Melting Point Calorimeter for Purity Determinations J. G. ASTON AND H. L. FINK, The Pennsylvania State College, State College, Pa., AND J. W. TOOKE AND M. R. CINES, Phillips Petroleum Company, Bartlesville, Okla.
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comparison of the data obtained on several compounds with this instrument with best vaIues obtained from the literature indicates a precision for heat capacities of about 3%, for heats of fusion about 5%, and for triple points about 0.03' C. In contrast to other methods of determining purity from melting or freezing points, this calorimeter can evaluate in one run the purity of a compound, about which the only information available is its molecular weight.
A melting point calorimeter to determine the purity of organic compounds which are liquids at or below room temperature is described. Its design follows that developed in precision low-temperature adiabatic calorimetry. Modifications have enabled the time required for a complete melting point determination to be reduced from several days in precision calorimeters to about 8 hours. The calorimeter requires only one individual to operate it. A
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platinum resistance thermometer, and the wooden spacers (set 120' apart) on the calorimeter and on the shields. The large filling tube and the wooden spacers make this calorimeter sufficiently rugged so that it may be shipped without danger of injury to the assembly. The primary purpose ot the large-size filling tube is to make the filling and emptying of the apparatus rapid without dismantling the calorimeter. For materials which are liquid a t room temperature, the sample is poured into the calorimeter through a funnel, which is made from a section of 0.3-cm. (0.125-inch) German silver tubing and fits into the calorimeter through the 0.6-cm. (0.25-inch) filling tube. The sample can be removed by being sucked out through another piece of 0.3-cm. (0.125-inch) tubing which fits down to the bottom of the calorimeter. The calorimeter bottom has been dished to facilitate removing the last few drops of the sample. The horizontal vanes inside the calorimeter have holes punched in their centers to permit these 0.125-inch tubes to reach the bottom of the calorimeter. In order to allow free passage of these tubes to the bottom, the platinum resistance thermometer re-entrant well was placed off center in the calorimeter.
HE purity of highly purified organic compounds which are liquids a t or below room temperature cannot generally be established by chemical means, so that it is necessary to make use of some physical method. At present, spectrometric methods are widely used for routine organic analyses but they are not satisfactory for analysis of materials which have a purity greater than 99%, since the precision claimed for infrared spectrometry is only 0.5% and for mass spectrometry 0.1%. To obtain a precision greater than O.l%, the purity can be determined by the freezing or melting point depression method. According to Johnston and Giauque (8), this technique is capable of a sensitivity of 0.0001 mole % impurity. Melting point determinations are widely used in organic chemistry to evaluate punty, but the usual procedure is some modification of the Thiele tube method. I n general, they are not very precise techniques, although a reproducibility of 0.03 C. has been claimed for one modification ( 5 ) . Greater precision in the determination of melting or freezing points below room temperatures can be achieved through the use of one of the various modifications of the time-temperature curve method (6, 11, 14, 15). Kone of these methods possesses the accuracy of determining the melting point which can be obtained with the Nernst calorimetric technique as developed for third law studies (16). I n addition to its greater accuracy, the calorimetric technique for the determination of purity by melting points possesses the advantage that all the necessary information, except the molecular weight of the material, can be obtained in a single determination. I n other methods, to evaluate purity from the melting point, it is necessary to know the freezing point of the pure compound and its cryoscopic constant or estimate it from the cooling data, with consequent loss of accuracy. As normally constructed and operated, low-temperature calorimeters require several days for one analysis and two operators are needed. The present melting point calorimeter has been designed to retain the advantages of the precise low-temperature calorimeters without retaining their disadvantages.
The complete melting point apparatus is shown in Figure 2. The top of the 0.25-inch filling tube is sealed to 8-mm. Pyrex tubing which has a standard taper joint a t the end. By removing the cap, made from one half of the standard taper joint, shown above the electrical connections numbered 10 and 11 (Figure 2), the long funnel can be slipped down through the filling tube into the calorimeter. A further connection is made to the Pyrex tub-
-FILLING T UEE
RING OF WIRES-
-CALORIMETER CRYOSTAl CAN+
APPARATUS
Following the general design of the precise adiabatic calorimeters (16),the calorimeter for this apparatus was built as shown in Figure 1. The similarity to the precise calorimeters can be seen a t once.
