MEASUREMENT OF LOW VAPOR PRESSURES
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2. The formation constants and the complexity constants have been calculated from the formation curves. 3. Copper forms only a 1:1 complex ion with triethylenetetramine. 4. Nickel forms a 1:1 and a 3 :2 complex ion with triethylenetetramine. 5. Approximate values of the heats of formation of the complex ions have been calculated. 6. The free energies of formation of the complex ions have been calculated. 7. The data available indicate that certain steric factors may play a significant role in the coiirdination tendency of triethylenetetramine. Acknowledgment is extended to Dr. Theodore H. Dexter, who made the absorption and continuous variation measurements reported in this paper. REFERENCES (1) ACKERMANN, It., P R U E , J . E.,
G.: Nature 189, 723-5 (1949). (2) BJERRUM, J . : Metal Anaininc Forniation i n Aqueous Solution. Theory of the Reversible Step Reaction. P. Hsase and Son, Copenhagen (1041). (3) RJERRUY,J., AND ANDERSEN, P.: Kgl. Dsnske Vidcnskab. Selskab., Mat.-fya. Medd. 22, No. 7 , 21 (1946). (4) JONABSEN, H. B., AND DOUGLAS, 13. E . : J . Am. Chem. 8 o c . 71, 4094 (1949). (5) JONASSEN, H. B . , LEBLANC, R . 13.. ~ I E I B O H A.M R,. , A N D ROC:.4N, R . h f . : J . .4m. Climi. SOC. 73, 2430 (1050). AXD SCHW.4RTZENBACH,
THE MEASUREMENT OF LOW VAPOR PRESSURES BY MEANS OF A MASS SPECTROMETER' A. W. TICKNER'
AND
F . P. LOSSING
Division of Chemistry, National Research Laboratories, Ottawa, Canada Received J u l y 8 , 1950 INTRODUCTION
In the study of the kinetics of hydrocarbon reactions, low-temperature fractionation is an important aid in the separation and identification of products. Fractionations are often most efficient a t pressures below 0.01 mm. of mercury, so that it is important to know the vapor pressure of the hydrocarbons down to 0.01 mm. or leas. The vapor pressures of many heavy hydrocarbons have been determined down to very low pressures by the effusion method of Knudsen (5) or by the use of the Rodebush manometer (7, 8). For the more volatile hydrocarbons, direct pressure measurements on the vapor are more convenient. However, direct measurements of vapor pressure are subject to errors due to the presence of volatile impurities. At temperatures where the vapor pressure is small, Contribution No. 2406 from the Kational Research Council, Ottawa. Research Laboratories Post-Doctorate Fellowship.
* Holder of a National
734
A . W. TICKNER .4ND F. P. LOSSING
the gas phase may contain relatively large amounts of volatile impurities, even though the hydrocarbon in bulk is extremely pure. Because the degree of purity required is prohibitive, few reliable vapor pressure data for the lower hydrocarbons exist below 1mm., and, in fact, few attempts to obtain such data have been reported. The National Bureau of Standards of the United States (11) has reported the vapor pressures of the lower hydrocarbons only down to 10 mm. In Stull’s compilation (14) values of the vapor pressure are given down to 1 mm. Delaplace (2) made use of the dependence of thermal conductivity on pressure, and has reported vapor pressures for the lower hydrocarbons down to 10-4 mm. Bourbo (l), using a McLeod gauge, has measured the vapor pressure of very pure acetylene down to lo-‘ mm. Smith (12,13) has published a convenient chart in which the available data at higher pressures have been extrapolated to 10-8 mm. All of these data are subject to the objection mentioned above. This difficulty would be avoided if the partial pressure of the hydrocarbon in question, rather than the total pressure, could be measured. Partial pressures can be measured by the mass spectrometer under conditions where a linear relationship exists between the intensity of the ion beam due to a given substance and the partial pressure of that substance behind the mass spectrometer leak. Honig (4) has discussed the conditions under which a linear relation can be obtained below a limiting pressure where viscous flow through the leak becomes important. MATERIALS
The methane, ethylene, ethane, propylene, propane, I-butene, Ti-butane, and 2-methylpropane used in this work were Phillips “research grade” materials, with a rated purity of 99.7 per cent or greater. The acetylene and carbon dioxide were commercial gases of better than 99 per cent purity supplied by the Dominion Oxygen Company. The acetylene was separated from the solvent acetone by low-temperature fractionation. The cyclopropane was an Ohio Chemical Company U.S.P. grade chemical, with a rated purity of 99.5 per cent; a middle fraction was used for these measurements. The n-pentane was from a sample supplied by the National Bureau of Standards, with an impurity content of 0.15 f 0.07 mole per cent. METHOD
The apparatus used for controlling the temperature of the sample is shown in figure 1. It is of a type in general use in these laboratories. Copper-Constantan thermocouples were placed in contact with the central Pyrex tube containing the material whose vapor pressure was being measured. Thermocouple No. 1, used for measuring the temperature of the condensed sample, was placed at the bottom tip of the tube. Thermocouple No. 2, situated about 2 cm. up the side of the tube, was used to check the temperature gradient up the tube. The central tube and thermocouples were covered with a layer of aluminum foil, to ensure a uniform temperature gradient, and over this was wound an insulating layer of glass tape. The heater winding was of No. 26 B. and S. gauge Chrome1 A wire. It was wound on with the turns becoming progressively farther apart near the bottom
MEASCREMEST OF LOW VAPOR PRESSURES
735
of the tube to ensure that the bottom tip of the tube was the coldest spot. The central tube assembly was surrounded by a Pyrex jacket, which could be evacuated to slow the rate of heat transfer and give better temperature control. The entire assembly could be immersed in liquid nitrogen or, for the low-temperature work with methane, in liquid hydrogen. The level of the cooling liquid was maintained nearly constant during operation.
