INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1952
3. The logarithm of the value of K‘ obtained by extrapolation of the plots of K‘ vs. 1/T to 1/T = 0 may be a linear function of the number of carbon atoms in the compounds for a homologous series. 4. From the above, K’ may be determined as a function of the temperature for a given compound, and the constants, L’and n, may be determined from one experimental isotherm, so that isothe+msat other temperatures may be predicted. At the present time, it is not possible t o calculate isotherms of a substance without a t least one experimental isotherm of that substance on the particular adsorbent; however, i t appears t h a t with further development of the behavior of the constants, L’ and n, such a calculation may become a possibility. NOMENCLATURE
A = a molecule of gas being adsorbed a, b = constants CA = concentration of adsorbed gas, moles per 100 moles carbon AG” = standard free energy change of adsorption A H “ = standard heat of adsorption K = thermodynamic equilibrium constant K’ = empirical adsorption constant related t o equilibrium constant
387
L = total concentration of active centers, moles per 100
L’
moles of adsorbent
= empirical adsorption constant related t o total number
of active centers available on adsorbent 1 = an active center m = aconstant n = aconstant p , P A = equilibrium pressure of adsorbed gas R = the gas constant R1, SI = a fra ment of a dissociated adsorbed molecule A S ” = stancfard entropy change of adsorption T = absolute tem erature B = fraction of afsorbent surface wvered LITERATURE CITED
(1)Brunauer, S.,“Adsorption of Gases and Vapors,” Vol. I, “Physical Adsorption,” p. 95,Princeton, N. J., Princeton University Press, 1945. (2) Glasstone, S., “Textbook of Physical Chemistry,” 2nd ed., p. 1199,New York, D.Van Nostrand Co., 1946. (3) Hougen, 0. A., and Watson, K. M., “Chemical Process Principles,” Vol. 111, “Kinetics and Catalysis,” p. 912,New York, John Wiley & Sons, 1947. (4) Langmuir, I., J . .Am. Chem. Soc., 40, 1361 (1918). IND.ENQ.CHEM.,42,1315 (1950). (6) Ray, G. C., and Box, E. 0.. (6) Sips, R.,J. Chem. Phys., 16, 490 (1948). RDCEIVED March 7, 1951.
Vapor Pressure of Benzene above 100”c. PAUL BENDER, GEORGE T. FURUKAWAl, AND JOHN R. HYNDMAN2 University of Wisconsin, Madison, Wis.
T
HE number of compounds for which there have been reported accurate vapor pressure data for the high temperature range is quite limited. To assist in supplying such information for materials of both industrial and academic interest there has been undertaken a t this laboratory a research program, the first results of which are reported in this paper. The vapor of benzene in the pressure range from slightly below 2 atmospheres to the critical point has been measured, with an estimated error of approximately 0.1 yo. The critical constants have been evaluated by means of compressibility measurements in the critical region. EXPERIMENTAL DETAILS
Apparatus. The apparatus constructed for this work was similar in design to that employed in studies of the compressibilities of gases by Beattie (Z), whose comprehensive description may be consulted for details. The sample under investigation was confined by mercury in a borosilicate glass liner in a stainless steel bomb. Initially liners of the design specified by Beattie and shown in Figure l a were used. The type shown in Figure l b was subsequently adopted because i t provided greater convenience and reliability in loading the bomb; loss of a purified sample through shattering of the liner on loading was almost completely eliminated. The thickness of the glass septum is not critical; diaphragms from 0.002 to 0.006 inch thick have been used successfully. The high temperature thermostat was essentially of the type described by Beattie ( 8 ) ; the bath fluid employed was the Socony-Vacuum Company’s Valrex oil A. Forced Ventilation was necessary with this medium a t temperatures above 250” C. Present address, National Bureau of Standards, Washington, D. C Arsenal Research Division, Rohm & Haas Co., Huntsville, Ala. 1
* Present address, Redetone
The control element in the thermostat was a resistance thermometer, of low thermal lag, which formed one arm of Mueller type direct current-operated Wheatstone bridge. The bridge output voltage a t unbalance was amplified by conventional means and applied as the control voltage in the phase-shifting thyratron circuit described by Reich (11)to regulate the current flowing through the control heaters in the thermostat. The temperature regulation obtained was f0.001”up to 200” C., &0.002” up to 250” C., and &0.003” up to 300” C. Temperature measurements made in rapid succession on each of several platinum thermometers in different positions in the bath verified the uniformity of temperature throughout the thermostat. All temperature measurements here reported are referred t o the International Centigrade Scale, and were made with research type platinum resistance thermometers obtained from the Leeds and Northrup Co. Several such thermometers, calibrated a t the National Bureau of Standards, were used interchangeably in the work. The resistance measurements were made with a thermostatted Rubicon Mueller bridge which was calibrated by means of an N.B.S. type standard resistor supplied by the University of Wisconsin Electrical Standards Laboratory. A thermometer current of 1.25 ma. gave a thermometric sensitivity of approximately 5 mm. per millidegree with the galvanometer scale a t two meters. A dead-weight gage of the type described by Keyes (7)was used in the pressure measurements. The piston-cylinder combination, of nominal effective diameter of 0.5 inch, was supplied by the gage division of the Pratt and Whitney Co. All weights of 100 grams and over were machined from stainless steel and calibrated against laboratory standards using a Paul Bunge balance of 5-kg. capacity and a sensitivity of the order of 5 mg. In the smaller sizes analytical weighb were used.
