Thermistors for Cryoscopy of Air-Sensitive Materials PAUL D. ZEMANY General Electric Research Laboratory, Schenectady, N . Y .
,A need arose for measuring the purity of samples of pentaborane by a simple method. By using a thermistor as the temperature-sensitive element for cryoscopy a very simple device was constructed that requires little attention or manipulation. I t can be used to measure the purity of compounds by their freezing points with a precision of +O.O04"C. Provisions have been made to prevent contact of air with the sample. Because of its simplicity and ease of operation, the apparatus may be useful for other purity determinations.
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of platinum a t 0' C. Thermistors for temperature measurements can be made of these materials with a wide range of characteristics, but the particular thermistor used in this application (Type 14B, manufactured by Western Electric Co.) has a resistance of 2500 ohms a t 20" C. as compared Ivith 25 ohms for a platinum resistance thermometer. With such a high resistance, problems of lead resistance, contact resistance, etc., do not occur. Resistances in the range of several thousand ohms can be accurately compared on very simple, rugged, and inexpensive apparatus. The temperature-sensitive portion of a thermistor is a bead, a fraction of a millimeter in diameter. In t h t type used, this bead is enveloped in one end of a glass tube 0.1 inch (0.25 cm.) in diameter and 3 inches long, with the leads opposed to the bead. Other kinds of thermistors are available ( 1 ) .
N ORDER to determine the purity of samples of pentaborane
by the depression of the freezing point, a device was required t h a t would incorporate methods of handling the sample entirely within a vacuum system, use relatively small (2-ml.) samples, utilize simple equipment, and be very easy to operate. In addition, it was desired that, thc method be capable of measuring purity t o about 0.01 mole % (10.01 ' C.). The device that was developed to meet these requirements may be useful for other purity determinat'ions. I t s useful range of temperature is from about +25" to -60" C., but this range may be extended in either direc,tion by simple modifications. THERMISTOR LEADS THERMISTOR WELL
FREEZING POINT CELL
The freezing point cell used is slightly modified from one used by Sewkirk (4). Details of its construction are shown in Figure 1. The evacuated jacket is left unsilvered to permit energy transfer by radiation. The rate of energy transfer can be controlled by using various freezing mixtures. The sample is stirred by the stainless steel helix, which is attached to the iron cylinder by two heavy wires. A shoulder on the cell rests on a solenoid. This solenoid is activated about 120 times a minute by the current passed through a Microswitch each time it is closed by a double cam attached to the shaft of a 60 r.p.m. motor. This lifts the iron cylinder and the attached stirring helix about 1.5 cm. When the contact is broken the stirrer drops to its normal position. FILLING THE CELL
IRON CYLINDER
The cell, which is sealed to a vacuum system, is filled by a vacuum distillation. I n the case of pentaborane, the samples are cuts from a fractional distillation, and this method of filling is permissible. If the samples contain nonvolatile impurities, other
IRRER SUPPORT RODS
THERMISTOR STAINLESS STEEL HELIX FOR STIRRING ACUUM SEAL TIP Figure 1.
;.E. F~TOELECTRIC RECORDER
Freezing Point Cell
The required simplicity in the temperature-measuring circuit was achieved by using a thermistor as the temperature-sensitive element (7). The characteristics of thermistors that make them particularly useful for this application may be illustrated by comparing them with the more familiar platinum resistance thermometers, which have been used extensively in cryoscopy ( 3 ) . Thermistors ( 5 ) have a 40/, change in resistance per degree change in temperature, as compared with 0.4% per degree for platinum, so that the relative change in resistance is ten times as great for small increments of temperature. The specific resistance of a typical thermistor material is lo9times as great as that
SvOLTs Figure 2. Circuit for Jleasuring Temperature with Thermistor
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V O L U M E 2 4 , NO. 2, F E B R U A R Y 1 9 5 2 means of filling must be devised ( 2 ) . In order to fill the cell completely, it must be immersed deeply enough to be cooled above the ring seal of the vacuum jacket, because the rate of heat transfer through the vacuum jacket is so slow. When all the sample has been condensed above the ring seal, the cell is closed off from the rest of the s stem, the refrigerant is lowered a bit, and the sample melts angruns down to fill the cell to a depth of about 5 cm., which is just sufficient to cover the top of the stirring helix a t the top of its stroke. The volume of sample required is about 2.0 i0.2 ml. With the cell filled by this method, the cooling is continued a t a rate of about 1' C. per minute until the sample is frozen while the freezing point is determined. The sample may be removed by another distillation. This is most easily accomplished by heating the sample with an infrared lamp after the refrigerant is removed. Instead of being sealed directly to the vacuum system, the cell may be fitted with a stopcock and standard-taper joints for attaching it to the system. For materials that may be handled in air, the cell is made detachable, with a large tapered joint in the body of the cell above the shoulder, but without a side arm.
