V O L U M E 23, NO. 5, M A Y 1 9 5 1
139
tion ranges, is required; and in which, in all accessible concentration ranges, the results are relatively insensitive to fluctuations in the composition of the matrix gas. For routine semicontinuous analysis of the water content of streaming industrial gases, a relatively simple apparatus can be used. 9 linear train of the following components promises to furnish the desired resulk
1. A flow controller, consisting of a diaphragm valve, to control the gas pressure at the inlet, and an orifice or capillary to govern the rate of flow under this pressure. 2. A small flowmeter to show that the flow is of the correct order of magnitude. The flow need not be closely controlled. 3. A reaction cell and measuring bridge, as shown in Figure 1. 4. A saturator, consisting of an intimate mixture of a salt hydrate and its anhydride, held in a constant temperature (ice) bath. The chosen hydrate should furnish an aqueous tension of the same order of magnitude as that to be measured. The output of the saturator must be accurately reproducible, but it need not be a perfect equilibrium mixture. The actual percentage of water in the hydrated gas can be determined &ally by an absolute (gravimetric) method. 5 . A second cell exactly like the first. (3. A guard tube containing a desiccant and leading to the vent. The null readings of both cells ought first to be determined by passing a perfectly dry gas through the train with the saturator
temporarily by-passed. The train can then be reconstituted and used without any further calibration; the data supplied by the second cell serve as a reference standard in terms of which the readings of the first cell can be interpreted. The calibration curves are essentially linear. Therefore the reading of the second cell with the gas of known water content delivered from th? saturator, taken together with the null reading of that cell, serves to define the calibration line appropriate to the composition and flow rate of the gas passing through the system. With this calibration line the original water content of the test gas can be easily and uniquely determined from the reading of the first cell. ACKNOWLEDGMENT
Much of the preliminary work in the development of this analytical system was carried out by J. A. and Peter Kafalas. LITERATURE CITED
(1) Cohn, Gunther, ANAL.CHEM.,19,832 (1947).
( 2 ) Greenhill, E. B., and Whitehead, J. R., J . Sci. Insfrunients, 26,
92 (1949).
(3) Metal Hydrides, Inc., Beverly, Mass., Pamphlet, "Calcium Hy(4)
dride.'' Walker, A. C., and Emst, E. J., Jr., IND.ENG.C H m f . , .IN.u.. ED., 2, 134 (1931).
RECEIVED September 7,
1950.
Precision Multiple Sorption-Desorption Apparatus W. 0. MILLIGAN, WARREN C. SIMPSON', GORDON L. BUSHEY2, HENRY H. RACHFOKD, JR.3, AYD ARTHUR L. DRAPER The Rice Institute, Hotiston, Tex.
A precision multiple apparatus has been constructed which is capqble of determining fifteen sorptiondesorption isotherms or isobars simultaneously, thus permitting the rapid accumulation of accurate sorption data. The apparatus is based on the gravimetric principle. Weight changes are followed by silica spring balances. A micrometer slide and microscope are moved into successive spring balance positions by means of a hand-operated or motordriven screw. The reproducibility of the readings is independent of the screw mechanism. Extensive evacuation and degassing ensure removal of air from
T
HE determination of the amount of adsorption of gases or
vapors on solids has usually been carried out by volumetric methods. Gravimetric methods possess the advantage of permitting the simultaneous examination of several samples, and do not require a determination of the volume of the adsorbent. Since the classic experiments of McBain and Bakr (8) who first employed fused silica spring balances, the gravi. metric method has come into widespread use. A multiple apparatus consisting of six McBain-Bakr balances has been described by Stamm and Woodruff (1.9). In this device, each of six silica spring balances may be rotated in turn into position before a common measuring microscope. Harris, Ott, and Arnold (2) have discussed a similar apparatus. In this laboratory there has been constructed a multiple sorption-desorption apparatus capable of following fifteen samples Present address, Shell Development Co., Emeryvilla, Calif. Present address, Department of Chemistry, University of Illinois, Urbana, Ill. 8 Present address, Humble Oil and Refining Co., Houston, Tex 1
2
the samples prior to the introduction of the sorbate. Equilibrium points are attained at a temperature constant to better than 0.001' C. and at prwures determined to within 0.007 mm. of mercury. Isotherms and isobars are usually obtained in the temperature range -20' to 60" C. and at pressures from 10-6 to 400 mm. of mercury. The apparatus is especially designed for the examination of hydrous oxidewater systems but is not limited to these materials. Adsorption and desorption isobars and isotherms have been obtained for over 500 samples, representing 15,000 equilibrium point readings.
