INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
September, 1944
The necking effect for materials with a high degree of complex
flow,as distinguished from the elongation of the viscous materials is accentuated in materials of high consistency; the two lower photographs of Figure 5 show asphalts AA and BB which have the characteristics given in Table V. The center picture was made at an elongation of 3.5 em., and the lower one was taken after asphalt BB failed. The high consistency of these materials made it necessary to use instrument 5B for the rheological data given in Figure 4. Since elongation under standard test conditions is the only property measured by the ductility test, it is futile to deduce other interpretations from the results. If any fundamental information concerning asphalt is to be obtained from the ductility test, it is necessary to measure the force required to deform the material as is done in tensile strength tests. This was done by Abraham (1) and by Grant and Pullar (7') with apparatus of special design. I t is obvious that extremely sensitive equipment would he necessary for materials having ductilities of 100 cm. or higher and for those soft maieiials with low ductility. I t is possible that even such special tests would be no more informative than the rheological data obtained over a range of shearing stresses in viscometers designed on scientific principles. LITERATURE CITE13 (1)
for Testing Materials, Standard Method of Test for Ductility. of Bituminous Materials (D113-39), Part 11, p.
(2) Am. Soo.
466 (1942). (3) Ibid., Test for Penetration of Bituminous Materials (D5-25), Part 11, p. 483 (1942). (4) Ibid., Test for Softening Point of Bituminous Materials (Ring and Ball Method) (D36-26), Part 11, p. 488 (1942).
(5) Barr, G., "Monograph on Viscometry", London, Oxford Univ.
Press, 1931. Fair, E. F., Jr., and Volkmann, E. W., IND. ENG.CHEX., AXAL.ED., 15, 240-2 (1943). (7) Grant, F. R., and Pullar, H. B., Proc. Assoc. Asphalt Paving
(6)
Tech., Jan., 1936, 124. (8) Mooney, M., and Ewart, R. H., Phvsics, 5, 350-4 (1934). ( 9 ) Pendleton, W. W.,J . Applied Physics, 14, 170-80 (1943). (10) Rhodes, E. O., and Volkmann, E. W., Zbid., 8 , 492-5 (1937). (11) Rhodes, E. O., Volkmann, E. W., and Barker, C. T., A m . Soc. Tasting Materials, Symposium o n Consistency, 1937, 30-46. (12) Saal, R . N. J., Pmc. World Petroleum Congr., London, 2, 515-23 (1933).
Traxler, R. N., IXD.ENG.CHEM.,30, 322-4 (1938). Traxler, R. N., and Moffatt, L. R., IND. ENG.CHEM.,ANAL. ED., 10, 188-91 (1938). (15) Traxler, R. N., Romberg, J. W., and Schweyer, H. E., Ihid., (13) (14)
14, 340-3 (1942). (16)
Traxler, R. N ~and , Schweyer, H. E., Proc. Am. Soc. Testing
i M a t e ? i ~ l36, ~ , 544-51 (1936). (17) Ibid., 36, 518-30 (1936). (18) Traxler, R. N., Sohweyer, H. E., and Romberg. J. W.. Ibid.. 40, 1182-1200 (1940).
Abraham, H., "Asphalt and Allied Substances", 4th ed., New York, D. J7an Nostrand Co., 1938.
Ther
829
P m e E N r m a t the annual meeting of the Society of Rheology, in New York,
N. Y.,
1848
ynamic J
SPECIFIC HEAT AND RELATED PROPERTIES KENNETH S. PITZER University of California, Rerkrley, Calif.
F
The values for methane (4), ethane (17), and propane (18) are based on reasonably definite molecular-structure parameters (vibration frequencies, internal rotation potential barriers, etc.). For the heavier normal paraffins a structural picture was assumed which is uniform throughout the series and which, evidence now available indicates, is substantially correct (10, 12). While approximations are made both in the basic picture and in the mathematical analysis, their nature is such as to lead to small and uniform errors which can be compensated by the use of adjusted structural parameters.
When this treatment was first presented, the data necessary to fix certain parameters was inadequate. There is still much to be desired, but considerable improvement has led to the following revised values. The vibration frequencies of the CH, group are now taken as 827, 1170, 1375, 1460(2), 2950(3) em.-' (degeneracy), which are the exact ethane values and are substantially thoso used previously. Although the propane frequencies (13) cannot be definitely classified between CHa and CH2 groups, certain frequencies group closely around the CH8 values and leave 940, 1278, 1338, 1460, and 2950(2) em.-" to be ascribed statistically to the CH2 group. The use of these frequency sets gives good agreement with the experimental gaseous heat capacity data on the higher paraffins. The potential barrier and reduced moment for the end methyl groups of a lonqer chain are now assigned exactly the propane values (IS), 3400 calories per mole and 4.51 X lou4"
Available experimental gas specific-heat values for the normal paraffins are in excellent agreement with curves calculated by methods previously published by the writer. Certain parameters in these calculations are revised on the basis of recent spectroscopic studies. Calculated entropies are still in excellent agreement w-ithmeasured values. The
corresponding results for heat content and the free energy function are also presented. The data for branched paraffins are too meager to allow generalizations except that the change in specific heat with isomerization is small. Entropy differences calculated previously have been confirmed, in so far as additional data are available.