349 Figure 3 shows the kind of record obtained. hlternatively, the deviation of the recorder from zero at the freezing point is a measure of the purity. By adhering closely to the procedure outlined above, heat leaks, variable temperature gradients, radiation errors, resistive heating of the thermistor element, heating by the stirrer, and other similar possible sources of error are compensated. 72000
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TEMPERATURE MEASUREMENT
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The circuit used for measuring the resistance of the thermistor is shown in Figure 2. The components were chosen to operate satisfactorily a t about the melting point of pentaborane, but can be used to -60" C., below which the resistance becomes too high to operate the recorder with adequate sensitivity, and to +25", above which the resistance is too low to give satisfactory results with high resistances in other portions of the circuit. In order t o operate a t higher temperatures (to about t150")the simplest change is to use a 14A thermistor, but lowering the other resistances in the circuit would also extend the range to higher temperatures. To work st temperatures belo\\ -60" C. more drastic changes in instrumentation are required, the simplest being a more sensitive galvanometer, which will extend the range somewhat. By using a parallel impedance bridge (6) very high values of resistance can be measured accurately, but the extreme simplicity of the device is sacrificed. ADJUST RESISTANCE TO BALANCE AT ZERO
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FULL SENSITIVITY
Figure 3.
Typical Record of Freezing Point Determination
Chlorobenzene distillation cut 6. Freezing point found a t 68,310 ohms. Freezing point of pure chlorobenzene was 68,280 ohms
A recorder is used rather than an indicating galvanometer, because it simplifies the gathering of data. The most convenient procedure for measuring the freezing point has been found to be as follows: The decade box is set to the resistance corresponding to the freezing point of the pure material. As soon as the cell is filled and the stirrer turned on, the recording is started, with the galvanometer shunt a t low sensitivity. As the sample cools the sensitivity is increased as required. Generally the freezing point of the sample is exceeded, because of supercooling, but in a minute or two the freezing starts spontaneously and the resistance levels off to a very nearly constant value. [Fortunately, supercooling was not a problem with these particular samples, but it may be allowed for by the method described by Glasgow, Streiff, and Rossini (3)when necessary. ] Then the resistance of the decade box is increased sufficiently to balance again a t the zero of the recorder or galvanometer. The resistance so obtained is taken as a measure of the freezing point and its deviation from the freezing point of the pure material is a measure of the purity of the sample ( 3 ) .
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Figure 4.
Calibration Curve for Impurities in Chlorobenzene
The measurements of the freezing point of an impure compound should be referred to the observed resistance for the freezing point for the pure material, and the resistance for known amounts of added impurities, rather than to an absolute temperature calibration. The calibration of several thermistors has not changed by any measurable amount in several months. Zeffert and Hormats (7) also report satisfactory stability. However, until the stability is verified by a great deal more experience, occasional checks are recommended. CALIBRATION AND PERFORMANCE
Chlorobenzene, melting point -45.3 ' C., and solutions of benzene in chlorobenzene &-ereused to become acquainted with the behavior of the device. The freezing points on a series of Samples, shown in Figure 4, are illustrative of the kind of data that can be obtained with the apparatus. The time required to complete a determination, including filling and emptying the cell by vacuum distillations, is 2 hours or less, the actual determination requiring about 0.5 hour. The only manipulation of the measuring device is adjustment of the sensitivity of the shunt as the sample cools and a single balancing of the resistance on the decade box immediately after freezing starts. The temperature of freezing can be determined with a precision of about &0.004" C., using the simple components described here. A more sensitive indicating device would probably determine the freezing temperature more precisely, but the author has no data that would k l l how reliable the results might be, since the various possible murces of error mentioned above might have an appreciable effect on the value obtained. A t the limit
ANALYTICAL CHEMISTRY
350 of precision in the apparatus described (&0.004" C.) they do not seem t o affect the results. As the device was intended to check the purity of samples of a single compound, the calibration procedure is determination of the resistance corresponding to the freezing points of synthetic mixtures of the material under investigation. I n the case of chlorobenzene, for example, the data obtained (Figure 4) are used in the form of a calibration curve; the purity of unknown samples may be determined by reference to the curve. A similar calibration curve was made for pentaborane, and could be made for other materials.
LITERATURE CITED
(1) Dowell, K. P., Elec. Mjg., 42, 84 (August 1948). (2) Glasgow, A. R., Jr., Krouskop, N. C., and Rossini F. D., ANAL. CHEM., 2 2 ,
1521 (1950).