simultaneously. Silica spring balances are used, and the measuring microscope is moved from balance to balance by means of a translatory motion, employing a screw. The apparatus is designed to yield accurate data in quantity with speed and convenience in over-all operation. APPARATUS
Fifteen fused silica springs are mounted in vertical positions in individual borosilicate glass tubes suspended in a thermostatically controlled bath. The springs employed vary in sensitivity from 0.3 to 40 mm. per mg. Each sample tube is approximately 1 meter in length, in order t o allow the use of the high sensitivity springs, and also to allow the springs to operate a t room temperature. This latter feature is essential to prevent the continuous sensitivity variations observed when silica springs are exposed to high pressures of water vapor a t elevated temperatures. Also, the springs exhibit a small negative temperature coefficient; in this apparatus, temperature corrections are employed. Figures 1
ANALYTICAL CHEMISTRY
740
In the determination of spring lengths, the microscope cross hair is adjusted tangent to the uniformly curved silica hooks comprising the ends of the springs. This procedure i8 oonsidered much superior to methods involving setting the cross hair on vertical silica fibers drawn to a sharp point. Measurement of the entire h g t h of the spring or the use oi the reference rod obviates the necessity of establishing n zero base point of reference. The measuring microscope may be moved to successive balance poaitinns by nieansof a hand-operatcd or motor-driven precision m e a . A lockiogdeviceprevents movement, of the microscope during readings at a particular balance position. The accuracy of the spring Figure 1. Sorption-Desorption A p p a r a ~ ~ s length measurements depends on the edibration of the micrometer slide and 2 illustrate photographically and diagrammatically the form scale, andnot upon theaocuracyof the screw mechanism employ^ of the apparatus. to shift the microscope from one balance position to mother. The silica springs are suspended from glass hooks attached t o hand-ground spherical joints. and the aluminum orphtinumbucketa are suspended by fused silica fibers. Changes in sample weight, and hence the amounts of sorption or desorption, are determined bv measurine the chance in the leneth of the 8Drine. At times %'reference rod, considing of a silica fiber suspbndd from the upper hook, is used to make possible exact measurement of,spring lengths in excess of the 100-mm. scale an tha measuring microscope. The spring lengths may be determincd to slightly better than *0.001 mm., using a Gaertner Model 11-342 micrameter scale.
Pressures are measured by means of a differential manometer filled with either mercury or Apieaon-B oil, depending on the ga6 or vapor used as adsorbate. -The manometer is maintained a t 25 + 0.01' C. in a water bath with glass windows. Manometer readines are made with an Ehrbaoh cathetometer. In most of the work carried out in this Iahoratory, m t e r vapor is used as the adsorbate, and the oil manometer makes possible determination of pressure to 0.007 mm. of mercury. The vacuum system consists of a liquid nitrogen trap, a trap filled'with phosphorus pentoxide spread within layers of borosilicate glass wool vhen water vapor is employed, or with charcoal or
-~ ~~
~
Figure 2. Sorption-Desorption Apparatus A. B. C. D. E.
Individual mountings for silica spring balance. Meanwing miomsoope Thermostatically controlled bath Precision screw to m ~ v B e Differential manometer
F. 6.
li-id nitrogen trap Drying trap € I DitTueion . pump I. Ionization ga*e 3. pirani Ease
K.
L.
M-M.
