94 1
V O L U M E 24, NO. 6, J U N E 1 9 5 2 the tubes by stirring currents and hit the walls only because the magnets "float" from side to side. Calibration tests were run to determine the temperature rise above bat,h temperature realized from stirring in comparison t o unstirred. Results showed that the temperature rise was so small that i t was negligible over the short time intervals of test runs. Also, assuming constant and like speed of stirring, the actual energy gained would be equal for each sample, and consequently would cancel out from each side of the basic equation. Variations from differences in physical properties other than C, of the sample liquids were considered negligible. An additional test was run to determine esactly the precision involved when totally different heat capacities were used in each sample tube. This test was run with approximately 50 ml. of ivater in one sample tube and 20 ml. of water in the other tube. The results were within the prescribed accuracy of the test, but ahowed that more accurate results were obtainable with matched hrat capacities. Irit,erchange of sample tuhes from one cylinder to the other to determine if the assumed slight deviations from actual concent,ricity were significant revealed no perceptible errors, and interchange of liquids in t,he sample tubes showed that the emissivity of each tube was the same. The standard liquid used for comparison was distilled water for temperatures below 100" C. and tetraaryl silicate, a special heattransfer liquid, for higher temperatures. The specific heat of this "standard" liquid had been determined previously b y a convrntiorial method over the entire temperature range. Artual specific heat values used were talcell from an equation of C, z's. temperature which was derived statistically from the data. . l c ~ u r a c yof this standard was easily within 1yo. Water was used to measure the values of specific heat of toluene, benzene, and tetraaryl silicate in preliminary t e s t s The wsulting calibrations showed an accuracy of 1%.
RESULTS
The specific heat at constant pressure of isopropyl phenol silicate was measured by this method a t approximately 50" C. intervals between 50" and 250" C., and the data were statistically fitted t o an equation of C, us. temperature for purposes of liquid thermal characteristic evaluation. Bt least three determinations were made at all points, and at least four deterniinations were made for the heating-type tests in which appreciable rvaporation occurred. Typical cooling and heating test data are presented in Figures 2 and 3. I t may be noted in Figure 2 that t'he data arr s u f f i h i t for calculation of C, from the basic test equation. T h k is not possihlr for data given in Figure 3, because i t is necessary to eliminate thcs evaporative effect by combining tests. Resultant values at the high temperat,ures were lower than expected because the evaporation eliminated a highly volatile and low density liquid componrnt. a t approximately 200" C., and thereby left a liquid of higher molecular weight. I t was determined previously that increase i n molecular weight was synonymous with der1 in this particular chemical family. Figure 4 is a graph of the specific heat curve for iioprop1.l phenol silicate derived from this st,udy in comparison u-it,h the curve for tetraaryl silicate. Data were converted from metric to English units for purposes of engineering usefulness. The a(*curacy of the isopropyl phenol silicate derivrtf curve is within 1 in the loa- range and 2% in the high range. Experimental d a t a are helieved to have an accuracy within 5%. CONCLUSION
The t,ePt and apparatus were developed for use during a time when only small volumes of esperimental liquids were available, but, in view of the accuracy realized and the subsequent value of the data obtained, the method is believed to be an advancemrnt. in specific heat measurement of liquids a t high temperatures.
T E S T PROCEDURES
LITERATURE CITED
Test temperature was est,ablished at 25" C. above or below the bath temperahre, and the sample tubes and liquids were preheated or chilled to 75" C. above or below the bath temperature, to permit a constant thermal exchange over the range of Z!C 10" C. from the test temperature. Temperature measurements were rec,ortied on each liquid, on alternate minutes, over the approsimate 60" C. range. Th(. highly polished sample tuhes were handled only with ahsorbent cotston a t all times to prevent heat leaks through partial 1)lac.k h d i e s . Stethoscopic determinatiori of positive stirrer :ic,tiori was found satiqfactory.