-SHIELDS WOODEN
The thirty horizontal copper vanes which are inside the calorimeter (approximately 15-cc. capacity) have not been shown in the drawing. In addition, the wires for the heaters on the shield and on the calorimeter, the difference couples, and the thermocou les are not indicated. The main differences between this and tEe precision calorimeters are the 0.6-cm. (0.25-inch) German silver iilling tube, the offset re-entrant well for the
STRAIN- F R E E 'PLATINUM RESISTANCE THERMOMETER
Figure 1. Detail of Calorimeter 218
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V O L U M E 19, NO. 4, A P R I L 1 9 4 7
The large millianimeter measures the ourrent in the odorimeter heater. Since the resistance of this heater is known a8 a function of temperature, readings of the large milliammeter can he readily converted t o energy input t o the calorimeter. The four small
calorimeter heater is 30,6. and 8 volts. respectid Y Figure 4 is a photograph of the compk:te assembly, showing the melting point apparatus, the Mueller hridge, and the control box for the heating circuits and potentiom6:ter. The freezing point Streiff. and Rossini %pparatusof the type described hy GlruJZOW. (6) can he seen on the right side of the vacuum line. From this photograph, one can readily see the relative space occupied by the two types of apparatus, Furthermore, the very compactness of the arrangement of the melting point apparatus which man operation feasible is clearly shown. The only contributions of this apparatus to the art or caiorimetric melting point determination are its speed and its need for only one operator. The speed has been achieved through the use of the large filling tube, permitting very rapid filling and emptying of the oalolorimeter. In addition, because a certain amount of accuracy has been sacrificed t o aimplify operations, the calculations have been considerahly simplified. All computations can he d e by means of a slide rule and special tables which have been constructed. (The principles and methods of the calculation of calorimetric data have been discussed in considerable detail by Sturtevant, 16). Operation by only one man has been achieved by measuring the energy input to the calorimeter on the large milliammeter. I n addition, the very compactness of the control and measuring circuits has made it easy for a single operator t o maintaiu the shields and odorimeter in b d m e e and concurrently follow the drifts of the calorimeter temperature by means of the Mueller bridge.
ing from the high vacuum line, so that the calorimeter can be evacuated after the standard taper cap has been replaced. Since the measurements are made in the absence of air, the data obtained are triule ooints. On the left side of Figure 2 is the cali~
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indicated and labeled. The lower setof connections are for thk platinurp resistance thermometer and the thermocouples. All these leads are connected to either the Mueller bridge or the control box (Figure 3) by multiconductor. shielded cable. With the exception of the Mueller bridge. all electrical measuring acd heating control devices have been incorporated into one control box, as shown in Figure 3. T h e box is made into three sections. The section on the left is the potentiometer for reading the e.m.f. of the thermocouples. The various selector switches for the thermocouples and the difference couples are shown clearly. The center section is a special taut suspension galvanometer with a sensitivity of 0.5 p per mm., which is used either with the potentiometer or as a direct reading instrument for the deflections caused by the e.m.f. of the difference couples. In operation, the shields are balanced, so that the difference couples produce a deflection of less than 1 mm. on the galvanometer scale. The right section contains the heating circuits for the cslorimeter, the shields, and the ring of lead wires (see Figure 1).
