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. In 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., 22, 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, 19, 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.
T
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.
N. J .
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 $660" C., so that the temperature recorder should find many other research applications in addition to the determination of product purity.
-
The measurement of purity cryoscopically may be considered to involve three general problems: Measurement of the freezing point for pure material. Determination of the relationship between freezing point and puritty. Elimination of errors in the application of the method. The f i s t problem, characterization of the freezing point,
To,for the pure material, involves either the preparation of a highly purified reference sample or the analytical estimation of To from the shape of the freezing or melting curve of a somel$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 o involves the estimation of T o ,the freezing point of 1 0 0 ~ pure 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
V O L U M E 24, NO. 2, F E B R U A R Y 1 9 5 2 and of reference standards that are used in other methods of analysis such as spectrophotometry and titrimetry. A number of different techuiques for estimating TOhave been proposed by Mair et al. ( 6 ) , Aston et al. ( 1 , a), Herington and Handley ( 4 ) ,Stull ('?'), and others, but all are modifications of the fundamental principles originally formulated by White in 1920 (10). I
IDEAL
35 1 Rossini and his coworkers (S, 9) have developed analytical equations for determining To - 2'1 from the shape of the timetemperature freezing and melting curves, so that the entire curves need not be determined, but the assumption of thermodynamic ideality of the solutions is still made and, as it is a difference method, temperatures must be measured with greater accuracy than is necessary when the entire curve is measured. In the calorimetric method of Aston and his coworkers ( I ) , the fraction of the sample that is melted is determined from the accurately measured heat input, and this function is related to the equilibrium temperature, either graphically or analytically, as in the time-temperature methods. Although the calorimetric method is probably capable of greater accuracy, it is not so rapid or as generally applicable as the time-temperature methods. CALIBRATION CURVES O F FREEkING POINT VERSUS PURITY
TIME
Figure 1. Ideal Freezing Curves at Constant Rates of Heat Transfer
Once To is estimated with sufficient accuracy using a moderately pure sample, the purity of this and any other sample of the same material may be determined using only the value for the freezing (or melting) point, provided that the relation between freezing point and purity is known.
A . 100% pure sample B . Impure sample of same material
The equilibrium time-temperature freezing curve for a 100% gure material may be represented by curve A in Figure 1. As eat is removed at a constant rate from the liquid sample, the tem erature falls linearly with time until the freezing point To, is reacked. Pure solid then freezes out and the phase rule requires that the temperature remain constant until the sample is entirely frozen. The further removal of heat then merely cools the solid. Curve B represents the equilibrium freezing curve for a somewhat impure sample of the same material cooled a t the same rate of heat transfer as in curve A . Solid freezes out below T Oat some temperature T,. As pure solid is removed from the liquid a t a constant rate, the liquid remaining becomes enriched in the impurities and the temperaturp continues to fall a t a gradually increasing rate until the sample is entirely frozen. Equilibrium melting curves are merely mirror images of freezing curves.
0
10
20
30 40 50 60 70 PERCENT PARA ISOMER
80
90
100
Figure 3. Comparison of Calculated (Solid Lines) and Experimental (Circles) Solid-Liquid Phase Diagrams
For thermodynamically ideal Wstems the freezing tenipereture, T,, of a mixture is related to the purity and T Oby Equation 1, the well-known equation for solid-liquid equilibrium:
TIME
Figure 2. Freezing Curve for Impure Sample, Showing How TO May Be Estimated
The curves of Figure 1 are reproduced in Figure 2 to shom- how
To may be estimated. If heat is removed from the sample at a constant rate during the freezing process, the extension of the portion of the continuous line n-hich represents cooling solid until it intersects the dotted horizontal ,Z' line approximates the time a t which the sample would have bccn completely frozen if no cooling had occurred. This means that the length of any fraction of the dotted T / line is very nearly proportional to the fraction of the sample frozen. If 2, is chosen as the midpoint of the dotted T , line, T , represents the temperature a t which the sample is half frozen and the concentration of the impurities remaining in the liquid is trike as great as in the original sample. Sow, if the impurity concentration is sufficientlv low so that it is proportional to the freezing point lowering, ( T , - T,) is exactly equal to (To - T I ) and T O may be readily estimated.
