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frequently coincide with the change in crystal symmetry a t the enantiotropic first-order transition points. The cooperation of Professor A. Reis, of Cooper Union, New York, and of Professor Lars Thomassen, University of Michigan, Ann Arbor, Michigan, in granting the use of their laboratory facilities for the preparation of a sample of glaserite, and of a high-temperature x-ray diagram, respectively, is gratefully acknowledged. REFERENCES (1) (2) (3) (4) (5) (6)
(7) (8) (9) (10) (11)
(12) (13) (14)
BRIDIG, M. A,: J. Am. Chem. SOC.63, 2533 (1941); J. Phys. Chem. 46, 747 (1942). FINBAK, C., AND HASSEL,0.: 8. physik. Chem. B32, 130, 433 (1936); B36, 25 (1937). GOSSNER, B.: Neues Jahrb. Mineral. Geol. A67,89 (1928). GRAHMANN, W.: Z. anorg. Chem. 81, 266 (1913). HENDRICKS, S. B., POSNIAK, E., AND KRACEK, F. C.: J. Am. Chem. Soc. 64,2766 (1932). JANECKI,E.: 2. physik. Chem. 64, 343 (1908). KRACEK, F. C., AND KSANDA, C. J.: J. Phys. Chem. 34, 1741 (1930). KRACEK, F. C., POSNIAK, E., AND HENDRIJKS, S. B.: J. Am. Chern. SOC. 63,1183,2609, 3339 (1931). MULLER,H.: Neues Jahrb. Mineral. Geol., Supp. Vol. 80, 1 (1914). XACKEN, R.: Centr. Mineral. 1910, 262. O'DANIELS, H., AND TSCHEISCIIWILI, L.: 2. Krist. 104, 124 (1942); Chem. Abstracts 36, 6863 (1942). PAULING, L.: Phys. Rev. 36, 430 (1930). PERRIER,C., AND BELLANCA, A.: Periodic0 mineral. (Rome) 11, 163 (1940); Chem. Abstracts 37, 303 (1943). SIMON,F.: Ann. Physik 68, 241 (1922).
SIMPLE AND DIFFERENTIAL CRYOMETERS FOR MEASURING THE DEGREE OF PURITY AND THE FREEZING TEMPERATURES O F LIQUID OR MELTED SUBSTANCES W. SWIETOSLAWSIW
MeZZon Institute, Pittsburgh, Pennsylvania Received June 8 , 19@ BASIC CONSIDERATIONS
The devices used for measuring the freezing temperatures of liquid or melted substances are ordinarily provided with a stirrer and the liquid with some amount of crystals is thoroughly mixed (2). The freezing curve shows the lowering of the freezing temperature with the increase of the amount of crystals. No direct measurements of the amount of the solid phase can be made in such devices (1). 1 Senior Fellow, Multiple Industrial Fellowship on Tar Constituents, austained by the Koppers Company at Mellon Institute, Pittsburgh, Pennsylvania.
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In the cryometers shown in figure 1 another method of measuring freezing temperature is used. This procedure is based on the principle that the liquid phase in the center of the apparatus (V) is in equilibrium with the solid phase which forms the tiny cylinder 1. In between that cylinder and the walls of the
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apparatus a tiny layer of the liquid phase is formed by partial melting.of the cylinder. These two layers, tiny liquid layer 2 and tiny cylinder wall 1, protect the liquid in 1,-against any penetration or loss of heat from the liquid inside of the cryometer A (B or C). S o stirrer is employed and the temperature may be read and found still constant during a long period of time. During the formation of the cylinder the liquid in the cryometer is thoroughly shaken and the apparatus is immersed in a bath in which is maintained some lower temperature than the freezing temperature of the liquid. In order t o be able to read the temperature established inside of the apparatus, one contact of the thermocouple is immersed in a small amount of mercury in tube 6. The following considerations indicate that no change in temperature may occur in the Lyhole space 1-or in the vicinity of tube G , because they are thermally protected by the two layers 1 and 2. I n fact, after the large tube in which part A is fastened, as shown in figure 1, is placed in the cryostat n i t h some higher or some loner temperature than the freezing temperature of the substance, the only phenomena which may take place are a partial melting of the outside walls of cylinder 1 or a partial freezing of liquid layer 2. The temperature changes in 5' and G may be expected after cylinder 1 is somewhere melted through or layer 2 is someivhere entirely frozen, so as to make it. possible to exert the thermal influence of the temperatiire in the cryostat on that in V. CRYOMETERS
Two kinds of cryometers may be applied for measuring the freezing temperatures of liquids or melted substances: a simple cryometer, C or B, and a differential cryometer, (figure 1). The first apparatus may be used if it is filled with some standard substance rvith a known freezing point or rvit,h a relatively pure substance. The differential cryometer is utilized where the substance under examination contains a large amount of contaniination or if a mixture of two or more liquids or a solution of solid substances is getting attention. The method of operating a differential cryometer containing a liquid nith large amounts, say 0.5 to 5.0 per cent, of impurities will non- be described. First, the cylinder of solid phase should be formed. For this purpose the liquid should be supercooled and then the crystals obtained. This operation can be performed by shaking the apparatus and by bringing the surface of the tube A into contact iyith dry ice, liquid air, etc., in order to produce some amount of the solid phase. .%iter the formation of crystals, the cryometer should be heated so as to leaye R 1.ery small amount of them. Then the apparatus is immersed in a bath vith a temperature low enough to produce the cylinder of the solid phase. This process is carried out by shaking the apparatus every time after it is removed from the bath in order to observe the formation of the cylinder. After a tiny continuous wall of the cylinder is observed, the groning of the wall should be accompanied by thorough shaking of the cryometer. By inclining the cryometer sideways it is easy to estimate by eye the thickness of the wall or at, least the amount of the liquid phase left unfrozen. If the wall is