TYPICAL DATA
I n order to a h k i n an indication of the type of data which can he obtained with this apparatus, the results of measurements on a sample of 6rans-hutene2 have been summarized in Figures 5 and 6. The measurements of heat capacities of the solid tl-ans-butene2 obtained in this sppmatus are shown as circles in Figure 5. When these results are compared with the data of Guttman and Pitzer (7). shown as the triangular mints. it can be seen that the
Figure 3. Control Box
ANALYTICAL CHEMISTRY
220
considcrably higher degree of skill and the use of more judgment on the part of the operator. I n spite of these disadvantages, the calorimetric method is extremely valuable, since it i s possible to evaluate purity from differences in melting point as determined by the slope of a straight line. Furthermore, the effect of air solubility is not a factor. On the other hand, in the case of ' compounds which h v e a pronounced tendency t o farm glasses, the freezing point tcehnique offers the considerable advantage of the opportunity far seeding. A t present, in thc two cases in which glass formation has been encountered, pentene-1 and isopropylbenzene, i t has . . ... . not been possible to inauce crystd i m t i o n of the samples in th e calorimeter. e t ,er Assembly In all purity determinations base,i on the depression of the freeaing or melting points, one of the brisic assumptions made is .L.*'L.: :'..:.-:-..:> --,..,.,-"-A-L I I LLE ~ L L I L ~ U I L Q1s ~ L ~ L U ~ U - ~ U L U Vmurulid-insoluble. L~ Therefore 6% or m e values aoramea wnn tne more precise proeeaure. o e the question arises in evaluating an analysis by this method as to cause of the relatively large amount of impurity in the sample how one knows whether or not solid solutions are formed between which was studied in the new calorimeter, the heat capacity curve the solvent and the impurity. The importance of recognizing the shows a very considerable amount of premelting. existence of solid solution in the analysis of organic chemicals by Figure 6 presents the fusion data obtained on this same sample tho freezing or melting point method is not to be underestimated, of trans-butene-2. From the curves shown, the triple point of for it has been found that six binary systoms of relatively closetmns-butened wss found to he -105.52" C. as oompared with the boiling hydrocarbons indicate solid solution formatiou (17). value of -105.55" C. found by Guttman and Pitzer (7). Considering the fact that the purity of the present sample was 97.68% as compared with 99.153'7~for tho sample studied by Guttman and 55 Pitzer, the agreement is to he considered very good. Similarly, the values for the hest of fusion are in good. agreement, thc authors' value being 2230 calories per mole as compared with 2332 calories per malc for Guttman and Pitzer. Far the sake of further comparison other data are listed in Table I. The precision as indioated by the above comparisons are what is to he expeetod from this apparatus. While tho above table shows that the pmcisian of heats of fusion is of the order of 57,, those values were obtained incidcntal to the determination of the melting point of the various samples. They are the summation of the small incroments of energy necessary to melt various fractions of the samplc being studied. If measurements were to be made primarily to dctermine the heat of fusion, it is fclt that the precision would be of the ordor of 1%.
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DISCUSSION' Tho melting point calorimeter represents more than jnst a versittilc analytical tool. Unlike the spectrometers, it is not necessary to have high-purity samples to us8 as standards. Any relatively pure material which is a liquid a t or below room temperatures and nhich will crystallize without seeding or "shocking" and does not. form solid solution can he analyzed without any information other than its molecular weight, providing one is sure one is on the right side of the eut.ectic. When there is no information available in Ihe literaturo on a sample which is bo he analyzed, the time required is approximately 6 to 8 hours. With some information a,vailable the time is cut to about 4 hours. Obviously, this method of determining the melting paint is not so rapid as thc time-bmperature ourve t,echniques (6, 11, 14, 15). Furt,hcrmore, it is not so w d l adapted to routine measurements, for it requires R
TE~PERATURE°C.
Figure 5.
H e a t Capacity of trans-Butene-2
V O L U M E 1 9 , NO. 4, A P R I L 1 9 4 7 Table I.
Material
The calorimeter possesses versatility beyond melting point determinations. I t is possible to measure heat capacities down to liquid nitrogen Impurity This temperatures (approximately 80’ IC.). Following research Others the extrapolation method used by Parks (IO),it is Mole % possible to obtain entropies which are good to 0.20 < 0.001 (9) about 3% for materials which are normally liquid 1.31 0.057 ( I d ) 1.67 < 0 . 0 0 1 (13) at room temperature. With the heat of combus0,081 tion, it is then possible to determine the free energy 0.01 O.OOl(2) of the material at room temperature. In addition, the calorimeter is very well adapted to the determination of vapor pressures up to 2 atmospheres at temperatures up to about 25” C. With its multiple utility, the melting point calorimeter can be a very valuable adjunct to any laboratory engaged in the synthesis and preparation of high-purity organic chemicals.
Comparison of Results Obtained in the Melting Point Calorimeter with Best Data in the Literature Heats of Fusion This research Others Cal ./mole 590 5 7 7 . 2 (9)
Triple Points This research Others
c.
c.