388
INDUSTRIAL AND ENGINEERING CHEMISTRY
The gage was calibrated against the vapor pressure of carbon dioxide a t the ice point, as recommended by Bridgeman (3). The carbon dioxide was prepared by the thermal decomposition of analytical grade sodium bicarbonate and dried by means of a dry ice-alcohol trap followed by four columns of magnesium perchlorate. The dry gas was condensed by means of liquid air and then resublimed six times to ensure the removal of permanent gltaes before it was transferred into the bomb. A precision of 1 part in 7000 was achieved in the gage calibration, in which several different samples of carbon dioxide were regularly used as a check. The sensitivity of the gage and pressure equilibrium detector was better than 0.001 atmosphere,
7 b
Vol. 44, No. 2
The volumnometer, which was required for the establishment of the isotherms in the critical region used for the location of the critical point, gave a maximum displacement of 200 cc. It was thermostated in oil a t 30’ f 0.02’ C. and was calibrated before use. Preparation of Materials. The water samples used were of conductivity grade. The benzene samples employed were drawn from three different sources: 1. Reagent grade benzene was treated with concentrated sulfuric acid until it gave a negative test for thiophene with isatin. The sample was then washed repeatedly with water and dried, first with calcium chloride and then with sodium wire. After distillation from fresh sodium wire the benzene was subjected to slow fractional crystallization according to the second method of Schwab and Wichers ( l a ) . 2. Two 200-gram samples of research grade benzene, from lot 57 were obtained from the Phillips Petroleum Co. This material had been certified by the National Bureau of Standards to contain 99.93 i 0.03 mole yobenzene, the most probable impurity being cyclohexane. 3. “Thiophene-free’’ reagent grade benzene was dried with sodium wire and distilled a t a 20 to 1 reflux ratio through a 30plate Oldershaw column. The constant boiling fraction was segregated for use.
The purified liquid was placed for degassing by repeated distillation under vacuum in an all glass apparatus, of the type described by Keyes (7), t o which the sample bulbs were sealed. After proper degassing a suitable quantity of liquid was condensed in the sample bulb, frozen out by means of a dry icealcohol bath, and the bulb sealed off under high vacuum.
EQUILIBRIUM
Figure 1. Types of Glass Liners Employed as Sample Holders A special lightweight weight pan was provided to extend the range of the dead-weight gage down to approximately 1.7 atmospheres. To permit a check on the calibration of the gage and to provide an independent means of pressure measurement a t the lower pressures, the mercury manometer shown in Figure 2 was constructed; the 10-foot borosilicate glass manometer tube, 14 mm. inside diameter, was obtained from the Corning Glass CO. The stainless steel tube forming the enclosed arm of the manometer was carefully machined to a uniform internal diameter so that the displacement of the mercury41 interface from its initial setting a t the electrical contact point could be calculated from the measured change in level in the calibrated glass arm. Copperconstantan thermocouples were distributed along the mercury and oil columns to permit the necessary temperature measurements. A11 measurements of length were referred to a Gaertner Type M-901 precision cathetometer. The probable error in pressure measurements with this manometer was approximately 1 mm., corresponding to a maximum percentage uncertainty of 0.05%. The dead-weight gage and the manometer were always found to be in good agreement, usually within 1 mm. In the vapor pressure measurements the contributions to the pressure of the hydrostatic heads of oil, mercury, and sample were carefully established, and a correction was made for the effect of capillary action in the pressure equilibrium indicator column. The correction for the partial pressure of mercury vapor in the bomb was made on the basis of Dalton’s law, with due allowance for the Poynting effect (9) where it was significant.
OIL INJECTOR
Figure 2.
General Design of Mercury Manometer Assembly
Measurements of the vapor pressure for various vapor-liquid volume ratios were made at several temperatures and confirmed the high quality of the samples used in the measurements. All mercury used was washed successively with acid and alkali and then distilled twice under vacuum. EXPERIMENTAL RESULTS
The apparatus and general operating procedures were tested by making measurements on water, both before other work was undertaken and a t intervals thereafter. The results of these
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1952
I
check runs were compared with values reported from the National Bureau of Standards (8),as obtained by interpolation of their tabulated data; the excellent agreement found is shown in Figure 3.