(1) Cork, hl. J.,
"Heat," 2nd ed., Ken, York. John \Vilex 6- Sons,
1942. (2) Smith, C. S., Trans. Am. I n s f . Mining M e t . Engrs.. 137, 236-44
(1940). (3) Spear, S . H., .4STM Bull., S o . 168,79 (1950); TP 207. (4) Spear, S.H.. "Measurement and Significance of Specific Heat of Thermal Insulating Materials," presented at .I.S.T.JI. spring
meeting, Symposium on Thermal Insulating JIaterials, Cincinnati, March 7,1951. RECFJVED for revipw Soveinher 0 , 19.51. Accepted IIari,Ii 19. 19.52.
Procedures in Ice Calorimetry H. T. SPENGLEK Carbide and Carbon Chemicals Co., Union Carbide and Carbon Corp., South Charleston, W . V a . i simple Bunsen ice calorimeter and bath were designed for the rapid and reasonablj precise deterniination of enthalpy and related thermal data. Contrasted to the elaborate power-measuring equipment required for an electrical calorimeter, this apparatus requires no auxiliarl equipment other than an analltical balance. In addition to the determination of heats of fusion, heats of reaction, and enthalpies above 0" C., which have pretiously been determined b) the methods of ice calorimetrj , this instrument has been applied to the determination of heats of vaporization at 0" C. and enthalpies a t temperatures down to - i 7 ' C. Because of its simplicity, versatility, and ease of operation, a calorimeter of this type should find many uses where data of moderate precision are required.
T
HE isothermal-type calorimeter wing the volumetric. change and heat of transformation (fusion) of thp i(-e&water a t 0" C. to measure thermal effec't? b v w ti wribed h>- Robert Bunsen in 1870 ( 1 ) . Since that time severa! other workers (4, 6, 7 , 14) have brought out LTarious motlifications and applications of t,his instrument. Thr, latest ani1 most rrfineii design is t,hat of Ginnings and Corrucvini (.il, n.ho ]lave applied their apparatus to obtaining enthalpieP in th(8 teniperature range 0" to 900" c. The so-called ice calorimeter i,c particularly uFeful i n thc determination of small and veq. s ] ~ \ \ -heat efferti;. Artirlcs covering the following applications have appeared in the lit Property Enthaliiies ah0X.e 'O C. H e a t of adsorption H e a t of fusion
Reference
(4) (12)
(a)
Propel ty Reference FIrnt of solution (6) H e a t of radioactive decay (76) H e a t of reaction (7)
942
ANALYTICAL CHEMISTRY
The calorimeter described in this article is an adaptation of the design of Ginnings and Corruccini; it was primarily intended for use in obtaining enthalpy data of moderate precision from 0' to 200" c. As familiaiity with this instrument was gained, its speed and versatility became apparent. Procedures were developed for the majority of the properties listed above and, in addition, enthalpies from the temperature of solid carbon dioxide to 0' C., and heats of vaporization a t 0' C. The calorimeter, sample, thermostat, stirrer, thermoregulator, and constant temperature bath are shown schematically in Figure 1. The calorimeter assembly consisted of a central metal tube closed a t the bottom, open a t the top, and surrounded along the closed end by two concentric glass jackets. The space between the metal tube and the inner glass jacket was filled liquid-full with air-free distilled water. A tube led from this chamber to a small reservoir containing a mercury seal, which in turn led to the outside through a small capillary orifice. The instrument was supported in a constant temperature bath automatically regulated to 0.00' & 0.005" C. A vapor thermostat above the open end of the central tube provided means of bringing the sample, contained in a glass capsule, to the desired temperature. For enthalpy determinations the procedure involved first forminn an ice sheath on the outside of the metal tube. After the sakple had been placed in a small sealed glass capsule, it was suspended on a thread in the central tube of the vapor thermostat, brought to temperature, and quickly lowered into the calorimeter. The resulting volume change of the ice-water system caused by heat from the sample was measured by weighing the quantity of mercury drawn into the calorimetric Bystem through the exit capillary from a tared beaker. A suitable calibration constant derived from the density of mercury, the difference in volume between ice and water, and the heat of fusion of ice was then used to obtain the total calories released t o the calorimeter. This value was adjusted for the heat contributed by the glass capsule to yield the enthalpy of the sample. ~
thermostating; however, for continuous operation an electrically controlled bath of aqueous isopropyl alcohol proved to be more convenient. The vapor thermostat for bringing the sample to temperatures above room temperature is shown in Figure 1.
