Topics in..
.
Edited by GALEN W. W I N G , Seton Hall University, So. Orange, N . J. 0 7 0 7 9 These artieles, most of which are to be c o n l r i ~ e dby guest authors, are inladed to s m e the readers of h i s JOURNAL by calling attention to new dwelopments in the themy, design, or availability of chaieal laboratory inslrumentaliun, or by presenling useful insights and mplanations of lopies Ual are of practical importance to those who use, or teach the use of, modern instrumentation and instrumental techniques.
XXXIII.
Recent Developments in
Part Two.
Some Associated Measurements (Continued)
RANDOLPH C. WILHOIT, Thermodynamics Research Center, Dept. o f Chemistry, TexosA&M University, College Station, Texos 7 7 8 4 3 THE MEASUREMENT OF ELECTRICAL ENERGY The calorie was originally defined in terms of the specific heat of water a t some specified temperature. However, for many years virtually all calorimetric measure ments have been based on electrical cdihrations. Since about 1930 ealorimetrio data. have been reported in units of either the joule or the thermochemical calorie. The thennochemical calorie is equivalent to 4.184 (exactly) absolute joules, or 4.1833 international joules. Electrical energy is u s u d y supplied to a calorimeter by passing a direct current through a coil of resistance wire placed in the calorimeter. The electrical energy consumed in time, 1, is
where I is the current and E the potential drop across the heater. The current may be determined by measuring the potential drop across a standard resistance connected in series with the calorimeter heater. Figure 5 is a diagram of the essential elements of a circuit suitable for measuring the electrical energy dissipated in a calorimeter. The power supply may consist of a large storage cell, or of a. filtered rectifier circuit designed t o deliver either constant voltage or constant current. Current is first passed through the dummy heater, whieh k similar to the calorimeter heater but located outside the calorimeter, in order t o adjust and stabilize the power supply. Switch number 1 is used to divert the current from the dummy heater t o the calorimeter hertter, and also t o start the timer. Switch number 2 k used t o connect the potential measuring instrument to either the calorimeter heater or the standard resistor.
7,-
Figure 5.
Colorirneter heater circuit
Accuracies in the measurement of energy of around one part in five hundred can be easily made with readily available components. Greater accuracies require that attention be paid to numerous sources of error, especially if a large quantity of energy is t o be delivered in a short time. These arise from transients in the power supply, from difficulties in obtaining a complete record of potential and current a t various times during the heating period, and from conduction of some of the heat out of the calorimeter. Measurement of the time of heating may also be a problem when accuracies approaching one part in ten thousand are the goal. The use of a. high quality constant current power s u p ply simplifies the recording of data since only the potential need be measured during the heating period. This measure-
ment is simplified still further by use of an electronic differential voltmeter or digital voltmeter. These instnlments usually have a recorder output whieh permits continuous recording of the voltage during the heating period. The design and construotion of a heater suitable for high precision measurements is very critical. The resistance of the heater changes as a. result of the rise in temperature which occurs when current is passed through it. To keep this as small as possible it should be constructed from a n alloy wire which has a law temperature coefficient of resistance. Manganin and constantan are often used to construct heaters which operate near room temperature. Wire made from Karma alloy, sold by the Driver-Harris Co. has a minimum temperature coefficient a t tempers, tines of 511-100DC, and also a higher resistivity than manganin or canstantan. Good thermal contact between the heater wire and the calorimeter is important in order to minimize the increase in temperature of the heater wire. This not only stabilizes the resistance of the heater but also reduces the conduction of heat away from the calorimeter. At the same time,, good electrical insulation must be maintained between the heater wire and the calorimeter. The heater wire should be spread over as large a surface as possible, consistent with the space limitations in the calorimeter. Immersion of the wire directly in a heat transfer liquid such as refined mineral oil or silicone oil is often helpful. The 3M Company S u p plies a line of fluorinated liquids which are also good for this purpose. The design and placement of the electrical leads between the calorimeter and the jacket need consideration. Generally four such leads are used; two for the current and two for the potential measure ment. The lead wires should be ss small as practical so that they do not conduct heat away from the calorimeter. The size of the potential leads is limited primarily by the requirement of mechanical strength and convenience in hndling. The cwrent leads, however, should he large enough to prevent generation of an appreciable amount of heat. The heat leakage can be reduced further by placing the lead wires in goad thermal contact with the outside surface of the calorimeter. The point a t which the potential leads a m connected to the current leads must also he earefullv considered. Other methods of supplying electrical energy quantitatively to the calorimeter may have certain advantages. For example an alternating current watt-hour meter can he used. Precision watt hour meters have been developed for purposes of standardization and calibration of
(Cmtinued on page A888)
Volume 44, Number 9, September 1967
/
A685
Chemical Instrumentation large amounts of electrical power. The Sangamo Electric Co., Springfield, IU., supplies a rotating standard watbhour meter which operates on 60 cycle current of 115nominal volts with a one half ampere current coil. This can be used satisfactorily dawn to power levels of around 10 watts. With suitable crtlibrrttion and care, rotating standard watt hour meters are good to an accuracy of one part per thousand. A number of circuits have been developed which supply power in the form of square wave pulses, with a precise quantity of energy carried by each pulse. The total energy delivered during the heating period can be measured with a pulse counter. The equipment described by Wm. Prengle, Jr., F. L. Worley, Jr., and C. E. Mauk (Sl), is a typical example. These methods introduce considerable convenience into the procedure, since no measurements are required during the heating period, and the total energy can be simply read at the end of the experiment. In addition, the power level can be varied during the heating. This is a considerable advantage in twin calorimetry, where the electrical energy is to he balanced against the heat effect being measured.