Ballnef d a m e Adsorbate sourae tubs Thyratmn =ontmUe=-thermmaGUletor
V O L U M E 23, NO. 5, M A Y 1 9 5 1 other adsorbents when adsorbates such as hydrocarbons are in use, a Distillation Products, Inc., water-cooled glass diffusion pump, tn-o-stage, filled with Octoil-S, and a mechanical fore pump. The ultimate high vacuum is measured with an ionization gage tube with a control circuit similar to that of Ridenour ( I O ) . The ionization tube is calibrated with a McLeod gage. The low vacuum is registered by a standard Pirani gage. The apparatus is constructed entirely of glass, and contains numerous stopcocks and spherical joints, all of which are handground for high vacuum use. Special lubricants such as ApiezonL grease are used. Preliminary tests were made using mercury cutoff stopcocks, but experience shows that the amount of grease vapor present in the apparatus is no more harmful than mercury vapor. In studying the phenomenon of sorption-desorption hysteresis, it is known (9) that the volume of the apparatus should be large compared with the volume of the samples, in order to provide essentially isobaric increments or decrements of sorbate. A 20-liter ballast volume is available in this apparatus, which permits an apparatus-volume to sample-volume ratio of the order of 10,OOO.
Gas or vapor adsorbate is introduced into the system through a stopcock from a tube which can be either heated or cooled; this tube is fitted to the system by a spherical joint. The pure water vapor used in most of the work carried out in this laboratory is obtained from the thermal decomposition of barium chloride dihydrate crystals placed in the source tube and carefully degassed a t liquid nitrogen temperatures before heating. The spherical joint and the ease of heating and cooling the adsorbate source make it readily adaptable to other gases and vapors. The constant temperature bath is especially designed for operation in the temperature range of -20" to +40" C. The tank consists of a double-walled, 15-gallon copper container with double glass windows to permit visual examination of the buckets during operation. The space between the copper walls is filled with Silocel insulation and a layer of drying agent, and is hermetically sealed with soft solder. -1dditional desiccant material is available between the double-glass windows to prevent deposition of moisture; this precaution is especially important when working a t the lower temperatures. The copper tank also has 1 inch of external cork, sealed m-ith an insulating paint. Temperatures below that of the room are obtained by a 0.75-horsepower Freon-12 compressor with a copper expansion coil within the bath. or runs continuously and the cooling effect is baladjustable fixrd heaters. A smaller heater ( 5 to in the anode rirouit of an FG-57 Thyratron tube controlled bj- a 200-ml. merc,ury-toluene thermoregulator a.ith Sichromc-mercury contacts in a 0.75-mm. bore capillary. The entire fluid ill t.he bath is circulated at a rate of about seven times per minute. The extreme constancy of the temperature in the bath is principally attributable to the large volume, the thorough stirring, the good insulation, and the seusitivity of the mercurytoluene thermoregulator. The choice of weight of the samples and the sensitivit,y of the springs depend primarily upon the amount of adsorption expected. Typical operating values for the adsorption of water vapor o n various hydrous oxides often require a sample weight of about 100 to 150 mg. in platinum buckets of about 75 to 100 mg.; a larger sample weight may lie used in aluminum buckets of about 10 to 25 mg. Aluminum buckets are fabricated from aluminum foil, and are pretreated with concentrated nitric acid to form a thin layer of alumina which renders the aluminum insensitive to the action of water vapor after accidental exposure to mercury vapor. Aluminum buckets not pretreated with nitric acid have often reacted with water vapor a t high relative humidities near saturation. It is believed that, the amount of mercury vapor often present in laboratories (because of accideutal spillage) is sufficient to catalyze the reaction with water vapor, especially in the absence of air a t high relative humidities. Blank isotherms on treated and nontreated aluminum buckets ha,ve demonstrated that the amount of water adsorbed by the invisible film of aluminum oxide is entirely negligible. Platinum buckets are fabricated from "dead soft" platinum foil, O.OOO5 inch thick, which is easily obtainable from dental supply houses. The platinum buckets are preferred, except in