It consisted of a glass tube 22 mni. in inside diameter and 400 mm. long, open a t both ends and surrounded by a jacket in which various compounds could be refluxed to produce desired temperatures. The jacket was insulated with magnesia and provided with a boiler and condenser. Obviously, it would be possible to connect this system to a pressure-controlling device and vary the boiling point of a given liquid for obtaining different temperatures; however, in practice, changing the thermostating liquid \$-as found simpler and about as rapid. The samples were contained in sealed borosilicate glass capsules approximately 50 mm. long made from tubing 19 mm. in outside diameter. A small hook of glass n-as formed on the ca sule seal and a fine thread attached. By means of this thread tEe sample was suspended about midway down the inner tube of the thermostat; the up er end of the tube \\-as closed x i t h a cork, which eliminated t f e effect of convection currents up the tube. In Figure 1 the capsule is shown suspended in position.
A comparison of the above apparatus and that of Ginnings and Cdrruccini will reveal several points of difference. The thermal gate which they used to prevent radiation down the central tube of the calorimeter was not included, because it
r n
iI L i
DETAILS O F CONSTRUCTION
The details of the calorimeter proper are shown in Figure 2. The central metal tube, A , was constructed of a 3/4-inch thinwalled, 18-8 stainless steel tube with the lower end welded shut. TFOcircular fins, B, of '/rt-inch stainless steel sheet were welded to the tube near the bottom to serve as supports for the ice sheath and to aid in heat transfer from sample to calorimeter. The central tube was surrounded, the l o ~ - e three r quarters of its length, by two glass jackets, C, D,75 and 100 mm. in diameter, respectively. These glass parts were suspended from metal flanges, E, F , welded to the central tube. Some initial difficulty was experienced in securely attaching the glass jackets to the flanges. The gla~-to-metalseals were made with soft rubber gaskets, and mechanical support was provided with adjustable metal stirrups, G, H, passing under the bottom of each jacket and through two guides on each of the flanges. The ends of the stirrups extended through the guides and were threaded and fitted with small nuts which could be tightened to exert the desired tension. The inner jacket, C, was filled liquid-full with redistilled, gasfilled with dry carbon dioxide free water while the outer jacket, ,D, gas by means of connection D ,. provided thermal insulation. The volumetric changes occurring in the inner jacket from formation or melting of ice were made to act against a water-mercury interface in the reservoir, I . This reservoir was connected to the inner jacket through a l/Anch stainless steel eductor tube, Z'. The tube passed through the flanges. -4hard polyethylene tube, J ,joined the end of the eductor to the glass reservoir. The plastic connector was used to minimize the danger of breakage, which is ever-present in a glass-to-metal joint. Subsequent calibrations showed that no "breathing" or leaking of the conneetor tube was detectable. The mercury reservoir was connected through a small capillary tube and stopcock, K , to a tared beaker, L, containing mercury. Gain or loss of mercury in the beaker yielded a quantitative measurement of the heat given or taken from the calorimetric system by the sample. A small stainless steel tube, M , leading from the outside through the flanges to the bottom of the inner metal tube, was used to introduce a continuous stream of dry carbon dioxide gas; this prevented diffusion of water vapor from the outside air into the calorimeter. Preliminary work was carried out using a water-ice bath for
XIT ILAR"
1 - 1 Figure 1.