(3) Glasgow, A. R., Jr., Streiff, A%. J., and Rossini, F. D., J . Research
dl'atl. Bur. Standards, 35, 355 (1945). (4) Xewkirk, A. E., General Electric Research Laboratory. private communication. (5) Pearson, G. L., Bell Lab. Record, 1 9 , 106 (December 1940). (6) Reynolds, S.I., and Race, H. H., Gen. Elec. Reu., 41, 529 (1938). (7) Zeffert, B. &I., and Hormats, S., A N ~ I CHEM., ,. 21, 1420 (1949). RECEIVSD July 13, 1951. Work supported in part by a contract from U. S. Army Ordnance.
Freezing Points in Determination of Product Purity C. R. WITSCHONKE Application Research Department, Calco Chemical Division, American Cyanamid Co., Bound Brook, Impurities which are uneconomical to remove from industrial products generally form ideal systems with the compound being produced. Freezing and melting points can thus be used for quantitative analysis of materials of lower purity than is generally realized. The cryoscopic method of analysis is briefly, but critically, reviewed on this basis, possible sources of error are discussed, and some applications of the method to industrial products are presented. A versatile, new automatic freezing point apparatus which was developed in this laboratory is described.
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HE measurement of freezing and melting points has long
been accepted as a qualitative method for estimating the purity of materials, but some of the quantitative applications of cryoscopic measurements seem to have been neglected by industry. Two factors appear to be largely responsible for this: the need for automatic temperature-recording equipment of suitable accuracy and dependability, and the difficulty in treating nonideal systems quantitatively. Recent advances in resistance thermometer instrumentation give promise that temperature recorders suitable for the determination of product purity from time-temperature curves will soon be available commercially at a cost comparable with other precision analytical instruments. I t is also gradually being recognized that the impurities which are uneconomical to remove from organic chemicals of industrial interest will probably form ideal systems because of their structural similarity to the desired product. The importance of the cryoscopic method lies in the fact that it detects the sum total of impurities in a sample and becomes more reliable as the purity approaches lOO$7& the very region in which most analytical methods become less exact, On the other hand, the method is not applicable if the material cannot be solidified, if the material decomposes a t its melting point, or if the impurities are not soluble in the melt at the freezing temperature. In addition, the cryoscopic method will not generally divulge the chemical identity of the impurities. As is true with other analytical techniques, the cryoscopic method is not a cure-all, but a recognition of its limitations serves only to increase the confidence in the applications to which it may be put. The purpose of this paper is to summarize the factors that must be considered in determinations of purity by the cryoscopic method, to point out how nearly most organic systems of industrial interest approach ideal behavior, and to describe a new freezing point apparatus and recording equipment developed and used in this laboratory.
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Its operation is based on the principle of maintaining a small constant temperature differential between the sample and the surrounding bath, so that constant, controlled rates of heat transfer are obtained. The temperature of the sample is plotted automatically w-ithan accuracy of ZtO.01"C. over the range -40" to $200° C. This range can readily be extended to cover any portion or all of the range 180" to $660" C., so that the temperature recorder should find many other research applications in addition to the determination of product purity.
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The measurement of purity cryoscopically may be considered to involve three general problems: Measurement of the freezing point for pure material. Determination of the relationship between freezing point and puritty. Elimination of errors in the application of the method. The f i s t problem, characterization of the freezing point,
To,for the pure material, involves either the preparation of a highly purified reference sample or the analytical estimation of To from the shape of the freezing or melting curve of a somel$hat impure sample. Once To is known with sufficient reliability, impure samples may be readily analyzed for purity by a simple freezing point measurement, provided that the relationship between the purity and freezing point is known or may be predicted. Generally, such composition-freezing point calibration curves are experimentally determined, but many organic systems of industrial interest are so nearly ideal that the relationship may be successfully calculated. The third problem in cryoscopic measurements is to eliminate any significant errors in the determination of freezing points and purities due to slow approach to equilibrium, polymorphic changes, nonideal behavior, and the formation of solid solutions, compounds, or eutectics. Each of the foregoing three problems is considered briefly below. ESTKMATION OF To
One of the most useful applications of the cryoscopic method involves the estimation of T o ,the freezing point of 1 0 0 ~ pure o material, using only a moderately pure sample. Once T Ois known, the purity of this somewhat impure sample can be readily deduced from its measured freezing point, T,, without the necessity of preparing a highly purified reference material. This is especially valuable in checking the purities of new synthetic chemicals, of isomeric mixtures that are difficult to separate