CALORIMETRIC STANDARDS The use of a standard substance to determine the heat capacity of the empty calorimeter by either a Type I or Type I1 calibration exneriment was described in Pert I. These chibrations use 8 standard absorbs an accurately known quantity of heat when it undergoes an isothermal transfamation. Reference substances are also used to test the performance of the calorimeter by comparing the measured value with the accepted value. Thus it is convenient to have two or more standards; one for calibration and others for testing. A Series of standards of differing properties makes it possible to choose one which is similar to the system being studied. On the other hand too much proliferation of standards is nndesirable, because of the uncertainties introduced in establishing their properties and the dEculties involved in obtaining intercomparison among different laboratories. Many standards have been introduced by the IUPAC Suboommittee on Thermodynamics and Thermochemistry and, in recent years, by the US. Calorimetry Conference, working in close cooperation with the National Bureau of Standards. The requirements for, and usefulness of, standards are different in different areas of oalorimetry. The list of standards in use is constantly being revised, although the introduction and aoecptance of new standards is a slow process. The current situation will be reviewed in various areas of application. A short bibliography is presented at the end of this section. Values of properties of some key comnounds. ineludinc same of those used as standard materidq will he found in reference (46). Heats of C m n h t i o n . Because of the great dficulty of direct electrical oalibration to the desired accuracy, combustion
A686
/
Journal o f Chemical Education
bustion of compounds containing chlorine. Heat Capaeily. Primary standards are used to determine the heat capacity of the emntv calorimeter and secondarv stand-
calorimeters are nearly always calibrated by burning a primary standard of aceuratelv known heat of eombustion. In who are conducting measurements. Benzoic acid is a. nearly ideal standard, and has been used for this purpose for over forty years. It is stable at room temperature, can be obtained in a state of high purity, and is nonhygroscopic. Furthermore it burns cleanly to water and carbon dioxide and its heat of combustion is highly reproducible. The National Bureau of Standards supplies samples certified for heat of combustion. The currently available batch, designated 39i, has a. heat of combustion of -6317.8 cal gm-I (vacuum) when burned under standard conditions. These are; a bomb whose internal volume is 300 ml per gram of sample burned, a reference temperatm of 25"C, and an initial state of 30 atm of oxygen with 1 ml of water present in the bomb. Standard samples are also supplied by the National Physical Laboratory in England. In addition to the primary standard, a number of secondary standards have been used for testing of eombustion cdorimeters. Several of these are described below. 2,2,4-Trimethylpentane is used far testing eombustion of volatile hydrocarbons. Certified sample available from the National Bureau of Standards. Naphthalene is used for testing combw tion of solid hydrocarbons. Certified samples formerly supplied by the National Bureau of Standards. Succinic seid is used for testing eombustion of solid materials containing carbon, hydrogen, and oxygen. Recent measurements and a review of older data is given in reference (38). They reoommend a standard state energy of combustion of -3020.50 + 0.50 cal gm-I (vacuum) a t 25'C. Sucrose is used to test eombustion of solid eamnounds eontaininc a laree oronor-
4. n-Heptane was recommended as a heat capacity standard by the Fourth Calorimetry Conference in 1949. It is suitable in both liquid and solid forms. It is not yet certified by the National Bureau of Standards, but they will supply limited quantities to qualified investigators. Diphcnyl ether was suggested as a comparison standard by Ginnings and Furukawa (34). I t can be used to a somewhat higher temperature than n-heptane, but hen not found wide application. Water can be obtained in a state of higher purity than can m y of the other standards. I t has long been recognieed as a standard in the range of 0-100'C. The relatively high volrttility is a disadvantage in some applications. Benzoic acid was recommended as a standard for the heat capacity of solids at low temperature by the Fourth Calorimetry Conference. I t has a high heat capacity per unit volume below 100°K. It is not suitable for use above 350°K hecause of its volatility and corrosiveness. a-Aluminum oxide (synthetic sapphire, or corundum) has been used as a. standard since about 1950. It is the only generally accepted standard for heat capacity at high temperatures. Limited quantities of the Calorimetry Conference sample may be obtained from the National Bureau of Standards. At the request of the Calorimetry Conference in 1964, the National Bureau of Standards has agreed to issue aluminum oxide as a certified standard reference material. This should be available within a year or two and will be in the form of annealed rods, sbont 0.075 by 0.20 inches. Copper was recommended by the Calorimetry Conference in 1965 for use below 20°K. Limited quantities of high
of Standards. Hippuric acid was proposed to test the combustion of nitrogen-containing compounds. I t has not been widely used in recent yearn. Sym-Diphenylthiourea and thioglycolic scid have been proposed for testing 'eombustion of sulfur containing compounds. Table 4.