where P is the mole per cent purity, To and T i are absolute temperatures as already defined, R is the molar gas constant, and L/ is the molar heat of fusion. P and L f relate to the solid phase freezing out whether it is present in excess or not. Literature values for L/ are frequently not available, in which case L, may be measured calorimetrically or cryoscopically. The purity P is relatively insensitive to changes in the value used for L! so that L, can often be estimated with sufficient accuracy by the use of the rule that the entropy of fusion, L f / T o ,is constant for chemically similar substances. This relationship appears to hold especially well for aromatic chemicals. The only assumptions involved in the derivation of Equation 1 are :
1. L f is constant over the temperature range TOto Tf Only one component freezes out 3. The solution is thermodynamically ideal 2.
The first assumption introduces very little error because of the relatively small temperature ranges generally involved. The
ANALYTICAL CHEMISTRY
352 second assumption, however, requires that no eutectics, compounds, or solid solutions separate out; the influence of these is discussed later. The last assumption is generally valid because the impurities which are most difficult to remove from organic materials are usually those which are structurally similar to the product that is desired-e.g., isomers and homologs-yet these are the very materials most likely to form ideal mixtures with the major component. An example of this is the system, 0- and p-nitrochlorobenzene, which is represented in Figure 3. The solid curves were calculated from Equation 1, while the circles represent experimental data reported by Holleman (6). The agreement is very nearly perfect even as to the freezing temperature and composition of the eutectic. Crude coal-tar naphthalene is another example. This material contains a mixture of aromatic and aliphatic impurities, yet ’behaves as if it were very nearly ideal up to impurity concentrations as high as SOY0, as shown in Figure 4. In this example the creosote oil was carefully fractionated to remove the naphthalene, and the average molecular
401 100
90
00
I
70
60
50
40
30
WEGHT PERCENT NAPHTHALENE
Figure 4. Comparison of Calculated (Solid Line) and Experimental (Circles) Freezing Point Diagram for Naphthalene Creosote Oil
1%eight of the naphthalene-free oil was determined in p-nitrochlorobenzene. The solid line was calculated using this value for the molecular weight of the oil and the circles represent experimentally determined points upon the addition of the oil to pure naphthalene. Many organic systems of commercial interest such as mixed xylenes, dichlorobenzenes, gasoline aliphatics, etc., behave ideally even when the impurity concentrations are large. Thus, once Toand L, are known, the freezing point-purity relationship may be calculated and one or two experimental points will usually suffice to verify the ideality of the system or at least will permit adjustment of the calibration curve for slight deviations from ideality.
POSSIBLE SOURCES OF ERRORS
Attainment of Equilibrium. Mathematiral analysis of timetemperature freezing and melting curves for the estimation of Torequires adequate attainment of equilibrium along the entire solid-liquid curve. A typical example of such an equilibrium curve is shown in Figure 5, which presents an experimentally determined curve for a highly purified sample of cyanuric chloride. On the other hand, the experimentally determined curve for a sample of diethyl dithiophosphoric acid which is reproduced in Figure 6 shows that in this case supercooling was significant and recovery of equilibrium was slow, even though the sample was well seeded. The temperature maximum attained after supercooling and represented by T,in Figure 6 is often used to characterize the freezing points of samples, yet the true equilibrium value is
FREEZING CURVE 0995 MOLE PERCENT WRE
4
0
1 I
V I
I:[,
143 143
,
I 50
0
Figure 5.
