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Chemical Instrumentation
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Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
XXXIII. Recent Developments in Calorimetry Part Three.
Some Specific Types of Calorimeters (Conclusion)
RANDOLPH C. WILHOIT, Thermodynamics Research Center, De~artmento f Chemisfrv. Texas A & M University, cd1lege Station, Texas j j 8 4 3 SOLUTION CALORIMETRY The heat eapncii.y 01 liquids n r d lhe enthalpy change resulling irom mixing of x solid, liquid, or gas with a liquid are me* swed in w. solulian calorimelcr. The magnitude oi encrgy change ranges from mien,ealoriea to kilocslories. In conlrast I," cornhnstion ealorimclers whic:h inchdn only a few stnndnrd types, sohllion enlorimeters m e found in many forms and sl.yles. The lovcl of accuracy varies widely for different kinds of measurements and conditions hnl is i n general eonsiderably less thxn that usually oblsined in
cc,mbcml.ion cnlorimei.ers. I n this sect,ion, 1,hreepopular t,ypes of oi~lo~.imele~x are duscribed which are s,litnblc for wcwk wil,h relalively nnnvolxtilo solvenls, stleh :%a water, in Ihe vicini1.y of t.oom temperni.u~m A very simple ealorirneler may he cow strucled by installing a. cover, st,irrer, the^ mnmeler, and mixing device in a. Dewat. flark. Those descrihed in the m~tnuelsof I>aniels, et al. (6d), and Shoemaker and Garland ( 6 3 ) are t,ypical of many which
tions in soht,ions, was described by Arnett,, Bentrode, Burke, and Dugglehy (68). A calorimeter made to their plans has recently been offelwl for sale by the Guild Corporalion. I t eonsisk of s. complete system inch~dingthermometer and eont rola. In this lype of apparatus the calorimeter, in eRect, consists 01 those p a r k of the inner glass wall, t,hermometer, heater, pipet, and other parts which a n in more or less direct condact with the sohttion. The jacket consists of the out,er wall of the Dewar flask and (he cover. Thus there is no sharp separation between calorimeter and jacket, nnd the heat cnpacit.y of the calorimeter is a function oi the amount of solulion it cont,ains. In addition, it is difficult to cont,rol evaporation. Although the accuracy can bc improved hy immeming the Ilewar flask, partly or completely, in a constant temperature bat,h, this type of calorimeter is ~lsuallyrestricted Lo measuremenis of low accuracy. Far greater accuracies, it is advisable to use s calorimeter which consists of a thin walled metal or glass vessel completely snr~ m ~ o d eby d an adiabatic or isot,herrnsl jacket,. Such enlorimeters are completely, or nearly, filled with liquid and may be of 1.hesingle or the twin type. I n this type of i~ calorimeter, the space between the d o rimeter and the iseket is often evacuated to reduce heat transfer.
Figure 10. Schematic Diogrom 8700 Colorimeter System (Courfesy LKB lnrfrumenfs, lnc.)
Fig.t9. Dewor Flmrk Colorimeter (O'Horo, Wu, and Helpler, J. CHEM. EDUC. 38, 51 3 11961 11
lmvc hcen nsed for ~ m d e ~ g l n d , ~indrocxie lion in physirnl rhcmisl~ry. A somewhat
Figure 14.
Calorimeter Cells for the Benzinger Calorimeter ICourterv Beckman Instruments Inc.)
Control in Science and Industry" (6). I t is strictly a static conduction twin calorimeter. The two calorimeters are in the form of cylindrical shells about 4 in. long. They are placed end to end concentrically inside the jacket. The space between the calorimeters and the jacket is ocoupied by thermooouple wire. The thermocouples are prepared from a helix of constantan wire. Half of each turn is plated with copper and acts as the copper lead of a thermocouple junction. Therefore each turn forms one pttir of junctions. The helix is coiled around the calorimeter and produces a thermooouple of about 10,000 junctions for each calorimeter. This arrangement provides a very large area of contact between the solution in the d o rimeter and the thermoeou~leiunctions. Figure 13. Benringer Microcalorimeter b u r l e v Berkmon lnrlrumenlr lnc.)
