Solution and Low-Temperature Heat Capacity Measurements
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Calorillleters for Heat of Solution and Low-Temperature Heat Capacity Measurements GEOLOGICAL SURVEY PROFESSIONAL PAPER 755 Calorimeters for Heat of Solution and Low-Temperature Heat Capacity Measurements By RICHARD A. ROBIE and BRUCE S. HEMINGWAY GEOLOGICAL SURVEY PROFESSIONAL PAPER 755 A description of the construction and operation of a calorimeter for determining the heats of solution in aqueous HF or HCl and of a low-temperature heat capacity calorimeter for cp measurements between 10 and 380 kelvin UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1972 UNITED STATES DEPARTM.ENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600115 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 40 cents Stock Number 2401-2182. CONTENTS Page Page Abstract ___________________________________________ _ 1 Heat of solution calorimeter __________________________ _ 13 Introduction ________________________________________ _ 1 Components ____________________________________ _ 13 Acknowledgments ___________________________________ _ 2 Submarine _________________________________ _ 13 Low-temperature heat capacity calorimeter_____________ _ 3 Calorimeter vesseL __________________________ _ 17 Cryostat components ____________________________ _ 3 Calorimeter top and closure seal ______________ _ 17 Submarine _________________________________ _ 3 Thermometer_______________________________ _ 17 Heater_____________________________________ _ Electrical lead wires _________________________ _ 4 17 Heat station ________________________________ _ 4 Stirrer shaft and motor drive _________________ _ 17 Helium reservoir ____________________________ _ 4 Puncture tube ______________________________ _ 19 Isothermal shield ____________________________ _ 4 Sample holder and impeller assembly __________ _ 19 Helium economizer __________________________ _ 4 Calorimeter support connecter ________________ _ 19 Floating ring _______________________________ _ 4 Operation and data reduction _____________________ _ 19 Vacuum elevator and heat switch _____________ _ 4 Corrections _________________________________________ _ 20 Adiabatic shield _____________________________ _ 4 Sources of heat exchange _________________________ _ 20 Calorimeter ________________________________ _ 4 ltadiation ______________________________________ _ 20 Thermometer_______________________________ _ 4 Conduction through solid connections ______________ _ Heater _____________________________________ _ 22 7 Conduction and convection through a gas __________ _ 22 Cryostat hoist_ _____________________________ _ 7 Corrections for heat exchange _____________________ _ 25 Vacuum systems ________________________________ _ 7 Corrections for superheating of the electrical heater __ 27 Adiabatic shield controller________________________ _ 7 Calculation of heat of reaction ________________________ _ 28 Assembly ______________________________________ _ 9 Heat of solution of Tris (C4HuNOa) ___________________ _ 28 Temperature-energy measurements circuit_ _________ _ 9 Operation and data reduction _____________________ _ Conclusions and recommendations for future improve- 11 ments ____________________________________________ _ Heat capacity results for Calorimetry Conference benzoic 30 acid _____________________________________________ _ 13 References cited _____________________________________ _ 31 ILLUSTRATIONS Page FIGURE 1. Diagram showing cross section of helium cryostat for heat capacity measurements between 4 and 400 K_ _ _ _ _ _ _ _ _ _ 3 2. Cutaway drawing of sample holder and heater bobbin-thermometer assembly_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ 6 3. Block diagram of adiabatic shield controller________________________________________________________________ 8 4. Schematic diagram showing electrical measurements circuit for calorimetry _________________ .:.___________________ 10 5. Temperature-time curve for a heat capacity measurement_ _________ .