length of heat-shrink tuhing is placed a t the other end to secure the cable. We have used this apparatus to measure heats of metal ion-ligand reactions2 with good results and few problems with manipulation. D. A. Aikens, R. A. Bailey, G. G. Giachino. J. A. Moore, and R. P. T. Tomkins, "integrated Experimental Chemistry." Vol. 11, Allyn and Bacon, Boston, 1978, Ch. 40.
Volume 58
Number 9
September I981
733