OR several years the writer has been interested in the specific heat of gaseous paraffin hydrocarbons. The accompanying figure and tables present a convenient summary of the present status of results. NORMAL PARAFFINS
INDUSTRIAL AND ENGINEERING CHEMISTRY
830
TABLE I.
ENTROPIES OR
Vol. 36, No. 9
X O R M A L P A R A F F I N HYDROCARBONS
C a l . / O K./Gram Mole
T, X. 298.16 231.09 272.66 298.16 371.5
Ethane (19) Propane (6') n-Butane ( 1 ) n-Pentane ( 0 ) n-Heptane ( 1 1 )
TABLE 11. T, K.
Scxptl. 54 85-0.15 60.46* 0 . 1 72 05*0.2 83.13 -0.2 111.77*O .3
S0SlC.d.
54.85 60.46 72.06 83,.27 111.5
TIIGRMODYNAMIC P R O P E R T I E S O F
GASEOUSNORMAL
PARAFFIN HYDROCARBONS
298.16
400
500
600
1000
800
1500 '
Molal Heat Capacity, Gram-Cal./' K. Methane Ethane Propane Butane Pentane Hexane Heptane Octane A per CHn
8.536 12.585 17 57 23.61 29 30 35 06 40 82 46 58 5.760
9.736 15.68 22.54 29.80 36.91 44.04 51.18 58.31 7.134
11.133 18.66 27.04 35.54 43.96 52.39 60.83 69.27 8.438
12.546 21.31 30.84 40,32 49.76 59.21 68.66 78.11 9.45
15.10 17.21 20.71 25.83 29.32 34.90 37.OS 41.83 49 26 48.23 54.20 63 51 59.37 66.57 77.76 70.51 78.94 92.02 81.65 91.31 106.27 92.79 103.68 120.52 11.14 12.37 14.26
Entropy, Gram-Cal./o K./Gram Mole Methane 44.50 47.16 49.48 Ethane 54.85 68.98 62.79 Propane 64.51 70.37 75.89 Butane 74.10 81.91 89.20 Pentane 83.27 92.95 101.03 Hexane 92.45 104.03 114.73 101.64 115.10 127.53 Heptane Octane 110.82 126.18 140.34 A per CHa 9.186 11.076 12.803 Heat Content ( H ; Methane Ethane Propane Butane Pentane Hexane Heptane Octane A per C
36.46 45.27 52 73 58.51 64.26 70.01 75 76 81.51 5.75
38.86 48.24 56.48 63.49 70.33 77.20 84.06 90.93 6.866
8.321 12.75 17.77 23.34 28.74 34.16 39 59 45.01 5.423
- F $ ) / T , Gram-Cal./' 40.75 50 77 59.81 67.93 75.77 83.65 91.54 99.42 7.885
Methane Ethane Propane Butane Pentane Hexane Heptane Octaue Cm
11.56 21.13 18.28 34.60 25.67 48.64 33.58 63.27 41.34 77.76 49.12 92.25 56.89 106,74 64.66 121.23 7.77 14.49
K./Gram Mole
42.30 45.21 47.65 52.84 53.08 57.29 61.11 69.46 62.93 68.74 74.10 85.85 72.00 79.70 86.72 102.03 80.78 90.28 98.92 117.75 89.60 100.90 111.18 133.52 98.42 111.52 123.43 149.29 107.24 122.14 135.69 165.06 8.82 10.62 12.26 15.77
Specifio Heat, Gram-Cal./O C./Gram or B.t.u./'
T, K. 6, ' F.
