J. S. BINFORD, JR.,J. M. STROHMENGER, AND T. H. HEBERT
2404
A Modified Drop Calorimeter. The Heat Content of Aluminum Carbide
and Cobalt(I1) Fluoride above 25"
by Jesse S . Binford, Jr., James M. Strohmenger, and Thomas H. Hebert Department of Chemietry, University of South Florida, Tampa, Florida
(Received August 19, 1966)
The construction and operation of a drop calorimeter with an electric resistance furnace and a copper block receiver is described. The change in block temperature is followed with a thermistor. Calibration is performed with or-alumina from the National Bureau of Standards. Between the temperatures of 400 and 1400" the average uncertainty is 0.7%. At lower temperatures the uncertainty in calibration is somewhat greater. The = 36.76T 3.50 X equation derived for aluminum carbide, A1,Cls, is ( H r - H298.15) 10-T2 (0.95 X 1O6/T) - 14,500 cal mole-' (298.15 < T < 1773"K, 1.6%). The = -0.45067' 10.83 X 10-T2 equation for cobalt(I1) fluoride, CoF2,is (HT- H298.15) (1.77 X 1O6/T) 4700 cal mole-' (298.15 < 7' < 1400"K, 39"). Cobalt(I1) fluoride melts at 1127" with a heat of fusion of 10,720 cal mole-' (2%).
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Introduction The thermodynamic functions of many well-known crystalline compounds are unknown above room temperature because of the lack of heat content data. Thus, chemical equilibria involving these compounds cannot be predicted with certainty a t high temperatures. The drop calorimeter technique continues to be the most reliable method for determining these heat contents. Such a calorimeter has been constructed using a rather simple bridge, having a thermistor as one arm, to measure temperature changes with high precision. Two simple high-melting compounds, aluminum carbide and cobalt(I1) fluoride, known to be stable a t high temperatures are chosen for this study. Description of Apparatus The basic design of the apparatus is similar to that of Southard,' later modified by Margrave and Grimley.2 A 7-cc capacity sample capsule made of platinum10% rhodium is suspended in a furnace tube on a platinum-lO~orhodium wire. Difficulty is sometimes experienced with the wire kinking during the drop. This is largely eliminated by using heavier wire between AWG 24 (0.020-in. diameter) and AWG 26 (0.016-in. diameter). Heat exchange between the wire and the capsule is minimized by using a small The Journal of Phyeieal Chemistry
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double-bore ceramic bead to connect them. The furnace consists of an alumina tube 18 in. long and 1.25 in. 0.d. with a l/s-in. wall wound with platinum10% rhodium AWG 20 (0.032-in. diameter) along 14 in. of its length. The furnace temperature is controlled manually by adjusting two variable transformers connected in series. A stabilizer on the line voltage keeps furnace temperature drift t o less than 1 deg/hr. The furnace tube is insulated with alumina cement and Fiberfrax, an aluminum silicate. The furnace wall is stainless steel and the ends are steel plates soldered to copper coils for water cooling. The outside furnace dimensions are 14-in. diameter and 16.5-in. length. The heating coil is 6 turns/in. except for a 0.5-in. length at each end which is 12 turns/in. This geometry produces a constant-temperature zone which varies less than 1" over a 1-in. length at 1050'. The sample capsule is 1 in. long excluding the filling spout and support wire. Sample temperatures are measured with a platinum-lO% rhodium thermocouple in contact with the capsule at 1200" and below. i l t higher temperatures the thermocouple tends t o stick to the capsule and it is positioned about 0.25 in. above its surface. (1) J. C . Southard, J . A m . Chem. Soc., 6 3 , 3142 (1941). (2) J. L. Margrave and R. T . Grimley, J. Phys. Chem., 62, 1436 (1958).
A MODIFIEDDROPCALORIMETER
2405
poses of calculating heat losses to the oil bath, the surThe thermocouple, originally calibrated by the National face temperatures are needed within 0.01", a requireBureau of Standards, is recalibrated frequently with ment well met by the termistor in this location. The short lengths of gold wire as described by Roeser and resistance of the thermistor is followed manually with Lonberger. At higher temperatures palladium wire a 9999.9-ohm General Radio Type 1432 decade which is used. serves as another arm of a Wheatstone bridge. The The top of the furnace is closed with a ceramic heat other two arms are 5000-ohm fixed resistors of the wirereflector and the bottom hole with a 0.75411. thick wound type. At 25" the thermistor has a resistance of water-cooled copper door. Heat exchange between 4200 ohms. A change of 0.001" causes a change of furnace and receiver is negligible when this door is 0.18 ohm in the thermistor. Some difficulty is usually closed. When testing a sample, 1 hr is allowed for it experienced with stability of thermistors. Olette to reach the temperature of the furnace before dropping has solved this problem by maintaining the current it into the receiver. From the hottest zone of the supply to the bridge constant within f0.020j,.4 In furnace to the bottom of the receiver hole is 24 in. The sample falls freely until it is inside the receiver the present work, the thermistor is preheated a t 60" for several days. The current to the bridge is held below hole and is then braked by air pressure to a stop. 20 pa by placing a 50-kilohm resistor in series with the The receiver, or calorimeter, consists of a nickelpower supply, a 2-v low-discharge storage cell. The plated copper block, 5l/, in. in diameter and 8 in. high, bridge unbalance is determined with a Leeds and with a hole 5 in. deep in the middle to hold the electrical calibrating heater and to receive the sample. A Northrup 9835 dc microvolt amplifier. One scale 0.75-in. thick copper door on top of the block opens to unit deflection on the amplifier is equivalent to a receive the sample and closes to trap the heat. After change of approximately 0.1 ohm or 0.001". The coming to rest slightly off the bottom of the hole, the thermistor is calibrated against a calibrated Beckman sample capsule is soon lowered enough to make contact thermometer. Although the resistance of the thermiswith the block in order to minimize the thermal equiltor does drift after several months, the entire calibraibration time. A sample of 23 g of aluminum oxide tion curve is shifted almost parallel so that it is not crystals from NBS a t 1000" in the furnace requires 20 necessary to recalibrate. The uncertainty owing to min to reach thermal equilibrium in the calorimeter. drift corresponding to a temperature change of 5" The copper block rests on Bakelite knife edges in a is only several thousandths of a degree and is proporbrass container. This container, 7 in. i.d. and 11 in. tionately less for small temperature changes. high, is completely immersed in an isothermal oil bath Experimental Section held constant to .tO.Ol". The brass container is Calibration. The method of Kubaschewski and filled with argon to minimize the heat-exchange rate Evans6is used to determine the temperature change of between bath and block. This rate, expressed in the block, which corrects for heat losses to the isoterms of block temperature change, is 0.002 deg min-' thermal surroundings. The heat capacity of the deg-'. The heat capacity of the block as determined block is determined by measuring current, potential, electrically is 1995 f 6 cal/deg. All electric potentials and time for a resistance coil in the block. Current is are measured with a Leeds and Northrup I