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too thick, it can be melted from within the apparatus by placing in tube 6 a glass rod heated to an appropriate temperature. If the wall has the proper size (for instance, if it is 0.3 to 0.5 mm. thick), the outside thin layer of liquid phase 2 should be formed by placing the apparatus for a short time in a bath with the temperature somewhat higher than the melting temperature of the substance. After these manipulations are completed, part A of the cryometer should be placed in a larger tube, fastened ivith stoppers 5 and 7, as shown in figure 1, and immersed in a cryostat maintained a t a constant temperature as close as possible to the freezing point of the liquid.
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FIG.2. Cryometric purification of p-xylene After some time, ordinarily 15 to 20 min., thermal equilibrium is reached between the liquid in V and the inside surface of the solid phase forming the cylinder, and then the freezing temperature t, of the liquid (figure 2) is determined. This temperature is lower than that corresponding to the equilibrium of the liquid with an infinitesimally small amount of the solid phase. This circumstance does not play any r61e if the purpose of the experiment is to remove the impurities as far as possible and to determine the freezing temperature of very pure (99.995-99.999 per cent) or a pure (99.95-99.99 per cent) substance. After the freezing temperature t. is determined, the larger protecting test tube is removed and that part of the apparatus is immersed in a bath at a temperature
sufficiently low to freeze the liquid. The cryometer is shaken vigorously and the walls of the cylinder gron. in size. The freezing stlops when about 90 per cent of the liquid is solidified. Rest, the remaining mother liquor is transferred into another tube, 8 2 , by inclining the cryometer. Then the freezing temperature of the liquid in A, containing a large amount of contamination is determined in the same manner. Xom the thick cylinder in A1 should be melted so ?s to leave a small amount of the solid phase, a small cylinder should be formed again, and the freezing temperature ti found in the same m y as before. l f t e r this determination, once more a thick cylinder is formed. Then the mother liquor (about 10 per cent of the total amount) is transferred into At and its freezing temperature t ; , is determined. These operations should be repeated several times so as to I t , , have two series of figures: t,, ti, fl, t 3 , t 4 , etc., in part AI, and t l , t z , t l , t4, etc., in part -42of the apparat,us. If the substance is pure after two or three reniovals of unfrozen liquid, the freezing temperatures become constant, for instance, t 3 = 14 = t 5 . Such a result indicates that the impurities have indeed been removed and that the liquid remaining in 9 1 is pure. It can mean, however, that no further purification is possible by repeating the same operation, because the solid phase contains some of the impurities or because they are still present on the surfaces of and in the spaces among the crystals.' I n order to examine the purity of the substance left in A1,the walls of the cylinder may be successively increased by depositing more of the solid phase, by very carefully shaking the apparatus. If no lowering of the freezing temperature is observed, there is an indication that the liquid is pure. It is easy to understand that the freezing temperature of the mother liquor transferred from AI into ABwill be the lowest t: after the first transfer and then it should increase after each transfer of the unfrozen liquid from AI into Az. If the freezing temperatwes t:, t:, t: are plotted against the volume of the liquid in Az,a curve is obtained which starts at the point t: < t, and which may end a t to, if the total amount of the liquid is transferred from A1 into A z . There is, however, no value in examining the total shape of the curve AP (figure 2). It is enough to reach such a degree of purity of the liquid in A1 that two consecutive transfers of the unfrozen liquid from A 1 into,Az mill not produce any change in freezing temperature. After this state is reached, part A1 should be cut off, the liquid frozen entirely, high vacuum should be produced, and the cryometer sealed as shown in figure 1 (C). S o w the apparatus may be used as a standardsimple cryometer with the reference substance of a known freezing temperature. Such cryometers, one filled with water, another with pure benzene, are used in everyday practice for fising the points 0°C. and +5.51°C. p-XYLESE AND BENZENE I n figure 2, two curves, i l l and Az, are presented to demonstrate how an Eastman p-xylene sample containing some impurities and some amount of water PURIFICATIOh' O F
* The cryometer may be modified so that the unfrozen liquid can be separated by centrifuge; thus the number of freezing operations may be reduced.