141 instances where the weight of the bucket must be reduced to a minimum. Considerable attention has been given to the question of the effect of traces of mercury vapor or grease on the samples. Shidei (If) observed that certain samples of alumina developed a slightly yellow color after exposure to a vacuum stopcock grease for many days in a high vacuum. In this laboratory, Simpson obtained six isotherms, simultaneously, which agreed closely for water vapor on a silica gel, using mercury cutoffs inst,ead of grease-sealed stopcocks, The mercury cutoffs xere then replaced with hand-ground stopcocks, lubricated with Apiezon-L grease, and the isotherms were repeated on the same samples. The resulting set of six isotherms agreed closely with the first set. The silica gels were then left in the apparatus for several weeks in high vacuum (about 10-8 mm. of mercury) and possible changes in n-eight were sought which might indicate a slow adsorption or deposition of grease from the stopcocks. S o detectable change in weight was observed, but on removal of the samples from the apparatus, they were slightly yellow in color, in agreement with Shidei's results for alumina. These and other preliminary experiments suggest that the use of s t o p cocks and spherical joints is justified. The inherent' ease and rapidity of operation permit many more isotherms to be obtained than would be possible if samples were sealed into the apparatus by a glass-blowing technique, and gases and vapors added and removed from the system by grease-free methods. APPLICATIONS
The apparatus was primarily designed for the rapid determination of numerous water vapor isotherms on fifteen samples, siniultaneously. In addition to sorption-desorption measurements, phase rule isotherms and isobars have been employed to detect definite hydrates or to follow changes in hydrates or definite hydroxides. This apparatus was originally constructed in 1942, has been modified in various %rays,and has since that. t.inie been in almost constant use. At present over 500 samples have been examined, the resulting isotherms and isobars represeiit,ing over 15,000 equilibrium point readings. Some of the results of this work have been previously reported ( I , 4-8). ACKNOWLEDGMENTS
The silica springs described in this paper are obtained from the Houston Technical Laboratory, 2424 Branard St., Houston 6, Tex. The authors wish to express their gratihde to the following companieR for the fellowships granted some of the authors during the periods indicated: The Eastman Kodak Co., Simpson, 194243, Bushey, 1946-47; The Procter & Gamble Co., Bushey, 194748, Draper, 1948-50; and the Humble Oil and Refining Co., Rachford, 1945-46, Draper, 19.50-51. LITERATURE CITED
(1) Draper and hlilligan, Teras J . Sci., 2, 209 (1950).
Harris, Ott, and Arnold, Division of Colloid Chemistry, 104th Meeting AM.CHEM.SOC., Buffalo, N. Y., September 1942. (3) McBaiii and Bakr, J . Am. Chem. Soc., 4 8 , 6 9 0 (1926). (4) Milligan, Bushey, and Draper, J. Phys. & Colloid Chem., 55,
(2)
44 (1951). (5) Milligan and Rachford, J . Am. Chem. Soc., 70, 2922 (1948). (6) Milligan and Rachford, J . Phys. & Colloid Chem., 51, 333 (1947). (7) Milligan, Weiser, and Simpson, Division of Colloid Chemistry, 105th Meeting AM.CHEM.Soc., Detroit, Mich., .Ipril 1943.' (8) Milligan, Whitehurst, and Bushey, Oficial Digest Federation Paint & Varnish Production C h b s , No. 283, 601 (-\ugnst 1948).
Pidgeon, Can. J . Research, 10, 713 (1934). Ridenour, Rev. Sci. Instruments, 12, 134 (1941). (11) Shidei, M e m . CoEZ. Sci. Kyoto I m p . Univ., 99, 42 (1924). (12) Stamm and Woodruff, ISD.Esc,. CHZM.,Asat.. ED., 13, 836 (9) (10)
(1941)
.
RECEIVED M n y 20. 19.50. Preliminary report presented before the Division CHE\fICAL of Colloid Chemistry a t the 105th Meeting of the AMEKICAN SOCIETY, Detroit, Mich.