General Arrangement of Calorimeter
V O L U M E 24, N O . 6, J U N E 1 9 5 2 was not proposed to operate a t the elevated temperatures (900' C.) which they employed. The taper in the bottom section of the inner tube, to allow free fall of the sample capsule, was also not included. This feature would have complicated the fabrication. Moreover, it wae found that free fall of the sample could be accomplished in the present instrument by arresting the capsule fall an inch or so from the bottom of the calorimeter tube with the thread and slowly lowering it the rest of the way. No detectable increase in heat leak was introduced by the thread,
943
the bottom of the calorimeter tube by the thread and slowly lowered to rest a t the bottom. Experience showed that a minimum of 90 minutes had to be allowed for complete heat transfer to or from the calorimeter. The end of a run was determined by frequent weighings of the mercury cup until the initial heat leak rate was obtained. A heat leak correction to the gross weight of mercury drawn into the calorimeter was made by subtracting a quantity of mercury equal to the product of the heat leak rate and the length of the run. This corrected weight of mercury was then used t o calculate the calories given up to the calorimeter by capsule and sample by means of the following formula: p = 64.631
CALIBRATION AND TESTING
The calibration constant of Ginnings and Corruccini was used to convert weight of mercury to international calories added to or subtracted from the calorimeter. Their value was 270.37 & 0.06 international joules per gram of mercury or 64.631 =t0.014 defined calories per gram. It was not considered practical or necessary to redetermine this constant as the Bureau of Standards workers had carried out this work electrically with high precision. Several test runs to obtain enthalpy values of materials of known purity and enthalpy indicated the factor was entirely adequate (see Table I). The temperature of the sample in the thermostat was checked by measuring t h e t e m p e r a t u r e , under similar conditions, of a small beaker of mercury suspended in the same position i n t h e t h e r m o s t a t . These tests also gave indication of the length of time the sample must remain in the thermostat to attain equilibrium. A 90-minute period was finally chosen as satisfactory for all test conditions. Temperature readings were reproducible to 10.l0c. PROCEDURES
The bath (Figure 1) was adjusted to 0" C. Flow of c a r b o n dioxide gas was started through the purge tube. The central tube of the calorimeter was filled n-ith acetone to within a few inches of the top, and Figure 2. Calorimeter the tared mercury beaker, Detail half-full, was placed under the capillary exit from the ice-R-ater jacket. The apparatus was allox-ed t o stand for approximately half an hour to equilibrate. Solid carbon dioxide was then slowly sifted into the acetone until an ice sheath of maximum thickness was formed on the outer wall of the metal tube. With care and practice it was possible to freeze the sheath in tear-drop form around the bottom of the tube and fins, where most of the heat exchange between sample and calorimeter occurred. For measurements requiring that the sample be heated above room temperature, a vapor thermostat was employed. The sample contained in a glass capsule was suspended for a suitable length of time (usually 1.5 hours) in the central tube of the thermostat by means of a thin thread. A loosely fitting cork closed the top of the thermostat tube to minimize convection effects. While the sample was equilibrating check weighings were made on the mercury cup to obtain a heat leak value. The heat leak was very erratic with a freshly prepared ice sheath. Check xeighings were periodically made until a constant value was obtained. The average heat leak was found to be equivalent to 0.0005 gram of mercury per minute. To start a run the sample tube was allowed to fall freely from thermostat to calorimeter. I t was arrested an inch or so from
where p
wm
= heat absorbed by the calorimeter, internationdl
calories
wm = weight of mercury, grams From the value of q is substracted the portion of the heat effect contributed by the capsule. For this the equation pc = Ktwe is used, where qe = calories released to calorimeter by borosilicate glass capsule Kt = enthalpy of borosilicate glass from experimental temperature to 0" C., calories per gram w c = weight of capsule, grams Dividing the calories of heat from the sample by the sample weight yielded the enthalpy value per gram.
Ordinarily, a new glass capsule n a s fabricated for each sample. In order to use this method it is necessary to assume that t h e enthalpy of borosilicate glass is essentially constant from batch to batch. Several dozen determinations scattered over 18 months ~ o u l dseem to support the assumption to xithin the precision claimed for this work.