i
,
~
I03 150 200 TIME MINUTES
-
250
Experimental Freezing Curve for Material of High Purity
represented by T,. This difference can become large as the purity decreases and supercooling becomes marked. In addition, materials such as o-nitrochlorobenzene and o-nitrodiphenylamine, for example, were found to require an hour or more to equilibrate after supercooling, even when extremely pure and well seeded. A number of techniques may be used to detect nonequilibrium conditions. Both freezing and melting curves can be run on the same sample, in which case any difference in the temperature of the transition is a measure of the deviation from equilibrium. The true equilibrium value is not always the mean of these two transition temperatures. A useful procedure for correcting for nonequilibrium conditions is to determine the freezing or melting curves at two different rates of heat transfer and then to extrapolate the resulting transition temperatures linearly to zero rate of heat transfer. The “constant teniperature differential” (CTD) freezing point apparatus developed in this laboratory and described under Apparatus is admirably adapted for this purpose. When the attainment of equilibrium is impractically slow, the constant temperature differential apparatus is especially useful, because heat transfer to or from the sample can be reduced to zero by thermostating the outside bath a t the sample temperature until equilibration is complete. The rate of approach of the sample to equilibrium may be determined by following changes in the sample temperature under adiabatic conditions. EXPERIMENTAL
-
FREEZING CURVE
Y
s -35-
-
Y
-36-
a
8
S O MOLE
-371
PERCENT
PVRE
-
MTHYLDlTHOFUOSPHORlC AClD
1
I
0
IO
I
20
30
!
I
40
50
- MINUTES Figure 6. Experimental Freezing Curve for Somewhat Impure Material TIME
Eutectic Formation. The cryoscopic behavior of a eutectic mixture is identical with that for a pure compound. In fact, the shape of a eutectic freezing or melting curve may in some cases be used to determine other impurities than those forming the eutectic--e.g., for determining the amount of a meta isomer in an essentially eutectic mixture of ortho, para isomers. On the other
V O L U M E 2 4 , NO. 2, F E B R U A R Y 1 9 5 2
353
hand, the presence of undetected eutectic solid phases flattens the solid-liquid portions of the time-temperature curves and leads to anomalously high purities. Thus, if the sample to be analyzed happens to contain exactly a eutectic mixture of impurities, none of then1 &ill be detected from the freezing curve. It can be shown fioin Equation 1, however, that this is a highly improbable situation in the 90 to 100% purity range, because an organic chemical that freezes between -40' and +200° C. can form eutectics with as low as 10% of an impurity only when the freezing point of the impurity itself is a t least 100' higher than the compound of interest. This difference in freezing points must become even greater when the impurity concentration in the eutectic becomes less, requiring considerable chemical dissimilarity between the compounds. Thus the possibility of encountering eutectic mixtuies in the 90 to 100% purity range becomes small when several different methods of purification are employed.
'
'
3 NITRO, 4 CHLORO TOLUENE
76 -
,
4
I
7 4 1
I
I
A
FREEZING
1
PI 1
998 MOLE PERCENT PURE
B
MELTING
-
?I
1
4
1 I
4t I
\\hen the amount of an iiiipurity in a sample exceeds the eutectic composition, owing to a high impurity concentration or to marked deviations from ideal behavior, the impurity will be the first solid phase to freeze out. The temperature coefficient of solubility for the impurity must be high when the impurity concentration in the eutectic is less than 2095, however, so the precipitation of the impurity Till not affect the slope of the liquid portion of the freezing cuive perceptibly and may not be detected. The presence of the impurity solid phase can easily be found by noting the turbiditj- it produces in the sample just above the observed eutectic bleak. If the composition of the sample is near the eutectic composition, the time-temperature curve of the eutectic may be used to determine the concentration of any other impurities. Solid Solution Formation. The cryoscopic method of analysis is based on the separation of a single, pure, solid phase. If any of the impurities in a sample form a solid solution with the major component, the relationship expressed in Equation 1 is no longer valid, unless it is adjusted by a factor dependent upon the distribution coefficient of the components betJwen the solid and liquid phases. This distribution coefficient is generally not laown, of course, so no correction can be made. Fortunately, in organic chemical systems the appearance of solid solutions is much more infrequent than the more normal eutectic type behavior. In addition, several methods are available for detecting the presence of solid solutions. If the impurity forming the qolid solution is present in appreciable qumtity, it may be