Substance
Laboratory, Argonne, Ill.
-
SOLUTION CALORIMETRY Solution calorimeters are nearly always calibrated by electrical heating. The acceptance and use of standards here are much more limited than in heat capacity and combustion calorimetry. In fact,
.
.
Heat C a ~ a c i t vStandards
Useful Range of Temperature, 'K
p-Fluorobeneic acid is suitable for tese ing combustion of compounds of low fluorine content, and m-trifloorotoluic acid for compounds of intermediate fluorine content. These last two compounds are discussed in reference (33). Trichlorophenol is used for testing com-
Melti:g
K
Point References
some investigators argue that reliance should be placed entirely on careful design and operation of the oallorimeter, rather than on the use of test reactions. However most calorimetrists h d that the use of a suitable reference material helps to (Continued on page AC88)
Chemical lnstrumentcrtion determine whether systematic errors have actually been eliminated. G u n (3.9) has listed desirable cha~acteristics of reference materials and has reviewed several specific standards. The solution of potassium chloride has, in effect, been used as a test material for endothermic reactions for many years. The heat of solution has been measured many times and the results are viewed in reference ( 4 0 ) Additional idolmation is given by Somsen, Coops, and Tolk (41). The agreement among different values is not as good as it should be for a. test material, and Gunn concludes that it is not as suitable for this use as was once believed. The heat of solution of KC1 is markedly influenced by traces of water occluded in the solid, and removal of the last traces of such water is notoriously difficult. The heat of solution is also iduenced by strains in the crystal which result from mechanical treatment or crystal imperfections. The rather high temperature coefficient of the heat of this process and the rather slow rate of solution are other disadvantages. The heat of solution of one mole of single crystal KC1 in 200 moles of water at 25°C is 4200 6 calories. Ray (46) has devised a rather ingenious method of using the evaporation of water for checking the operation of certain types of solution calorimeters. The water is removed from a glass helix evaporator, immersed in the liquid in the calorimeter, bv mesns of s. vacuum numu.
acid solution. I t offers many advantages as a calorimetric standard and also as an acidimetrio standard. It can be obtained in a. highly pure state and dried by heating to 80°C and then cooling in a vacuum desiccator. The heat of solution of one mole of THAM in 0.1M HCl at 26'C is -7107 f 3 calories, when the find concentration is 5 g per liter. I t is fairly insensitive to the concentration of HC1 and THAM. Limited quantities for testing eitlorimeters may he obtained from Dr. E. J. Prosen of the National Bureau of Standards. The bureau is preparing to issue certified samples for use in calorimetry and acidimetry. Gunn (3#) recommends the use of the reaction of sulfuric acid withexcess sodium hydroxide solution for testing the sccur%cy of calorimetem. Solutions of sceurately known acid concentration can be prepared by dilution of constant boiling sulfuric acid. The concentration of the sodium hydroxide, or the presence of sodium csrbonate, has only a small effect on the beat evolved. Shomate and Huffman (43) have reported values for the heat of solution of magnesium in hydrochloric acid. This reaction m y be used for testing cslorimetem designed to study this type of reaction. Measurement of the heat of solution of metals in acid is difficult because the hydrogen came.; away some r a t e r vapor. The hrnt of dolution of surcinic acid ijr dilute llCl m a y have some value at n tejt
A688
/
Journml of Chemical Education
reaction far an endothermic solution of s solid.