389
I
I
I
I
SAMPLE I
+0.02
en
e
+0.01
c n w
I
I
100
140
I
E
I
I
220 TEMPERATURE, "C (INTI
0
' " 0
I
180
260
I
300
Figure 4. Comparison of Observed and Calculated Values for Vapor Pressure of Benzene
B
0
E
G-0.0I a$ '2
a=
\
\
-0.02 -
I 20
The compressibility isotherms determined in the critical region and shown in Figure 5 have been interpreted to give the following values for the critical constants:
o
\
?'
I
160 200 240 TEMPERATURE, "C. (INTI
Critical temperature = 288.94 f 0.05' C. (Intern.) Critical pressure = 48.34 f 0.03 standard atmospheres Critical volume = 0.253 f 0.03 liters per mole
28(
A less comprehensive check run gave results entirely consistent with these values.
Figure 3. Comparison of Results Obtained in MBasurements on Water with Those Reported from National Bureau of Standards The vapor pressure of benzene was measured a t over 200 temperatures in the course of eight runs. Three runs were made on material from sample 1 before the manometer and small weight pan were constructed. Subsequent runs were made 011 the certified research-grade benzene and sample 3. The extension of the measurements in these runs to pressures below 2 atmospheres made possible a comparison with the results previously reported by Smith (19),which have been confirmed in this laboratory by measurements made by Quinn (10). The values reported here converged to good agreement with those of Smith and Quinn in the common range covered by the investigations. The analytic representation of these results was hence based on the equation of Smith for the lower pressures, a modifying deviation function being added a t higher temperatures:
t
5 125' C .
log P = 4.02441 125" < t log P = 4.02441
-
48.40 c al
e
288 84 I
I
0.21
1211.215 -t 220.87
I
I
0.23
I
0.25
I
I
0.27
I
I
0.2s
VOLUME, liters per mole
+
< 289'
I
Figure 5.
Isotherms for Benzene in Critical Region
C.
+ 4.35 X - t 1211'215 + 220.87
(t
6.24 X 10-9 ( t
-
125)
+
- 125)$
where P = vapor pressure, standard atmospheres, and t perature on International Centigrade Scale.
I :
DISCUSSION
(2)
tem-
Deviations of the experimentally observed values from those calculated from the equations are shown in Figure 4; Table I gives calculated values for the vappr pressure of benzene at 10' intervals from 100" t o 280' C. Figure 4 shows that the measurements on samples 2 and 3 were given the heaviest weight in the correlation of the results at the lowest temperatures. This procedure was followed because in these runs the widest pressure range was covered, and the measurements for sample 2 were made on material of unquestionable purity.
A comprehensive summary of earlier measurements in the high pressure range was assembled by Gornowski, h i c k , and Hixson (6) for comparison with their work. Of these it is pertinent here to consider only the results of Young (16) and von H u h (6) in addition to those of Gornowski et al. With the present work these constitute four investigations, none of which can properly be cited as unequivocal support for any other. Comparison in Table I shows that from 130' t o over 200' C. the values of Young and of Gornowski, h i c k , and Hixson differ by only 0.1 to 0.2570. Young, however, also reported measurements extending into the lower pressure range which, by comparison with the well confirmed results of Smith may be shown to be consistently low by approximately 1%. The agreement noted between Young and Gornowski et al. hence does not ac-
INDUSTRIAL AND ENGINEERING CHEMISTRY
390
TABLEI. COMPARISON
VALUES OF VAPOR PRESSURE OF BENEZENE OTAINED IN VARIOUS INVESTIGATIONS
Temp., ’ C. (Intern.)
This work
100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280
1.777 2.311 2.959 3.738 4.663 5.751 7.016 8.474 10.148 12.051 14.205 16,628 19,345 22,38 25 I75 29.50 33.64 38.21 43.27
i
OF
Pressure, Standard Atmospheres Gornowski yon Huhn (6)
1.820 2.983 4.660 6.988 10.‘1’1
i4,’is 19.35 25.76 33.58 42.93
et
(6)
.
.