66.93 92.45 118.28 144.22 169.59 195.03 220.45 245.88 25.43
Kg.-Cal./Gram Mole
2.397 3.323 4.365 5.549 2.856 4.295 6.010 8.003 3.512 5.558 8.040 10.925 4.646 7.370 10.636 14.41 5.664 9.046 13.081 17.73 6.689 10.730 15.540 21.08 7.713 12.414 17.998 24.42 8.787 14.098 20.456 27.77 1.024 1.684 2.458 3.346
Free Energy Functions, (If: Methane Ethane Propane Butane Pentane Hexane Heptane Ootane Aper CHr
- H:),
51.64 55.61, 59.21 66.42 73.23 79.39 81.13 90.95 99.77 96.01 108.87 120.31 110.33 126.21 140.27 124.73 143.60 160.30 139.12 161.00 180.32 153.52 178.40 200.35 14.396 17.40 20.03
298.16 400 500 600 800 77.00 260.3 440.3 620.3 980.3 0.532 0.607 0.694 0.782 0.941 0 419 0.521 0.621 0.709 0.859 0.398 0.511 0.613 0.699 0.841 0 406 0.513 0.612 0.694 0.830 0.406 0.512 0 609 0.690 0 823 0.407 0.511 0.608 0.687 0.818 0.407 0.511 o . 6 ~ 0.685 0.815 0.408 0.510 0.606 0.684 0.812 0.411 0.509 0,.602 0.674 0.794
F./Lb. 1000 1340.3 1.073 0.975 0.949 0.933 0.923 0 916 0.911 0.908 0.882
1500 2240.3 1.291 1.161 1.117 1.093 1.078 1.068 1.061 1.055 1.016
gram cm.* The potential barrier for skeletal internal rotations is determined, as before, from the experimental entropy values. These revisions shift this parameter from 3600 to 3300 calories per mole. Although a value smaller than the 3400 calories per mole of the end methyl groups may seem improbable, it should be remembered: (a) that this is an adjusted parameter which absorbs all inaccuracies of independently obtained parameters and approximations in the method of calculation; and (b) that the steric repulsion term which is independently introduced in this method effectively raises the barriers in all actual cases. Except as stated above, the parameters and formulas given in 1940 (10) are used in the present calculations. Table I compares the observed and calculated entropies of gaseous normal paraffins. Table I1 presents the calculated heat capacity, entropy, heat content, and free energy functions. The 8
I"
300
400
TPK.
500
600
Figure 1. Molal Heat Capacity of Gaseous Normal Paraffin Hydrocarbons from Propane to Heptane Kistiakowmky and Rice (7); Q Dailey and Felsing (3); Q Pitzer (11, 14); t, Eucken and Samtedt (5).
molal heat capacity curves are shown in Figure 1 together with experimental results. Table I1 also gives the specific heat over the range 298' to 1500' K.or 77" to 2240' F. The accuracy of the results can be judged from Table I and Figure 1. The specific heat curves should not err by more than?% except at the highest temperatures and arc probably more accurate than this. The values are based on the thermochemical defined calorie of 4.1833 international joules and R 1.98718 calories per degree. Otherwise in so far as the difference is significant, the physical constants of Birge (8) (1941) are used rather than those from International Critical Tables used heretofore. =5
BRANCIIED-CHAIN PARAFFINS
Both experimental data and theoretical development for branched-chain paraffins are less complete than for the straight chains. The entropies (and free energy functions) calculated in 1940 have been confirmed by all experimental values obtained since. Most notable was the value of Schumann, Aston, and Sagenkahn (16) for isopentane which agrees exactly with the calculated entropy value of 82.0 calories per O K. at 298.16' I(. Data were also obtained for 2,3,4-trimethylpentane (14 ) with satisfactory agreement. It now appears that the heat capacities implied by the 1940 tables are somewhat too high for branched-chain paraffins just as they are for the normal series. Without doubt the cause is the same-namely, the use of too low values for the hydrogenbending vibration frequencies. However, the spectroscopic studies have not yet progressed to the point where reliable sets of frequencies can be assigned (even in a statistical sense) to the various groups of atoms. Nor are there enough reliable gas heat capacity data to allow generalizations. Available data show isobutane (a), isopentane (5), and neohexane (5) to have molal heat capacities a fraction of a calorie per O K. lower than their normal isomers: the four trimethylpentanes (8) all have slightly greater heat capacity than N-octane.
September, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
Thus it appears that the best available values for the differences in entropy or in the free energy function between isomers are still those published by the writer (IO) in 1940; the absolute value of the heat capacity of branched paraffins not yet studied experimentally may best be assumed to be the same as the normal isomer. In particular there is no need to revise the calculations of isomerization equilibria made by Rossini, Prosen, and the writer (16). No attempt is made in this brief note to compare these results with the numerous empirical or semitheoretical calculations which have been published nor with old experimental values where more accurate ones exist. The recent papers of Stull and Mayfield (IS) give many references to such work. ACKNOWLEDGMENT
The results of the present paper are being combined with recent heat of combustion values of Prosen and Rossini for the normal paraffins to give free energies of formation and related data, in connection with Research Project 44 of the American Petroleum Institute, a t the National Bureau of Standards. The writer wishes to thank E. J. R. Prosen for carrying out some of the calculations here presented.
CAROTENECONT
831
LITERATURE CITED (1)
Aston, J. G . , and Messerly, G . H., J. A m . Chem. SOC.,62, 1917
(2) (3)
Birge, R. T., Rea. Modern Phys., 13, 233 (1941). Dailey, B. P., and Felsing, W. A., J . Am. Chem. Soc., 65, 42, 44
(1940). (1943).
