Automatic Instrumental Methods for Determination of Critical Solution Temperatures JOHN R. MOSLEY, CLAUDE A. LUCCHESI',
and RALPH
H. MULLER
University o f California, Los Alamos Scientific Laboratory, Los Alamos,
Several instrumental means for the determination of critical solution temperatures using photoelectric detection and control are described. Heater control is effected either through a Schmitt trigger circuit or with limit switches mounted on standard recorders. Temperature is recorded on standard recording potentiometers. Precision of measurement to 1!=0.01' C. is obtainable although limitations in volume measurement and reagent purity may easily Iimit useful results to about f0.05" C.
A
PPARENTLY, Brown ( 2 ) was the first to use photoelectric methods for the automatic determination of critical solution temperatures-specifically, the aniline point of petroleum products. In his application a simple photoelectric relay wzm used to control the heating or cooling of the test sample and cause it to attain temperature equilibrium automatically. The pre.sent work was undertaken for two reasons. First, the authors believe that Brown's technique can find analytically useful extensions to many other chemical systems and, second, so many commercially recording techniques and instruments are now available that several new and original approaches are possible. This brief report affords the analyst several choices for the automatic recording of critical solution temperatures and for analytical information which such systems can provide. Each method reveals its own advantages as well as limitations and can best be adapted to the particular instrument which is a t hand. When a binary liquid-liquid system which exhibits the critical solution phenomenon is heated to a certain critical temperature, it passes from a state of miscibility to immiscibility, or vice versa, depending upon the particular system under study. This critical temperature depends not only on the components of the system, but also on their relative concentrations. Within certain concentration limits, depending on the system in question, the determination of the critical solution temperature constitutesasensitive quantitative analysis of the system. Conversely, when one component and the concentrations are given, a qualitative analysis may be made for the other component. This analysis is the basis for the ASTM ( 1 ) aniline point determination. Both methods require a previous calibration in which components and concentrations are known, but the literature contains many examples and even empirical equations for some series of compounds. If the system is stirred while in its immiscible state, a turbid mixture results; when the miscible state is reached, this turbidity disappears. Simple photoelectric methods can be used to detect this transition and to initiate a variety of control functions.
N. M.
parts: a main heater consisting of a 200-watt glass-insulated tape wrapped around the outside of the glass container, and a 10-watt control heater which was a coil of Xichrome wire immersed in the liquid. Both heaters were energized initially to bring the system up to the critical temperature rapidly. When the system reached the critical temperature for the first time, the main heater was permanently removed from the heating circuit by a lock-in relay. Temperature Determination. A copper-constantan thermocouple with reference junction a t 0" C. and a Type B Rubicon potentiometer were used for most of the temperature determinations. Control and Measurement. The transmission of light through the liquid system was measured with a Type 021 phototube with a projection type lamp, 75 watts a t 110 volts, as a light source. This lamp, however, was operated a t only 40 volts to provide a phototube signal of desired magnitude. The phototube was shielded from stray light with a metal box which had a window and collimating tube facing the liquid mixture. Several methods were used for the control and measurement of the critical solution temperatures. METHOD I. The phototube signal controlled a Schmitt trigger (6) circuit, which in turn controlled the heater. The mixture was heated to its critical temperature, around which it then hovered, alternately heating and cooling with a variation in temperature of about fO.05' C. The average was taken as the critical solution temperature. The temperature was measured either with a mercury thermometer or various combinations of thermocouple, Rubicon potentiometer, galvanometer, amplifier, and recorder. The potentiometer provided a wider range and higher sensitivity than obtainable with direct recording of
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EXPERIMENTAL
Glass Sample Container. A cylindrical shape was selected in order to use ordinary standard-taper glassware. The dimensions were adjusted so that a vessel using a 55/50 standard-taper top had a total volume of about 300 ml., and a 200-ml. sample was used each time. The upper portion of the vessel was fitted with a reflux condenser, two wells for heating coil outlets, and a hole for inserting a thermocouple. Magnetic stirring was used. Heating System. The heating system was divided into two 1
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Figure 1. Temperature vs. time with Schmitt trigger A. B.