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two sets of thermocouples are connected in opposition, as in the Calvet calorimeter. An overall view of the apparatus is shown in Figure 13. The calorimeter cells have been cleverly designed. Three types of glass cells are pictured in Figure 14. Metal cells are also furnished. They serve to measure the enthalpy change resulting from the mixing of two liquids. Initially the cells are positioned with the long axis horizontal, and the solutions are placed in the space between the outer and inner walls. I n two of thp+ wlls B partuiw whivlt runs either reparule- the rquatt~riullyor 1w.gil~ali11~ll.v two rdwiuns uf i ) l ~ p . ~ x i r u ~ equal w l y "01ume. I n the other type one solution is placed in the lower part of the space, and a much smaller volume of solution placed in
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Chemical Instrumentation small depressions in the inner wall, which initially are a t the top. In any case, the solutions are mixed hy rotating or tumbling the entire calorimeter and jacket. As in the Cdvet calorimeter. the eleo temperature between the two calorimetem. This signal is recorded by a pot,ent,iometric strip chart recorder. However in t,he Bensinger calorimeter, the conduction hetween the cdarimeter and its jacket is such that they reach the same temperature within a. few minutes. Thus following the absorption or evolution of heat, the difference in temperature reaches a maximum and then falls back to zero. Since all the heat is conducted to or from the jacket, and the rate of heat conduct,ion is proportional t o the temperature difference, the area under this cunre is a measure of the total heat evolved or absorbed by the reaction. Bensinger refers to this procedure as the "heat-burst" principle. Accuracies of 1.5-2y0 and resolutions of about calorie can be attained far solutions of 1-15 ml. Of these two calorimeters, the Calvet is more versatile and is capable of better overall accuracy. The two calorimeters have about the same range of sensitivity. Benainger has been primarily interested in using this apparatus for measuring the enthalpy ohange in enzyme catalyzed reactions, and it is largely restricted to the mixing of two liquid solutions, or possibly a. liquid and a solid. Within its range of ttpplication the Bensinger calorimeter is more convenient to use, especially with small volumes of solution. Many experimenters have built their awn mierncalorimeters of these and other types. Undoubtedly they can be built more economically than the cost of the commercid equipment, praviding the cost of the penon's time is not included. However, if such a. prnjeot is undertaken, one should be prepared to devote several years to the effort.
DYNAMIC CALORIMETRY The widespread use of differential thermal analysis in recent yearn has given impetus to the development of a whole series of dynamic calorimeters for studying phenomena which occur in condensed phases, especially solids. I n terms of number of people involved, this is probably the most active area. of calorimetric research today. Undoubtedly the ease and speed with which measurements can be made and the ready adaptability of these instruments to automation are major rear sons for its popularity. This type of calorimeter can be used with better accuracy and reproducibility not only for all types of measurements made in the typical differential thermal analysis apparatus (64) but also for the measurement of heat capacities, heats of transitions, and heats of reactions. They can also sometimes he used for detecting energetic effects in many miscellaneous phenomena. which would he difficult to study with any other type of calorimeter. These include study of nonequilibrium effects, energy stored in meta(Continued on page A870)
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Chemical lnstiumentcrtion Because of the dynamic nature of these techniques, the calorimeters and their samples are small in order to reduce the effect of temperature gradients. They are not generally ~ u i t e dfar studying equilibrium thermodvnamic orooerties of mate-
calorimeter;, very similar to the nsud IITA apparatus, t o those similar to vacuum jacketed adiabatic calorimeters used for specific heat work. A large number of these types of calorimeters have been described in the literature and applied to many types of materials over a wide range of temperatwes. Several are being manufactured by instrnment companies. The accuracy of measurements made with these instruments is difficult to judge, but it is usually fairly low. Even with the same instrument, the accuracy may vary widely for different kinds of measurements and different conditions. The dvnamic mode