Carbon tetra-22.89 rhloride . ~.~.-. cis-Butene-2 -138.72 Butadiene-1,3 - 1 0 8 . 9 8 Isoprene -145.91 Cyclopentane -93.47 0
221
- 2 2 . 8 7 (9) -138.90(12) -108.92(fd) -146.8(4)0 -93.46(2)
1685 1870 1115 136
1743 ( I d ) 1908(15) 1155(5) 1 4 4 . 1 (2)
Freezing point
RECIPROCAL
OF F R A C T I O N
MELTED
3
2
4
ACKNOWLEDGMENT
The authors would like to express their appreciation to the Leeds & Xorthrup Co. and to Dr. Fairchild of the Tagliabue Mfg. Co. of New York for making available special taut suspension galvanometers of high sensitivity, to T. Decker for the construction of the calorimeter, and to F. Maloy and G. Szasz for the construction and calibration of the platinum resistance thermometer. LITERATURE CITED
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rttibLN I
Figure 6.
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(1) Aston, Fink, and Cines, J . Am. Chem. SOC.(in press). (2) Aston, Fink, and Schumann, Ibid., 65,341 (1943). (3) Bekkedahl and Wood, J . Research Natl. Bur. Standards, 19, 551 (1937). (4) Bekkedahl, Wood, and Wjociechouski, I b i d . , 17, 883 (1936). (5) Francis and Collins, J . Chem. Soc., 1936, 137. (6) Glasgow, Streiff, and Rossini, J . Research S a t l . Bur. Standards, 35, 355 (1945). (7) Guttman and Pitzer, J . Am. Chem. Soc., 67,324 (1945). (8) Johnston and Giauque, Ibid., 51,3194 (1929). (9) Johnston and Long, Ibid., 56,31 (1934). (10) Kelley, Parks, and Huffman, J . Phys. Chem., 33, 1802 (1929). (11) Schwab and Wichers, in “Temperature, Its Measurement and Control in Science and Industry”, p. 256, Iiew York, Reinhold Publishing Corp., 1941. (12) Scott, Ferguson, and Brickwedde, J . Research Natl. Bur. Standards, 33, 1 (1944). (13) Scott, Rands, Brickwedde, and Bekkedahl, Office of the Rubber Director, Rept. CR119 (1943). (14) Skau, Proc. Am. Acad. Arts Sci., 67,551 (1933). (15) Smittenberg, Hoog, and Henkes, J . Am. Chem. SOC.,60, 17 (1938). (16) Sturtevant, in Weissberger’s “Physical Methods of Organic Chemistry”, Vol. 1, Chap. X,New York, Interscience Publishers, 1945. (17) Tooke and .Iston, J . Am. Chem. Soc., 67,2275 (1945).
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MCLICU
Fusion Data on trans-Butene-2
To investigate the solid-solubility of impurities in the freezing point technique, it is possible to add a small amount of the compound which is suspected to be the impurity and determine the molal lowering for that addition. Assuming that the heat of fusion is known, it is then possible to tell whether or not the added impurity forms solid solutions with the solvent. While such a procedure is accurate and correct, it is time-consuming, for, each time a sample were to be analyzed, it would be important to know whether its method of preparation was such as to allow variations In the materials n-hich are present as impurity. Obviously, although such a procedure can be very time-consuming, without exercising such precautions the analytical results are alTFays subject to some doubt. The procedure of investigating solid-soluble impurities in the calorimetric determination of nielting points is different. I t has been pointed out ( 1 ) that the ”premelting” characteristics of a solid solution are different from a mivture m-hich forms a eutectic. Because of this difference, in the case of a solid-soluble impurity, the calculation of the amount of the impurity from “premelting heat capacities”, as outlined by Johnston and Giauque (a),will be appreciably less than the value calculated from melting point loxering data. Therefore, to investigate the possibility of solidsoluble impurities in a neTT sample, it is necessary only to determine the heat capacities of the solid in the premelting rcgion. For this reason, in evaluating purities in unknown system., calorimetric determinations of melting points are of pal t i d a l value.
PRESENTED before the Division of Analytical and 3Iicro Chemistry at the 109th Meeting of the AMERICASCHEMICAL SOCIETY, Atlantic City, N. J.
Suggested Abbreviation for Analytical Chemistry Khen references to articles published in AS~LYTICAL CHEW are made in literature citations, particularly in connection uith manuscripts that are intended for publication in the journals of the A h i E R I c a s CHEMICAL SOCIETY as well as in other races n-here the use of an abbreviation for the name of this journal is desired, AXAL. C”Ear. is suggested as probably the most satisfactory. This is consistent with the abbreviations that are used in the List of Periodicals abstracted by Cheniicnl .lbstracts, which was made official in this respect by the Intcrnational Union of Chemistry several years ago. ISTRY