Present address, Shell Development Co., Houston 2 5 , Tex.
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Control of heating Control of heating and thermocouple
1441
V O L U M E 2 7 , NO. 9, S E P T E M B E R 1 9 5 5 thermocouple electromotive force on standard strip chart recorders. 1. The first method of recording temperature involved the use of a Becknian Photopen recorder. This recorder follows the beam of light reflected from the potentiometer galvanometer and traces 9 a record of the deflections on standard recorder paper. Sufficient amplification f r o m t h e length of the light beam was obtained so that there was about 1.3 microvolts per division on the chart corresponding to about 0.03" per division, and the t e nperature could be read t o about ~k0.006" C. The texperature is read as a potentiometer setting plus a value read from the chart paper. 2. The second variation in measuring the temperature was to feed the unbalance voltage from the galvanometer binding posts of the Rubicon potentiometer to a Leeds & Pu'orthrup direct current microvolt IO a m p l i f i e r . This amplified unbatance voltage then was recorded on a 2.5-mv, full-scale Brown 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 recorder. Temperatures could be obtained from T E M P E R A T U R E - 'C. the potentiometer setFigure 2. Temperature c's. tine: Dlus chart readturbidity in arbitrary units in[ i g a i n t o a b o u t &0.006" C. Figure 1, A , is an example of the record obtained. Again, the temperature scale is to be added to the base tenperature obtained from the potentiometer setting. 3. A third variation to improve the readability of the temperature from the chart was to connect another relay to the Schmitt trigger so that when it turned off the heater, it also shorted the thermocouple leads, and the recorder pen moved to zero. When the heater was turned on again, the recorder pen returned t o a position corresponding t o the critical temperature. Thus a series of rusps was obtained from which the critical temperature could be read with no necessity for calculating or estimating the average value of a sawtooth line. Figure 1, B, is an example of this method. METHOD 11. A disadvantage of using a Schmitt trigger circuit for control in a turbidity measurement is that the circuit can be set to trigger at practically any degree of turbidity. Thus the critical te nperature obtained is varied somewhat by the rather arbitrary determination of triggering level. A calibration could be made with known mixtures, but this calibration would be affected if later determinations mere made with dark-colored substances. A more fundamental approach is indicated by an investigation of the actual changes in turbidity near the critical temperature. 1. A Speedomax S - Y recorder, 10-mv. full scale, was available for this portion of the investigation. The thermocouple electromotive force was recorded on the X axis with the aid of the Rubicon potentiometer and the Leeds & Xorthrup direct current amplifier. The phototube signal was passed through a simple cathode follower t o the Y axis of the recorder. As shown in Figure 2, a very sudden change occurs in the turbidity value a t a certain te nperature, after which the turbidity changes more gradually until a clear solution is reached, as shown in Figure 3 recorded a t loner sensitivity. The slowness of the total turbidity change is probably due to lack of thermal equilibrium, and the temperature a t the sharp break should be taken as the critical solution temperature. This sharp break in the turbidity curve occurs before any change is apparent to the eye. 2 . While the X-Y recorder affords a precise method of critical temperature determination, manual control of heating is required, and the recorder chart must be moved for every heating and cooling cycle to avoid overlapping the curves. This latter disadvantage can be avoided by the use of a multipoint recorder t o record both temperature and turbidity against time. A
Speedomax 10-mv. 10-point recorder with odd points wired to record turbidity and the even ones temperature was used to produce Figure 4. The critical temperature is found by observing the time a t which the sudden change in turbidity took place, and reading the temperature a t this time from the same chart. A continuous record of the same type may be made with a tm.0pen recorder. Figure 5 is a record obtained with a Brown twopen recorder, 10-mv. full scale. The pens are offset on the time scale so one may pass under the other. The turbidity curve was
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Figure 4. Temperature and turbidity US. time with manual control of heating, using 10-point recorder
ANALYTICAL CHEMISTRY
1442
arbitrarily cut off a t point .I because the important part of the curve is the initial sharp change, and the time of return depends merely on the manual control of the heating current. 3. The next step was to use the sharp change in turbidity to control the heater. For this purpose, the two-pen recorder was fitted with a standard Brown mercury switch controller on one pen. This mercury switch activated a high-current mercury plunger-type relay which controlled the heater. When the output of the cathode follower, a measure of turbidity, was connected to the controlling pen, and the temperature recorded with the other pen, the record reproduced in Figure 6 was obtained. The control point was set slightly after the sharp break in the turbidity curve to ensure positive control of the heater. Slight drifts in the turbidity record are apparent, but they are so small compared to the total turbidity change (many times full scale) that they have negligible effect on the control point. The temperature can be read as an average of the extreme values, or the time a t the sudden change in turbidity may be observed, and the temperature a t this time determined on the same chart, allowing for the time offset of the pens. This latter method affords the most preqise of all those mentioned for the determination of critical sooution temperatures. The successive values agree within d~0.01, and even the total excursions are only 2t0.025'.