static calorimeters. Figure 15 represents symbolically the principle of operation of five types of differential calorimetew. The main commall features are the operation of the calorimeter with a cont.iouonaly changing temperature and the use of continuow i.ecording of significant variables and of closedloop control of the operation. The figure shows a schematic drawing of each of the calorimet,ers and their jackets, the location of the main thermometers and heaters, the type of control used, the variables which are recorded, the appearance of a typical record resulting from a sharp endothermic transition, the basis for calor~lationof the heat of transition, and the assumptions which are implied in the interpretation of the record. These drawings show thermocouples being used to measure temperature diffexences although matched resistance thermomet,em can he, and somebimes are, employed. The Model 900 Differential Thermal Analyzer msnufaetored by UnPont is a typical example of a calorimeter of the type represented by Figure 15A. It is essentially a DTA appsrat.us modified to f~rrnish quantitative information about heat effects. A more detailed drawing of this calorimeter and jacket b shown in Figure 16. Since two calorimeters are used and heat is transferred to the calorimeters only by conduction from the jacket, it may be termed a differential dynamic conduction calorimeter. A picture of the calorimeter cell is shown in Figwe 17. I t operates from the control circuits in the Model 900 Differential Analyaer. A sample i,f 1-200 mg is placed in one calorimeter and a reference substance in the other. The differ-
ence cdolorimetcr on an X-Y recorder. The jacket temperature is pmgrammed to
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with an isothemd jacket. The reference calorimeter is heated a t a constant rate. The temperatures of the two calorimeters are sensed with platinum resistance thermometers, and the electricd power to the sample calorimeter he&r is controlled by a closed loop so as to maintain the two calorimeters a t the same temperatures a t all times. If the two calorimeters exchange heat with the jacket a t the same rate, the electrical energy added to the
Figure 16. Crorr Section, Calorimeter Cell (Courtesy E. I.duPon1 de Nemourr 8 Company)
increase at a constant rate from 1 to 30" deg. min-'. The Differential Scanning Calorimeter, Model DSGIB. of the Perkin-Elmer Gorp., ha3 receivkd wide publicity for the past two or three years. I t is shown in Figure 18. I t operates on the principle of Figure 15B. Since the jacket remains e r sentially a t room temperature, this apparatus is a dynamic differential calorimeter
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Figure 17. Calorimeter Cell (Courtesy E. I. duPonf de Nemavrr & Cornpony)
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Chemical Instrumentation sample oalorimeter in any period of time is equal to the change in enthalpy of the calorimeter and sample. A signal proportional to the electrical power is generated and may bbe displayed on a strip chart recorder. With suitable calibration this may he converted to heat capacity of the sample, and the area under this curve may he converted to an enthalpy change. Samples are placed in shallow covered aluminum cups which may he used either sealed or unsealed. The cups containing reference material and sample are placed on s m d , diik-shaped, metal calorimeters. The calorimeters are thermally isolated from the surroundings. The whale assembly i~enclosed in a. cylindrical jacket which has a glass window a t the top to permit viewing of the cdlorimeters. The jacket may he either evacuated or filled
Figure 18.
Differential Scanning Cdorimeter lCovrtery Perkin-Elmer Carporotionl
with any gaseous atmosphere. Gaseous reactants may be added during the run, and gaseous products withdrawn for andysis. The instrument is compact, highly automated, and quite versatile. The reference calorimeter may he heated or cooled a t a constant rate, or its temperature may he
held constant for isothermal studies. Samples are usually in the range of 1-30 mg, and full-scale sensitivities in the range of 1 3 2 millicdories per second may he selected. I t covers the range of -100 to 500°C. This type of calorimeter is potentially more accurate than the conduction type illustrated in Figure 15A. The Dynatech Cow. offers a dynamic adiabatic calorimeter, the Model QTA-N7. I t corresponds to Figure 15C, and a detailed cross seet,ion is shown in Figure 19. Physically it is similar to a. static adiabatic calorimeter. The single calorimeter is suspended within an aneroid jacket. Electrical energy is dissipated in s. heater in the calorimeter a t a constant rate. The temoereture of the calorimeter is detected difference in temperature between the calorimeter and the jacket, controls the power to the jacket heater so that the jacket temperature follows that of the calorimeter. Assuming negligible temDerature nradients and sen, heat exchanee
be evacuated to reduce heat conduction through the gas p h a ~ e . Two calorimeters, one for solids and one for liquids, are supplied. I t operates in the range of -180 to 320°C. Specifications call for an accuracy of 2-5%, depending on temperature, in the meesurement of specific heat. The Deltatherm D5500 dvnamic adia-
paratus. However from the control &andpoint, the roles of the calorimeter and jacket are ~ v e r s e d . This is illustrated in Figure 15D. The jacket is heated a t a constant rate. The signal from bhe thermocouple, placed between jacket and calorimeter, controls the power input to the calorimeter heater so that the calorimeter follows t,he jacket. The electrical power to the calorimeter, which represents the instantaneous heat capacity of calorimeter and heater, is recorded. A record similar to that for Figure 15B is produced. This appears to he a fairly elaborate instrument and should he as versatile as the Perkin-Elmer calorimeter. The sample sise is considerably larger. I t operates in the range fromroom temperatnre t,o 8 0 0 T .