Table I.
Enthalpies, Heat of Vaporization, and Heat of Fusion Te%Eik,ure Enthalpy, Calories per G r a m O o t o t o C. Experimental Literature Reference
Substance
A.
Enthalpies above 0' C. 28.0 5.2 55.0 11.4 28.1 0.92 5.5,O 1.79 55.0 55.0 55.4 29.5 97.0 18.6 176.2 33.6 25.0 11.3
Sodium chloride
11ercury Water Heptane Borosilicate glass Ethylene'oxide
B.
Enthalpies -77.0 -77.0 -77.0
Borosilicate glass Benzene Heptane
C. n'ater
below 0' C. 12.5 24.5 37.5
5.2 11.4 0.93 1.82 .54.9 29.4 19.4 35.2 11.3
23:s 37.2
H e a t of Vaporization, 0' C. H e a t of Vaporization 599 595.9 H e a t of Fusion, t o C. H e a t of Fusion 5.5 30.0 30.19 52.6 31.5 ..
(9) (5) (5)
(8) (3)
(io) (11)
(IS)
D. Benzene Maleic anhydride
(10)
..
The preceding steps may be combined to yield
Ht - Ho where
w
64.631
[W
- ( d ~ / d t ) t ]- K I W ~ WS
gross weight of mercury drawn into calorinieter, grams ws = weight of sample, grams d w / d t = rate of heat leak, grams of mercury per minute t = length of r u n , minutes Ht - H , = enthalpy of sample from t to 0" C., cal. per gram Kt and w chave the values shown above. =
For measurements below room temperature the thermostat shown in Figure 3 was employed to bring the sample to tempera-
ANALYTICAL CHEMISTRY
944
ture. Only the temperature obtainable with solid carbon dioside-acetone mixtures was used for the work reported in this paper; however, other low temperatures could have been obtained with freezing mixtures such as calcium chloride-ice or by using a jacketed system for circulating a therniostating liquid. S o special technique vas required for ob5DL1D CAREON DIOXIDE4CETONE MIXTURE taining heats of fusion, as they were deterniined by several enthalpy runs over a t e m perat u re rang e n - h i c h i n c l u d e d the ALUMINUM FDlL melting point. Csually enthalpies a t two temperatures a b o v e MAGNESI lNSULATl0 a n d t w o below t h e m e l t i n g p o i n t were sufficient. The enthalpy temperature curve for maleic anhydride is shown in Figure 4 from 98" to 0' C. The heat of fusion is represented in this figure by distance A B , 31.5 calories Der gram. Figure 3. Low Temperature Tests of heats of Thermostat vaDorization were carried out on water. The water was placed in the bottom of the calorimeter and allo\+-etl to come to temperature. A slow stream (20 to 30 ml. per minute) of carbon dioxide gas was passed through a drying tube and then allowed to enter the bottom of the calorimeter by way of the spiral tube (.If in Figure 2). After a short preliminary period to saturate the water with gas, the run was started by weighing the mercury beaker. The gas stream carrying the evaporated water v a s passed through a tared phosphorus pentoxide tube. At the end of the run a weighing of the oxide tube gave the quantity of water evaporated, and a weighing of the mercury beaker (corrected for heat leak) yielded a measure of the heat effect. The heat of vaporization follows directly from these measurements.