detected from a fl:ttteniny in the time-temperature curve. In most cases the impurities likely t o be present in appreciable quantities are known from the preparative method used to obtain the sample, so that these impurities may be added to the sample in known amounts and anomalous behavior detected from abnormal changes in the freezing point. All major sources of error in the cryoscopic method, including the appearance of solid solutions, tend to yield abnormally high purities. Thus, bhould solid solutions be present and remain undetected, the impurities that are measured represent a minimum value. Polymorphic Behavior. Many organic materials can exist in two or more crystalline states, each form exhibiting its own freezing point. When the freezing points differ widely, no difficulty is encountered in cryoscopic methods of analysis, because it is immediately apparent from the freezing temperature which form is separating out. When, however, the freezing points of the polymorphs lie close together, the purity analysis will be in error if the wrong solid phase appears. An unusual example of this was found when freezing and melting curves were run a t constant rates of heat transfer on a sample of 3-nitro-4chlorotoluene. Two crystalline modifications appeared which froze only 0.5' C. apart, as demonstrated by the time-temperature curves presented in Figure 7. After approximately 30 minutes near the freezing point, the metastable p modification was observed to convert spontaneously to the Q form. Other Sources of Error. Compound formation between an impurity and the material being analyzed introduces two sources of error, one due to the large deviations from ideal behavior and the other due to the anomalous increase in the molar concentration of the impurity. Here again the very factors that introduce errors in the cryoscopic method tend to simplify the separation of the complicating impurities, so that the probability of error is greatly reduced. In any case, however, care must be evercised in applying the cryoscopic method to strongly acidic or basic materials. In most analyses the weight per cent impurity is desiied rather than mole per cent. When the average molecular weight of the impurities is known or may be approximated with sufficient accuracy, the conversion between the two concentration units is readily calculated. When, however, the average molecular weight of the impurities is unknown, it is sometimes practical to determine the average molecular weight of the entire sample in another material, and to use this value to convert from mole per cent to weight per cent. CONSTANT TEMPERATURE DIFFERENTIAL APPARATUS
In the classical Beckman method of measuring freezing or melting curves, the stirred sample is separated from a bath of fairly constant temperature by an air space which serves to regulate the rate of heat transfer. AB the temperature of the sample changes, the temperature difference between the bath and the sample also changes and the rate of heat transfer is, of course, continually varying. Rossini ( 6 ) and his coworkers a t the National Bureau of Standards reduced the magnitude of this variation by using a large temperature differential (of the order of 50" to 100' C.) between the bath and the sample and evacuating the space in between. This requires that pressures of the order of 1 to 10 microns be maintained constant throughout the experiment. Stull (7) circumvented the problem of maintaining uniform low pressures by using several Dewar-type sample tubes presealed a t the desired degree of vacuum. Aston (1)and his coworkers applied known increments of heat to the frozen sample in a calorimeter and measured the temperature rise after each addition. Thus, instead of plotting time against temperature, the fraction melted was plotted against temperature. There is probably little to choose between the calorimetric method and the National Bureau of Standards
ANALYTICAL CHEMISTRY
354
method, because both have applications to which they are peculiarly suited. Another solution to the problem of maintaining constant rates of heat transfer is to use an atmospheric pressure air baffle as in the Beckman method hut to maintain a relatively small (0" to 10' C.)constant temperature differential between the sample and the bath. Although a t first glance this may appear to be unduly difficult to accomplish, preliminary experiments in this lahoratory indicated that such was not the cme in the temperature range of interestnamely, -40" to +200" C. As a result, the manually controlled apparatus shown in Figure 8 was developed.