Dennison, D. M., Rev. Modern Phys., 12, 175 (1840). Eucken, A., and Sarstedt, B., Z. physik. Chem., B50, 143 (1941). Kemp, J. D., and Egan, C. J., J. Am. Chem. Soc., 60,1521 (1938). (7) Kistiakowsky and Rice, J . Chem. Phys., 8 , 610 (1940). ( 8 ) Leininger, R. F., unpublished measurements, Univ. Calif. (9) Messerly, G . H., and Kennedy, R. M., J . Am. Chem. SOC.,62, (4)
(5) (6)
2988 (1940). (10)
Pitzer, K. S., Chem. Rev., 27, 39 (1940); J . Chem. Phys., 8 , 711
(1940). (11) Pitzer, K. S., J . A m . Chem. Sac., 62, 1224 (1940). (12) Ihid., 63,2413 (1941). (13) Pitzer, K. S., J. Chem. Phys., 12, 310 (1944). (14) Pitzer and Scott, J. Am. Chem. SOC.,63, 2419 (1941).
(15) Rossini, F. D., Prosen, E. J. R., and Pitaer, K. S., J. Research Natl. Bur. Standards, 27, 529 (1941). (16) Sohumann, S. C., Aston, J. G . , and Sagenkahn, M., J . A m . Chem. Soc., 64, 1039 (1942). (17) Stitt, F., J . Chem. Phys., 1, 297 (1939). (18) Stull, D. R., and Mayfield, F. D., IND.ENG.C H ~ M35, . , 639, 1303 (1943). (19)
Witt, R. K., and Kemp, J. D., J.Am. Chem. SOC.,59, 273 (19371,
ALFALFA
etention on Dehvdration and Storage J
Various treatments are reported which stabilize the carotene in alfalfa during the process of drying. Blanching the fresh alfalfa with steam, prior to drying, furnishes complete protection for the carotene, and considerable protection is afforded when fresh ground alfalfa was treated with certain chemicals before it was dried. Two types of chemicals are used-namely, antioxidants and substances which are known to inactivate enzymes. Diphenylamine and hydroquinone are the most effective of the first type; thiourea and sodium cyanide are more effective than any other substances tested of the second type. The carotene content of alfalfa meal decreases as the temperature of storage is increased. However, essentially no change has been noted in the carotene content of alfalfa meals stored at 3" C.
M
ORE and more attention is being directed toward the
retention of natural nutritional constituenta in feeds as well as in foods since the practice of fortifying them with various food concentrates has come into common use. The primary purpose of this study on the carotene content of alfalfa is to find a practicable method to stabilize this precursor of vitamin A during dehydration and storage. The alfalfa used was stripped from the plants in the field. I n this way, principally leaves were collected since the bulk of the stems were avoided, and more nearly uniform samples were available for analysis. Stripping the plants has an additional advantage in that the carotene content of the material is much greater than if the whole plant is used, and thus the effect of a particular treatment can be noted more readily. The dehydration of the various samples was carried out in a Freas circulating air oven a t 65" C. The fresh and blanched samples were placed on towels on trays so that contact with metal
RALPH E. SILKER, W. G. SCHRENK, AND H. €1. ICING Kansas Agricultural Experiment Station, Manhattan, Kans,
was avoided. (The effect of certain metals will be reported in B later paper.) The dehydrated alfalfa was ground in a Wiley mill using a 2-mm. mesh screen; samples were taken for analysis, and the remainder was stored in glass jars with tight-fitting covers. A greater loss of carotene accompanies this slow method of dehydration than that of the rapid commercial dehydrators, but the different samples of a given experiment which are compared were subjected to the sbme conditions. METHOD OF ANALYSIS
The method used for det,ermination of carotene was essentially that of Wall and Kelley ( I S ) , which is a modification of the method of Moore and Ely (IO). Duplicate samples were run except for a short period when help was not available. Five grams of the fresh material (stemmy portions were broken into small pieces to avoid excessive splashing when the Blendor was started) was placed in a Waring Blendor with 60 ml. of a 1-1 Skellysolve B-alcohol mixture, and sufficient alcohol was added to form a foaming mixture. The moisture content of the alfalfa determines the amount of alcohol needed; 125-150 ml. were usually required. Blending was carried out for 10 minutes, and a nearly colorless pulp remained on filtration. The Blendor and pulp were carefully washed with Skellysolve B, and the extract and washings were treated with 100 ml. of water to remove most of the alcohol and cause the Skellysolve B layer to separate. Addition of a small amount of anhydrous sodium sulfate to the extract eliminated the formation of an emulsion. After separa-