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( 3 ) to behave in a similar manner. However, a t the aniline point of heptane (equal volumes of aniline and heptane) the pressure effect is evidently less, since a critical temperature of 87.98' C. was obtained, compared with literature values of 68" ( 7 ) , 70.6" (j), 68.9' ( 2 ) ,and 69.2" ( 1 ) .
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Table I.
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Temperature and turbidity vs. time with manual control of heating
Summary of Results for Hexane-Aniline System
% Hexane 10.40 10.40 10.40 13.20 13.20 19.40 19.40
Experiment Result, C. a t 590 Mm. H g
38.70 38.70 38.68 44.88 44.88 56.92 56.98
Literature Value C. (4) at
760'Mm. H g 30.0 30.0 30.0 40.0 40.0 50.0 50.0
RESULTS
Apparently very few studies have been made of critical solution temperatures at pressures below atmospheric. The effect of low atmospheric pressure (about 585 mm. of mercury a t Los Alamos) immediately became apparent with the investigation of the aniline-hexane system, results for which are shown in Table I. The system of n-butyl alcohol-water was found to be not completely miscible at any temperature below atmospheric pressure, and so, no longer exhibits the critical solution temperature phenomenon. The sec-butyl alcohol-water system was known
As is shown in Table I, measurements of critical solution temperatures may be repeated with new mixtures with variations usually less than f0.06". A practical limit probably has been reached in the measurement of those temperatures in that errors arising from the volume measurements and impurities in the reagents are apt to be greater than the observed precision in the actual temperature measurements. A mercury thermometer may give misleading results as to the degree of control owing to its large lag. In one instance where a thermometer was checked against the thermocouple, the thermocouple showed a variation of & O . O 5 O C. with Schmitt trigger control while a thermometer with 0.1' C. divisions showed a variation of less than 0.01' C.-Le., no movement of the mercury column. However, where high precision is not desired, a thermometer might be a simpler method of obtaining an average value for the temperature a t the control point. Another point to be observed is that there is some preferential distillation of the more volatile component, even with the reflux condenser, and the mixture constantly changes in composition. This effect was observed both with the aniline-hexane system, where the critical solution temperature decreased with time, and with water-triethylamine a t low triethylamine concentrations where the critical temperature increased m-ith time. Therefore, to obtain the true critical temperature for the initial mixture, the temperature reading must be taken as early as possible. If the time when temperature control begins is taken as zero time, the temperature may be read a t this time, or a series of temperature readings may be plotted against time and extrapolated back to a temperature at zero time. Either method yields substantially the same result. Because of this distillation effect, overshooting the critical temperature is to be avoided the first time it is approached. A sufficiently low heating rate may be used to avoid this overshooting, or if the approximate critical temperature is known, additional switches on the recorder may be used to turn off the main heater 5" to 10' C. before critical temperature is reached, and additional heating done only with the small control
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V O L U M E 27, NO. 9, S E P T E M B E R 1 9 5 5
1443 Petroleum Products and Lubricants," D611-44T,p. 41 (September, 1945). ENG.CHEM..ANAL.ED.,18. 739 (1946). Brown. C. W.. IND. Findlay, Alexander, "The Phase Rule," p. 99, Dbver Publishing Corp., New York, 1951. Keyes, D. B., arid Hildebrand, J. H., J . Am. Chem. Soc., 39, 2126 (1917). Ludeman, C. G., IND. ENG.CHEM.,AN.~L.ED.,12,446 (1940). Muller, R. H., and Lingane. J. J., ANAL.CHEM.,20, 795 (1948). Orrnandy, W. R., and Craven, E. C., J . Inst. Petroleum Technol., 12, 89 (1926).