__________________________________________ 12 6. Deviation plot of Cp for Calorimetry Conference sample of benzoic acid_______________________________________ 14 7. Deviation plot of U.S. Geological Survey measurements of the Cp of the Calorimetry Conference sample of benzoic acid_______________________________________________________________________________________________ 16 8. Cross section of HF solution calorimeter___________________________________________________________________ 18 9. Drawing of sample holder and loading pedestaL____________________________________________________________ 20 10. Temperature-time curve for the heat of solution of 4.6 g of hydromagnesite in 4 N HCL _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ 21 11. Coding sheet for heat of solution experiment_______________________________________________________________ 24 12. Graph showing heat of solution of Standard Reference Material724, Tris, between 25° and 35°C_________________ 29 TABLES Page TABLE 1. Time-voltage log of the heat capacity measurement shown in figure 5------------------------------------------ 12 2. Least squares values for the heat capacity of the Calorimetry Conference sample of benzoic acid___________________ 15 3. Observed heat capacities of Calorimetry Conference sample of benzoic acid_____________________________________ 15 III IV CONTENTS Page TABLE 4. Printout of computer reduction____________________________________________________________________________ 22 5. Fortran IV-G computer program for reducing heat of solution data ________________________ ---_- __ ------------- 23 6. Thermal and electrical constants of solution calorimeter at 30°C_______________________________________________ 25 7. Heat of solution of Tris (C4HnN03), in 0.1 N HCL---------------------------------------------------------- 30 CALORIMETERS FOR HEAT OF SOLUTION AND LOW-TEMPERATURE HEAT CAPACITY MEASUREMENTS By RICHARD A. RoBIE and BRucE S. HEMINGWAY ABSTRACT energy data for minerals. Unfortunately there has not This report describes the construction and operation of an been an equivalent increase in the number of calori aqueous-solution calorimeter for enthalpy of reaction measure metric studies of minerals, the principal source of free ments and a low-temperature calorimeter for measuring heat energy values. In the hope that we might attract others capacities between 10 and 380 K (kelvin). Heat capacity data are used to calculate the entropy from the third law of thermo into the "business," we present a description of the dynamics. construction and operation of two calorimeters used The heat capacity calorimeter is of the adiabatically shielded in laboratories of the U.S. Geological Survey for de type and uses precooling by solid nitrogen together with the use termining the enthalpy of formation, AH" and "Third of helium gas (for heat exchange) to reach temperatures of 52 K. Law" entropies, S, of minerals. These are a solution Liquid helium is used as the lowest temperature refrigerant. The calorimeter, a liquid-helium reservoir, a gas heat exchanger, and calorimeter for measuring heats of reaction and a heat an automatically controlled adiabatic shield are suspended within capacity calorimeter capable of precise measurements a cryogenic vacuum submarine. The submarine is sealed with in the range of 10 to 380,K (kelvin). reusable soft aluminum gaskets and is supported within an 18.6- The Gibbs free energy of formation, AG" is probably liter metal Dewar flask. The operation of the calorimeter was tested by measuring the heat capacity of the Calorimetry Con the single most useful thermodynamic property of a ference benzoic acid sample. Our results, between 29 and 300 K, phase inasmuch as it provides a direct measure of the are within 0.15 percent of the values obtained by six previous chemical stability of that phase. The Gibbs free energy investigations on this same material. of a phase may be determined from equilibrium data The vacuum-jacketed aqueous-solution calorimeter for heat of such as aqueous solubility and cell voltage or by calori solution measurements in strong acids, including HF, is made of gold-plated copper and has a copper resistance thermometer metric techniques from the relation wound on its outer surface, an internal heater, and a combined (1) sample holder-stirrer. It may be operated either at constant pressure to obtain the enthalpy change, llH, or at a constant vol where AH1 is the enthalpy of formation and AS is the ume to obtain the change in the internal energy, !!U, at temper atures up to 100°C. It has a temperature sensitivity of ±0.00005°C. entropy change. Heats of solution as small as 4 calories have been determined with Since the work of Julius Thomsen and Marcellin a precision of ±0.05 cal. Berthelot in the 1880's, aqueous-solution calorimetry Our result for the heat of solution of U.S. National Bureau of has been, and remains so today, the single most im Standards Standard Reference Material 724, Tris, C4HuNOa, in 0.1 N HCl at 25°C is: portant technique for the precise determination of the heats of formation and reaction involving complex !lH = -7098.0±11 cal mole-1 inorganic materials. The temperature coefficient of the solution reaction is The molar heat capacity at constant pressure, Cp, is the amount of heat necessary to raise the temperature d!!H I dT = 42.7 cal mole-1 degree-1 of 1 formula weight (mole) of a material by 1 degree. This value was obtained