Fif;. 1. Low-temperature thermostat
By adjusting the voltage across the heater winding any temperature above that of the surrounding bath could be maintained. To obtain the necessary degree of control, the line voltage mas regulated by a Sorenson Model loo0 regulator. The required voltages were obtained from this regulated supply by means of an adjustable autotransformer. The thermocouple used for the precise measurements of temperature was calibrated in the completed apparatus, using nitrogen, oxygen, and carbon dioxide. Each of the substances used for this purpose was condensed into the trap and subsequently vaporized in a closed system containing a barometric leg. The temperature of the trap was raised until the pressure in the system became equal to that of the atmosphere. When small opposite changes were made in the heater voltage, the pressure increased or decreased slightly. Under these conditions the rate of heat transfer through the layer of glass separating the thermocouple from
736
A . W. TICBNER AND F. P. LOSSINQ
the sample should be a minimum, and any temperature gradient through the glass should be nearly the same as during an actual vapor pressure determination. The thermocouple was recalibrated several times throughout the course of the work. The deviations from the accepted values of the electromotive force for copper-Constantan couples at these calibrating points were found to be a linear function of the temperature and, in accordance with the work of Scott (lo), the points were joined by a straight line which was extrapolated to cover the temperature range used. I n this manner the temperature of a sample could be measured with an accuracy of better than +0.3"C. a t temperatures a t or above the boiling point of nitrogen. At the lower temperatures used with methane, the uncertainty in the thermocouple calibration, together with the fact that the change in thermocouple E.M.F. with temperature becomes smaller, caused the accuracy of the temperature measurements to be less. The mass spectrometer used in this work was a 90"deflection instrument of the type described by Thode (15). A single glass diaphragm leak with an aperture 25 microns in diameter and 40 microns long was used. In a separate experiment it was shown that the intensity of the ion beam was a h e a r function of the partial pressure up to pressures of about 0.04 mm. in the chamber behind the leak. Above this pressure, deviations from linearity became appreciable. Therefore, all vapor pressures above 0.030 mm. were measured by expanding a portion of the equilibrated vapor into the leak chamber from suitable calibrated volumes. The volume of the leak chamber used was about 2 1. Vapor pressures up to 0.03 mm. were measured by admittingthe vapor directly to the leak chamber, pressures from 0.03 mm. to 0.1 mm. by expanding the vapor from a 500-ml. volume into the leak chamber, pressures from 0.1 to 1.0 mm. by expanding the vapor from a 30-ml. volume, and pressures from 1 mm. to 10 mm. by expanding the vapor from a 1-ml volume. Pressures above 10 mm. were measured directly on a mercury manometer. The ion peak used for measuring partial pressures must be free from any ion contribution from the impurities present. This was accomplished by selecting a part of the mass spectrum in which the ratios of the peak heights remained constant down to the lowest pressures measured. Since the interfering impurities are materials more volatile than the substance being measured, in general the impurities only contribute to peaks lower in mass number than the parent peak of the substance in question. Thus the parent peak is usually suitable. From a knowledge of the peak height corresponding to a given temperature, the sensitivity of the instrument to that peak in deflection per micron pressure, and the expension ratio, the vapor pressure can be calculated. The sensitivity of the instrument to the various hydrocarbons was measured by expanding samples of known pressure and volume into the calibrated leak chamber. DISCUSSION OF RESULTS
The results obtained are given in figures 2 and 3.3Selected values of the vapor pressures are given in table 1. A comparison with other data at pressures down 8 Large copies of figures 2 and 3 may be obtained from the authors in care of the National Research Laboratories at Ottawa.
0 I/T
10‘
FIG.2. Vapor pressures 10000
F
100.0
o
Ttcknrr
a n d Lorrlng
10.01-----
,“L
o.o.k 0.1
0.001
30
60 I/T
10 IO*
FIG.3. Vapor pressures 737
BO
90
100
738
A.