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The Harrop Laboratories describe a calorimeter based on the principle of Figure 15E which appears to he still in the
developmental and testing stage. This may be described as a constant rate dynamic oondwtion calorimeter. It is designed principslly for temperatures up ta 120O'C. The calorimeter does not contain an electrical heater. All the heat is transferred to it by conduction and radiation from the jacket. The rate of heat transfer is related both to the temperature
Figure 20. Adiabatic Calorimeter (Courtesy Dynakch Corporation1
difference between calorimeter and jacket and to the absoh~tet,emperature of the s y s tern. A t higher temperatures it is almost Figure 19.
Cross Section, Adiabatic Calorimeter (Courtesy Dynotech Co~poration)
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Chemical Instrumentation all due to radiation. The rate of heat transfer is established as a function of ing a meawrement, an analog computer uses the signal from the thermocouple to control the jacket heater so that the rate of heat conduction from the calorimeter to the jacket remains at some selected eonstant value. This is an unusual type of iacket control. and its further develo~ment
able.
SUMMARY Calorimeters come in many shapes, sizes, types, and degrees of sophistication. The only common feature is the objective of measurement of an energy change. Those described in this review are only a sample. I n common with other branches of experimental science, the great proliferation of instruments which has taken place in the past 20 years or so has had a great effect. I n spite of the simplicity of the principles and the wide choice of automated instrumentation now available, accurate chlorimetry still requires much skill and patience. Because of the elusiveness of sources of error, accuracies oi measurement are frequently misjudged. Experience has shown that reliance can be placed on calorimetric data only when they have been confirmed in two or more independent laboratories. Although several types of calorimeters have recently been placed on the market, additional types are still needed in other areas. Further progress in thermochemical measurements depends not only on the availability of instrumentation, but e q ~ ~ a l on l v the development of and v&ssii~eanalytical techniques and on the produetiolr of pure compounds.
Bibliography (52) F. DANIELS,J. \V. \VILLI.\MS, P. BENDER,P. A. .\LRERTY, AND C. D. CORNWELL, "Experimental Physical Chemistry," 6th ed., McGraw-Hill Book Co., New I-ork, 1962. N D C. W. GAR(53) D. P. S H O E M . ~ EAR LAND, "Experiments in Physical Chemistry," McGraw-Hill Book Co., New York, 1962. (54) W. W. WEXDL.~NDT,"Thermal Methods of Analyiii," Intemcience Publishers (division of John Wiley & Sons, Inc.) New Ywk, 1064 (55) J. JORDAN AND T. G. ALLEMAN, Anal. C h m . 29,9 (19.3) AND R. M. I Z ~ T P , (56) J. J. CHRISTENSEN J . Phya. Chem.66,1030 (1962) (57) J . J. CHRISTENSEN, R. M. IZATT, L. D. HANSEX, AND J. A. PARTRIDGE, J. Phys. Chem. 70,2003 (1966) (58) E. M. Amett, W. G. Bentrude, J. J . B u r k e P. ~ ?,Id. ~ ~ Duggleby, J . Am. Chem. Soc. 87, 1511 (1965).
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