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was subjected to purification in a differential cryometer. The percentages of the mother liquor removed were calculated, knowing the height of the liquid phase a t room temperature. For this purpose both parts A1 and Ap were previously calibrated. Temperature ti on curve A2 could not be determined because the amount of the liquid in A2 was too small. After four consecutive freezings and transfers of the unfrozen liquid, whereupon 38.2 per cent of the liquid was removed, the remaining liquid in AI had a constant temperature 13.20'C. f 0.01'. Higher accuracy could not be attained because the limit of the accuracy with which the potentiometric measurement could be made was reached. The value agrees perfectly with that given in the literature. However, in order to avoid any error which could be associated with the secondary phenomena which often accompany potentiometric measurements, the difference between the freezing temperatures of p-xylene and benzene was directly determined. For this purpose the contact of the thermocouple was transferred several times from the simple cryometer containing pure benzene into another with p-xylene (AI). In this case two readings on the dial were used for calculating the difference between the two freezing temperatures. The difference 7.68'C. was found, which gives the value 7.68' 5.51' = 13.19'C., accurate within f0.01'C. of the temperature found by determining directly the difference between the freezing temperatures of water and p-xylene. It should be emphasized that from both cryometers, one filled with benzene, another with p-xylene, the air was not removed, and that therefore small corrections could be made after the experiments were repeated in vacuum. Experiments were also conducted with pure and with purposely contaminated benzene. In the case of chemically pure benzene it was found that, after three successive removals of the unfrozen liquid, no change in freezing temperature was observable. After pure benzene had been contaminated with 0.35 per cent of m-xylene, it was necessary to repeat the successive removal of the mother liquor four times in order to obtain a product of freezing point within 0.00G"C. of that of pure benzene. Some of these experiments were carried out by using a Beckmann thermometer. For this work the cryometer of type C (figure 1) was employed.
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OTHER USES
These differential cryometers may be utilized for many other purposes, such as for determining the freezing temperatures of binary and ternary eutectics and of mixed crystals. A SIMPLE CRYOMETER WITH ADDITIONAL SCALE
The simple cryometer B illustrated in figure 1functiow in a similar way. The only differences are that cylinders of different sizes are successively formed and that, every time after the freezing temperature is determined, the volume of the remaining liquid phase is measured by turning the cryometer upside down. It is too early to suggest a scale enabling one to determine the degree of purity of substances by using for this purpose two curves similar to those presented in
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figure 2. It seems, however, that at least the difference t , - t, = At may be considered as characteristic for the purity of the sample examined. The shape of curve Az, especially the loiyest and several of the highest points, may also show the influence of contamination on freezing phenomena. For instance, after most of the contamination is removed from A I , the mixture in Az is diluted with practically pure substance. The observations made up to this time indicate that numerous typical cases should be examined before any standard figures for determining the degree of purity may be given (1). SUMMARY
I. Simple and differential cryometers for determining the freezing temperatures and degrees of purity of liquid or melted substances have been described. The freezing temperature is measured after an equilibrium is established betmen the liquid and the surrounding cylinder of the solid phase. The solid phase is protected against supercooling by a thin liquid layer formed in between the cylinder and the n alls of the apparatus. 2. In differential cryometers a thick cylinder of solid phase is formed after each freezing-point determination and the remaining liquor is transferred into another part of the apparatus XJ,here its freezing temperature is measured. After this pi ocedure is repeated several times two curves may be established, one of which merges with the straight line perpendicular to the axis of temperature and corresponds to the freezing temperature of the pure substance under examination. 3. The degree of purity may be estimated by examining the shape of both cuiyes. At present no conventional formula can be given for expressing the degree of purity of a substance before or after its cryometric purification. REFERENCES (1) AMERILAXINLTITLTE OF PHISICS. Temperature: zts Ueasurement and Control rn Sczence und Indu.stry, pp. 258, 259. S e w York (1941). (2) NAIR, B J . Bur. Standards J. Research 9, 458-72 (1932).