heater. I n any case, the distillation effect is small, amounting t o only about 0.5" per hour in this case for the aniline-hexane system. ACKNOWLEDGMENT
The authors wish to express their indebtedness to D. M. Olson for mechanical assistance and for the preparation of the drawings. LITERATURE CITED
(1) Am. Soc. Testing Llaterials, Philadelphia, Pa., "Standards on
RECEIVED for review December 11, 1954. Accepted M a y 6, 1955. Work done under the auspices of t h e U.S. Atomic Energy Commission
Sampling for In-line Instrumentation A Degasser for Obtaining Air-Free Samples U. L. UPSON Hanford A t o m i c Products O p e r a t i o n , General Ekctric Co., Richland, W a s h .
The degassing device described was designed for use on sample streams containing up to 98q0 air, as obtained with an 8-cubic feet per minute air jet and air lift, and delivers a sample stream of at least 99.9570 liquid content, comprising 50 to 8 0 7 ~ of the liquid content of the input stream. For the optimum orifice size, which yields this wide-range operation, a minimum liquid input flow of 0.05 gallon per minute ( ~ 2 0 0ml. per minute) is required for positive sample flow (at 8- to 12-inch lift above the degasser input) when an air flow of less than 0.5 gallon per minute is involved. A t air flow rates of 0.5 to 1 gallon per minute, the degasser functions on as little as 0.015-gallon-per-minute liquid input. For 0.05- to 1-gallon-per-minute liquid content in the input stream, air contents up to at least 2 gallons per minute can be tolerated. Higher flow rates were not obtainable under test conditions.
through the sample cell, while the air is bled off through the orifice, the two paths converging into the jet stream. The orifice diameter must be large enough that the pressure drop for air flow alone is not sufficient to depress the air-liquid interface to the liquid output port, yet small enough that when a small amount of 7 2 " Dia.
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N OBTAINING samples from process lines and vessels, particularly where corrosive or dangerous conditions are involved, one common engineering practice is to employ jet aspirators to lift the sample stream to the sampling position. In many instances, introduction of air or an inert gas into the input riser is necessary to ensure adequate lift; and frequently air is admitted inadvertently through leaks into the below-atmosphericpressure system. In such cases, the gas content of the sample stream may amount to 90% or more of the total flow. I n many instrumental applications it is necessary or desirable that the measured sample be air-free, or that the sample cup be completely filled a t all times. To this end, the degasser described herein was developed. This device was designed to operate in the flow range obtainable with an air jet of 8 cubic feet per minute (capacity, 3 gallons per minute). The data given here are for water inputs only, but similar operation has been obtained for both organic and high-concentration aqueous streams. Special designs and modifications for specific unusual applications are discussed.
Orifice Plug
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liquid is included in the air stream, the pressure difference is sufficient to ensure flow through the sample leg. With the proper orifice, the liquid level maintains itself just below the orifice position over a large range of air and liquid flow rates. A purpose of this study was to determine the orifice dimensions necessary to attain this condition and still cause an adequate fraction of the liquid phase to pass through the sample cell.
PRINCIPLE OF OPERATlON
EXPERIMENTAL
Degassing of an air-liquid mixture is effected by the disengagement of phases in the degasser chamber through gravity separation during low velocity flow of the liquid phase (Figure 1). The liquid thus passes out from the bottom of the chamber and
A full-scale model was constructed of Lucite to permit visual observation of the phase conditions within the degasser during operation. Interchangeable orifice plugs were made up having orifice dimensions of "/I6 inch long by '/u, '/a2, ' / I O , and a/32 inch in diameters-sizes calculated to bracket the usable range. Simi-