The freezing and melting curves shown in Figure 9 were obtained on the same sample a t identical temperature differentials using the constant temperature differential apparatus. The melting curve has been reversed on the time scale for ease of comparison. The equilibrium portions of the curves were perfectly linear, so that it was a simple matter to interpolate to the true freezing and melting points, even though the slopes of the lines did not differ greatly. The difference of 0.1" C. between the observed freeaing and melting points showed that the samples were not perfectly equilibrated even a t the low rates of heat transfer used in these experiments. When marked deviations from equilibrium or polymorphic changes occurred, the constant temperature differential apparatus proved to be especially useful, because the rate of heat transfer could he rapidly reduced to zero until equilibrium was established, as evidenced by a constant sample temperature, and then the timetemperature curve could he continued a t the original rate of heat transfer. AUTOMATIC APPARATUS
Figure 8.
In order to simplify the operation and extend the usefulness of the constant temperature differential apparatus, automatic temperature recording and controlling equipment was developed in cooperation with the Thwing-Albert Instrument Co. Two Model L-50 resistance recorders were modified, one to record the sample temperature (absolute recorder) and the other to record and control the temperature difference between the sample and the bath (differentialcontroller). ks a detailed discussion of the equipment is being planned for a future publication, only a general outline of the construction is presented here.
Constant Temperature Differential Freezing Point Apparatus
a
used in the sezme'tuhe. The &p of the sample tnhe cor&ined rubber stopper with openings for the glasespiral reciprocatin.; type stirrer, for the platinum resistance thermometer, for introducing an inert atmosphere, and for seeding or sampling the oantents of the tube. A male 45/50 T joint near the top of the sample tube fitted the female joint at the top of a glass jacket 45 mm. in outside diameter, so that a. 3-mm. air gap separated the two tubes. The air gap between the two tubes was maintained a t a& mospherio pressure by means of a smmtll opening near the top of the sample tube. The bath surrounding the jacket consisted of a 4liter, unuilvered Dewar containing a stirrer, thermoregulator, thermometer, electrical heating coil, and coil for circulating a cooling fluid. cis-Decalin usas used for bath the bath liquid and the circulating fluid because of its low viscosity at -40' C., low freezing point (-125' C,),and high boiling point (193' C,). The Circulating fluid was cooled to temperatures as law as -25" C. in an Aminco refrigerated bath and, when needed, the precooled fluid was further cooled to -80" C. by passing it through a Dewar containing a dry iceddecdin mixture. The path and rate of flow of the circulating fluid were controlled by means of two 0.25-inch needle valves. When bath temperatures above 170" C. were desired, the cis-Decalin hath fluid was redaced with mineral oil.
eo.
0
2
MOLL PERCENT
4
6
8
10 12 14 16
18 20 22 24 26 28
TIME-MINUTES
Figure 9.
Freezing and Melting Curves Obtained from Same Sample
CTD apparatus used. Magnitude of temperature differential was ~ a m for e both ourves. For clarity, time s o a k Sor melting o u n e has hcsn reversed
differentids of less t h a n i ' C. hktween the bath and the'sample.
intervals tcfollow changes in the sampre temEerituture. During a run the sample and bath temperatures were measured a t regular timiintervals and recorded. The difference was c d culated and the heater voltage or the thermoregulator setting was adjusted to maintain the desired temperature differential. When readings were taken every minute, sufficient time was available to plot the time-temperatnre curves in addition to carrying out the other control operations.
Figure 10. Compound Platinum ResistanceThermometer Element for I m m e r s i o n in Sample
35s
V O L U M E 24, NO. 2, F E B R U A R Y 1 9 5 2 The pioneering work of Stull(8) in recording resistance~thermometry served as a vduahle guide in designing the ahsolute recorder. The recorders were operated by the converted and amplified direct current signal from the Wheatstone bridge circuits. Only 1.7-mil current passed through the thermometers in order t o eliminate errors due to heatinr. The compound resistance element shown in Figure 10 was immersed in the sample. The lower two thirds of this element consisted of a 125-ohm platinum coil which semsd as one arm of the bridee circuit in the ahsalute recorder. The upper, GO-ohm section a n a another identical GO-ohm section, which was immersed in the outside bath, served as apposing arms in the differential controller bridge circuit.