W. TICKNER AND F. P. LOSSING
I
p Y
MEASUREMENT OF LOW VAPOR PRESSURES
739
to 0.001 mm. has been made for propane in a previous publication (16). In general, this is typical of the agreement where other data are available. It can be seen from figures 2 and 3 in this paper that the results obtained here are in good agreement with the data of the Sational Bureau of Standards (11). For acetylene the agreement with the values of Bourbo (1) is good, although the values obtained in this work are slightly lower. This could easily be accounted for if the acetylene used by Rourbo contained a very small amount of a volatile impurity. .4 comparison with the chart of Smith (12, 13) shows that his extrapolations lead to values which are too high. This is to be expected, since the lowest data on which his extrapolations were based are probably high, owing to the presence of volatile impurities (see, for example, figure 1 of reference 16). In some cases the high values which he obtained are partly due to the change in the slope of the line at the melting point, as for ethylene. Generally, the values of Smith appear to be too high by a factor of 2 or 3 at a pressure of 0.01 mm. It was possible in some cases to obtain approximate checks on the values reported for the melting points, since the slope of the vapor pressure curve undergoes a sharp change at the melting point. In fact, with cyclopropane and ethylene it was possible to obtain values of the equilibrium vapor pressure for some distance along the “supercooled” liquid curve. An abrupt rise in the temperature of the sample was noted when the sample began to freeze. The values of the triple points obtained are: for methane, -182.8OC. (cf. -182.48’C. (11)); for ethylene, - 169.1%. (m.p. - 169.15OC. (3)); and for cyclopropane, - 127.4’C. (m.p. - 127.53”C. (9)). For other gases, such as ethane, 1-butene, n-butane, and 2-methylpropane, where the triple points are close to the bottom of the pressure ranges measured, the effect was not observed. I n these cases the values obtained for the vapor pressures may not have been sufficiently accurate to show the change in slope, or the liquid may have been supercooled down to the lowest temperatures employed. The lower limit for which vapor pressure measurements mere practical with the size of leak used and the resulting sensitivities was mm. The method could be extended to much lower pressures by using leaks of larger diameter. In addition, the “thermolecular pressure difference” investigated by Knudsen and others (6, 17) begins to become appreciable for some gases below about 0.01 mm. (see, for example, the lowest values obtained for methane and acetylene). The inside diameter of the low-temperature trap was 1.G cm. The amount of the effect is doubtful in the intermediate region and the simple square-root relation does not apply. Conversely, this method could be used as a means of investigating the Knudsen effect in the intermediate pressure region. The method is extremely rapid, since pressure readings can be obtained as fast m equilibrium can be established. I t was found that a complete curve for one hydrocarbon could be obtained in about 8-10 hr. The quantity of material needed is only about 0.02 mole for the present apparatus, and this could be considerably reduced with a suitable design of trap and expansion bulbs.
740
H. B. VAN DER HEIJDE AND A. H. W. ATEN, JR.
REFERENCES (1) BOURBO, P. S.:J. Phys. Chem. (U.S.S.R.) 18, 253 (1944);J. Phys. (U.S.S.R.) 7, 286 (1943). (2) DELAPLACE, R.: Compt. rend. 201,493 (1937). (3) EQAN, C. J., AND KEMP,J. D.: J. Am. Chem. SOC.69, 1264 (1937). (4) HONIQ, R. E.:J. Applied Phys. 16, 646 (1945). (5) KNUDSEN, M.:Ann. Physik 28,999 (1909);29.179 (1909). (6)KNUDSEN, M.: Ann. Physik 51,205,633(1910). (7) RODEBUSH, W.H., A N D COONS,C. C.: J. Am. Chem. SOC.49, 1953 (1927). (8) RODEBUSH, W.H., A N D HENRY,W. F.: J. Am. Chem. SOC.62,3159 (1930). (9) KNOWLTON, J. W.,AND ROSSIXI,F. D.: J. Research Natl. Bur. Standards 4S, 113 (1949). (10) SCOTT, R.B.:J. Research Natl. Bur. Standards 26,459 (1940). (11) Selected Values of Properties of Hydrocarbons, National Bureau of Standards, A.P.I. Project 44 (1949). (12) SMITH,R . V.:U. S. Bur. h h e s Inform. Circ. No. 7216. (13) SMITH,R . V.:Pctroleuni Refiner 22, 19 (1943). (14)STULL,D. R.: Ind. Eng. Chem. SQ, 517 (1947). (15) TEODE, H.G., GRAHAM, R. L., AND HARKNESS, A. L.:J. Sei. Instruments 24,119 (1947). (16) TICKNER, A. W.,AND LOSSINO, F. P.: J. Chem. Phys. 18, 148 (1950). (17) WEBER,S.,AND ScHafmT,G.: Commun. Phys. Lab. Univ. Leiden No. 246C; 7th Congr. Intern. Froid, 1st Comm. Intern., Rapports et Commun. June, 1936,61-73.
ADSORI'TIOS CURVE OF A RARE EARTH I N AN IONEXCHANGE COLUMN H. 13. VA?; DER IIEIJDE
AND
A. H. W. ATEN,
JR.
Institute ,for .!'itclear Research, Amuterdam, Holland I