Mixing Calorimeters The mixing of benzene with carbon tetrachloride has been suggested as a mesns of testing calorimeters which measure the heat of mixing of volatile liquids. However it has been found to be unsatisfactory because of the reaction of carbon tetrachloride with mercury, which is often present in such cdorimeters. The systems hexm-cyelohexane and wateracetone have also been suggested and are being investigated. No general concensus has been reached as yet.
Vapor Flow Calorimeters Waddington, Smith, Williamson, and Scott (64)have suggested csrbon disulfide as a suitable references material for testing vapor flow calorimeters. They have published accurate values of the vapor heat capacity and other thermodynamic properties. Water and benzene might also be used for this purpose hut have some disadvantages. Water does not boil as smoothly in the boiler as do most organic liquids, and the heat capacity of steam is highly dependent on pressure. The heat capacity of gaseous benzene is not as well established as is that of carbon disulfide beosuse its more complicated molecular structure causes greater uncertainties in the statistical calculstions.
Bibliography (31) W. PRENGLE,JR., F. L. WORLEY, JR., AND C . E. MAUK,J . C h n . Eng. Data, 6,395 (1961). T. B. DOUGLAS, (32) G. T. FURUKAWA, R. E. MCCOSKEY.AND D. C.
Part Three-Some
GINNINCG.J . Res. Natl. BUT Std., 57, (1956). (33) W. D. GOODAND D. W. SCW, in H. A. Skinner (ed.), "Experimental Thermochemistry," Vol. 11, WileyInterscience, New York, 1962. (34) D. C. GINNINGS AND G. T. FWUKAWA. J . Am. Chem. Soc... 75,. 522 (1953). H. E. STIMSON, AND (35) N. S. OSBORNE, D. C. GINNINGS, J . Res. Nall.Bur. Std., 23, 197 (1939). (36) E. D. WEST,Trans. Farday SOC.,59, 2200 (1963). AND G. L. KINWON, (37) J . W. EDWARDS Trans. Faraday Soc., 58, 1313 (1962). (38) E. D. WEST AND D. C. GINNINQS, J . Res. Natl. BUT.Std., 60, :309 (1958). (39) S. R. GUNN,J . P h p C h a . , 69,2902 (1965). (40) V. B. PARKER,''Thermal Properties of Aqueous Uniunivalent Electrolytes," National Standard Reference Data Series. Nst. Bur. Standards, 2, (April 1,1965). (41) G. SOMSEN,J. COOPS,AND M. W. Tom, Rec. Tvav. Chim., 82, 231
fit
11Qfi2)
(42) J. D. RAT, Rev. Sei. Instr., 27, 863 (1956). (43) C. H. SHOMATE AND E. H. HUPFMAN, J. Am. C h n . Sac., 65,1625 (1943). J. C. SMITE. (44) G. W. WADDINGTON, K. D. WILLIAMSON,AND D. W. SCOTT,J . Phys. C h a . , 66, 1074 (1962). (45) G. P. SOMAYAJULU,A. P. KUDCHADKER, AND B. J. ZWOJAIN~KI, "Thermodynamics," in Ann. Reo. Phys. Chem., 16,213 (1965). (46) G. PILCHER,AND L. E. S ~ N , Phil. Trans. Roy. Soc. (London), A248.23 (1955).
Specific Types of Calorimeters
A brief survey of various kinds of ealarimeters and fields of thermochemical research was given in Part I of this series. Some basic terminology was also explained. Part I1 consisted of a discussion of tbermometry and the measurement of electrical energy, as applied to calorimetry, and %list bf calorimetric standards. Some additional constructional details are given in this final section, and several commercial instruments are described.
SOME DESIGN OBJECTIVES
It is difficult to state, in general terms, desirable design objectives which apply to all of the many types of calorimeters. * Temperature gradients disappear most quickly from materials of high thermal diffusivity. Some values of this property were listed in Table I1 of Part 11. At room temperature the thermal diffusivity of glass is about 6.2 X lo-"* sec-I and its thermal conductivity is 2.76 X 10-zeal deg-1 em-'set-1.
The following criteria are offered to help evaluate static calorimeters which have either an isothermal or adiabatic jacket. Of course, a. balance must d w a p be made among wst of construction, accuracy, and convenience of operation.
Characteristics of the Calorimeter (1) In all Type I experiments (measurement of heat capacity) or in Type I1 experiments (measurement of energy change a t constant temperature) involving small energy changes, the heat capacity of the calorimeters used should be small compared to the beat capacity of the sample. (2) The temperature of the outside surface of the calorimeter should remain miform at all times. This uniformity is accomplished by making the oukr wall in a simple symmetrical shape. Materiala of high thermal diffusivity sre desirable.' Irregular projections and large masees of a poorly conducting m a t e d such as glass or plastic should he avoided. The cover is (Catinued on page A890)