The absolute recorder is shown in Figure 11 The full scale range of the instrument was 10' C. with 0.5" overshoot at each end. Fhnges were itutomatically shifted a t each end of the pen travel and the decade in u8e was indicated by a second pen, which was displaoed slightly on the time soale. In order to record temperatures ;aholut~lyover all the decades, the slight nonlinearity of the temperature coefficient of platinum was compensated for by shifting both ratio arms in the bridge circuit for each decade. Switching both ratio arms also tended to e2meel any errors due to contact resistances in the switches. By choosing proper values far the ratio arm resistors, temperatures were recorded with an accuracy of +0.01' C. throughout the full temperature range of -40" to +ZOO" C. This range can, of course, he readily extended to cover any portion or all of the platinum thermometer range of -190' to +GG0" C. The stability and sensitivity of the ahsolute recorder were both better than
n.mDc.
Figure 11. PrecisionAbsolute Temperature Recorder
The temperature differential between the bath and the s&mple was fixed by setting a control pointer an the differential controller. The pen carriage operated a microswitch fixed to the control pointer, and the microswitch, in turn, controlled the heat input to the bath. Full scale temperature differential ranges of *2",,5", loD,and 20' C. could he chosen by switching in the proper slldewire shunts. 10-
9-
1 I
9.0
Figure 12.
An automatically recorded f i e mple of nitrobenzene is presented i,, r L B y L E r51LLp51rruu~e is s h o r n vertically and time horizontally from right to left. As the temperature of the liquid sample fell, the ranges were shifted automatically until the SUnple began to freeze in the range shown on the vertical scale. The temperature of the sample increas d, owing to recovery from supercooling, and then remained constant until the sample was almost completely frozen. When the sample was frozen solid, the temperature decreased rapidly and, a t the lower limit of the scale, the instrument shifted to the next lower range. The pen tracing a t the bottom of Figure 12 indicated the temperature decade in use. The advantages of accurate temperature records, as compared with visually observed and manually recorded data, for characterieing the freezing paints of industrial products are a t once apparent. In addition, temperature recorders with an accuracy of O.0lo C. should also find many research and production applica, tions in fields other than that of cryoscopic measurements. LITERATURE CITED
J. G., Fink, H. L.,Tooke, J. TV.. and Cines, M. R., ANAL.CHEM.,19,218 (1947). (2) Baton, J. G.. and Messerly, G. H.. J . Am. Chem. Soc.,
(1) Adon,
62, 1917 (1940):
FREEZING CURVE TRACED 0 Y AUTOMATIC RECORDER
(3) Glasgow, A. R., Streiff, A. J., and Rossini, F. D., J . R e s e a r c h N a t l . BUT. Standads, 35,355 (1945). (4) Herington, E. F. G., and
8-
Handley, R., 3. Chem. Sor., 1950, 199. (5) Holleman, A. F., P m . A d . Sei. Amsterdam, 11, 248 (1908). (6) Mair, B. J., Glasgow, A.
7-
R-
R., and Rossini, F. D., J . Research Natl. Bur.
/I
0
Curve Drawn by Absolute Temperature Recorder for Plant Sample of Nitrobenzene
Standads, 26,591 (1941). (7) Stull. D. R., IND. ENC. CHEM.,ANAL. Eo., 18, 234 (1946). (8) Stull. D. R., Rev. Sci. Insbuments, 16, 318 (1945). (9) Taylor. W. J., and Rossini, F. D., I . Research Natl. Bur. Standards, 32, 197 (1944). (10) Vhite. W. P.. J . Phw. C h m . , 24,393 (1920). R ~ C E I V E DNovember 20. 1950. Presented in w r t before the Division of Anelytiod Chemistry a t the 118th Meeting of the A m ~ r c m C n ~ x r cSOCIETY. ~~. Chicago, Ill.