I
... 3.717 4.629 5.699 6.943 8.379 10.028 11.908 14.038 16.441 19.139 22.17 25.56 29.35 33.58 38.23 43.34
young (15)
::::: 1.757
3.711 4.632 5.700
:!:;:
10.040 11,901 14.013
~~:~~~
22,14 25.46 29.19 33.33 37,96 43.13
tually provide support for the latter results, particularly in view of the lack of agreement a t higher temperatures. A parallel situation is found in the comparison of the authors’ data with that of von Huhn. A reasonably satisfactory agreement exists for teniperatures from 140’ to 260’ C., while for the highest and lowest values the differences increase. The experimental techniques employed by von Huhn could be expected, however, to yield least accuracy a t the lowest temperatures, so the comparison is possibly more satisfactory in this case than in that preceding. Of major concern is the lack of agreement between the present results and those of Gornowski et al. At the highest temperatures the comparison is complicated by the question of the necessity of correcting for the presence of mercury in the vapor phase. Because the discrepancy persists a t lower temperatures, where the indicated correction is negligible even if required, consideration of the work of Smith ( 1 3 ) ,Quinn ( I O ) , and Zmaczynski (16) is apposite. The lowest temperature for which data are given by Gornowski et al., is 130’ C., whereas the range of Smith’s work extended to 104’ C., that of Quinn to 110” C., and that of Zmaczynski to 121.7’ C.; the following comparison hence involves an undesirable but unavoidable short extrapolation. Although the coverage of recent work ( 4 , 14) on the vapor pressure of benzene a t the National Bureau of Standards has been limited to pressures up to atmospheric, their equations lead to calculated values a t temperatures up to 110’ C., which are in excellent agreement (maximum deviation less than 0.101,) with the directly observed values of Smith and Quinn. Extrapolation of the data of Zmaczynski to 130’ C. gives a value for the vapor pressure of benzene which is 0.270 higher than that of the present work, which in turn lies 0.6% above that of Gornowski el al. Furthermore, comparison of the results of Zmaczynski with those of Smith and Quinn suggests that the data of Zmaczynski may be
Vol. 44, No. 2
slightly high at the higher temperatures; this, if true, would yield an even more favorable comparison with the authors’ results. On the other hand, it is acknowledged that the results of measurements on sample 1 show a trend to the high side at lower temperatures which could be interpreted as favoring Zmaczynski’s results. Because these measurements were not prosecuted to low enough temperatures however, it is impossible to eliminate the possibility that the trend was due to some undetected small error. In any case, the general comparison indicates a divergence of no more than 0.2% at most between the present data and the only extant results of adequately confirmed accuracy. This convergence of results to agreement with those of Smith, Quinn, and Zmaczynski and the accuracy attained in the authors’ measurements on water provide the best available evidence for the validity of the present work. It does not appear profitable to speculate on possible explanations for the discrepancies between the various sets of values reported for the critical constants because of the difficulties inherent in such measurements. ACKNOWLEDGMENT
This research was supported in part by the Research Committee of the Graduate School of the University of Wisconsin from funds supplied by the Wisconsin Alumni Research Foundation, The authors also wish to express their appreciation to J. B. Davis, Lloyd Lincoln, and Leonard Perwitz, whose assistance in the construction of the apparatus was a major contribution to the success of,the work. LITERATURE CITED
(1) (2) (3) (4)
Beattie, ,J. A,, Proc. Am. Acad. Arts and Sci., 6 9 , 3 8 9 (1934). Beattie, J. A,, Rev. Sci. Instruments, 2, 458 (1931). Bridgeman, 0. C., J. Am. Chem. Soe., 49, 1174 (1927). Forziati, A. F.. Norris, W. R., and Rossini, F. D., J . Research
Natl. Bur. Standards, 43, 555 (1949). (5) Gornowski, E. J., Amick, E. H., Jr., and Hixson, A; N.. IND. ENG.CKEM.,39, 1348 (1947). (6) Huhn, von W., Forsch. Gebiete ingenieurw., A2, 109 (1931). (7) Keyes, F. G., Proc. Am. Acad. Arts and Sci., 68, 505 (1933). (8) Osborne, N. S., and Meyers, C. H., J . Research Natl. Bur. Standards, 1 3 , l (1934). (9) Poynting, J. H., Phil. Mag., 1 2 , 3 2 (1881). 3
(10) Quinn, G., unpublished results, Univ. Wisconsin (1949). (11) Reich, H. J., “Theory and Applications of Electron Tubes,” p. 514, New York, McGraw-Hill Book Co., Inc., 1944. (12) Schwab, F. W., and Wichers, E., J . Research Natl. Bur. Standards, 2 5 , 7 4 7 (1940). (13) Smith, E. R., Ibid., 26,129 (1941). (14) Willingham, C. B., Taylor, W. J., Pignocco, J. M., and Rossini, F. D., Ibid., 35,219 (1945): (15) Young, S., Proc. Roy. Soc. Dublin, 12, 374 (1910). (16) Zmaozynski, A., J . chim. phys., 27, 503 (1930); Trabajos del IX Congreso intern. de Quimica, Quimica Fisica, 2 , 225-36 (1934). See also Swietoslawski, W., “Ebulliometric Measurements,” p. 66ff., New York, Reinhold Publishing Corp., 1945. RECEIVED June 1,1948.