-
1
-
-1possible objection may be made that the dissolved gas will alter the heat, of vaporization of the liquid. If the gas id appreciably soluble in the liquid, this will be true. Consequently, in extending this procedure to materials other than water (and carbon dioside as the carrier gas) care must be taken to select a gas that is essentially insoluble in and nonreactive with the liquid. There is much to recommend helium as a satisfactory answer for most cases. The problem of recovering the evaporated sample can be solved, in general, by useof the freezing technique. If the sampleladen carrier gas is passed through an efficient condensate trap, all the vapor can be removed and accurately weighed on an analytical balance. In certain cases it might prove more practical to remove the sample from the gas b>-means of a scrubber, and measure by appropriate analysis. Tests of heat of reactions which proceed a t 0" C. are readily carried out in this instrument. The heat of neutralization of nitric acid with sodium hydroxide was determined as an example. ,1quantity of standard aqueous caustic was placed in the calorimeter well. B sample of acid slightly less than the stoichiometric amount equivalent to t,he caustic was placed in a glass ampoule and introduced into the calorimeter. Biter sufficient time had elapsed t o allow the acid and base to come to 0" C., the ampoule was broken with a glass rod, previously chilled to t'he same temperature. Except when t,he ampoule is being broken, the top of the well is closed with a cork. The quantity of heat evolved was determined as outlined above. An average value based on several runs was -13.6 kg.cal. per mole of water formed; this compares with a value of - 13.69 kg.-cal. from the literature (2).
100
P
-
d 60
'
3
k
____
- -
MELTING POlNT=S2.6%. HEAT
OF
FUSION
0-A
31.5 sal/pm
1 0
0
Figure 4.
20
40 60 ENTHALPY, co1Jgm
80
100
Enthalpy-Temperature Relation of Maleic Anhydride
Cnless the reaction under study is hnown to go to completion, a portion of the reaction mixture must he removed and analyzed in order to establish the extent of reaction. Some gas-liquid reactions are also adaptable to determination by this method. The liquid is placed in the calorimeter and the gas to he reacted is slowly introduced through the carbon d i o d e purge tube (M, Figure 2). DISCUSSION
ii summary of typical data obtained Rith the instrument described above appears in Table I. Section h lists enthalpy results above 0" C. on samples for which comparable data appear in the literature. The agreement with two exceptions is good: the average accuracy is about 1% . Section B shows data determined from -77" to 0" C. Here, the average accuracy is from 1 to 2% of the values measured. After completion of the low temperature work it was discovered that Snietoslawski ( 1 4 ) had reported one ice calorimeter determination from -14" to 0" C. The authors' work shows the feasihilit>-of extending this range to -77' C. The heat of fusion data nere found to be about as accurate as the enthalpy determinations. The value for the heat of vaporization of water agreed t o within 0.5% of the accepted value. I n general, the precision of measurement with this instrument is directly proportional to the actual quantity of heat being determined. LITERATURE CITED
Bunsen, R., Ann. P h y s i k , 141, 1 (1870). Getman, E. H., and Daniels, F., "Outlines of Physical Chemistry," 7th ed., p. 483, Sew York, John Wiley & Sons, 1945. Giauque, FT'. F., and Gordon, J., J . A m . Chem. Soc., 71, 2177 (1949). Ginnings, D. C., and Corruccini, R. J., Bur. Standards J . Research, 38, 583 (1947). Ginninps, D. C., Douglas, T. B., and Ball, A. F., Ibid., 45, 23 (1950). Heiber, IT., and Muhlhauer, F., 2. anorg. Chem., 186, 97-118 (1930). Heiber, W., and Reindl, E., 2. Elektrochern., 46, 556-8 (1940). "International Critical Tables," 1st ed., T'ol. 2, p. 93, Sew York, McGraw-Hill Book Co., 1999. Ibid., Vol. 5, pp. 100-15. Oliver, G. D., Eaton, M., and Huffman, H. M . , J . Am. Chem. SOC.,70, 1502 (1948). Parks, G. S., Huffman, H. M., and Thomas, 9. B., Ibid., 52, 1032 (1930). Patrick, W , A, and Greider, C. E., J . Phus. Chem., 29, 1031 (1925). Perry, J. H., "Chemical Engineers' Handbook," 3rd ed., p. 769, S e w York, McGraw-Hill Book Co., 1950. Swietoslawski, W.,"Microcalorimetry," pp. 66-9, New York, Reinhold Publishing Gorp., 1946. RECEIVEDor review February 7, 1952.
Accepted April 14, 1952.
