IlVDCSTRIAL A S D ENGINEERISG CHEMISTRY
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a small pre-dip tank to hold the methanol. The fruit in its foreward movement could be carried into and out of the methanol on a simple carrying device, for since apples sink in methanol they would be quiclcly and thoroughly covered. The results indicate that the wax-solvent action would be rapid enough to permit the cleaning machine to be run a t about its customary rate. The apples used in these experiments were unusually difficult to clean. With these the 1minute solvent period mas more than ample, and it is probable that 30 seconds or less would ordinarily be sufficient.
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By using a sloping drain board for the outgoing carrier there should be very little carry-over of the methanol. It could readily be used over again, as it filters easily from the sediment that collects from the dipped apples. After a long period of use it might need to be distilled to separate it from the dissolved wax. This could be done in a still of simple construction as no fractionating column would be necessary. Literature Cited (1) Heald, S e l l e r , and Overleq,
Tvash. Agr.
Expt Sta , Bull. 226 (1928)
Critical Solution Temperatures of Systems of Sulfur Dioxide and Normal Paraffins' W. F. Seyer and Eric Todd UXIVERSITY OF
B R I T I S H C O L U M B I A , T'AXCOUVER,
BRITISHC O L U M B I A
pek-oletini hydrocarions, it seemed desirable t80investigate under what conditions this consideration is true. JIaiiy binary systems of partially miscible liquids have been jtudied with interesting results ever since Abaschew and Alesejen ( 1 ) first opened up this field. Much of this work was actuated by a desire to find a general equation by means of n-hich a curve defining the limits of the coexistence of the two liquid phases could be constructed. The similarity of the critical phenomena in pure liquids to the critical solution temperature (C. S. T.), as well as the similarity in shape of the curves, suggested that this might be possible. This similarity in shape is due largely to the way in mhich the results are plot'ted. By plotting the mol fraction as molal percentage against the temperature a t which one phase disappear., somewhat, different curves are obtained than when the usual method as first recommended by Rothmund I.?) is used. The straight percentage method of plotting tends to mask somewhat the influence of the molecular weights of the components on the C. S, T., which, as d l be shown in this paper, is a n important factor in dealing with partially miscible liquids. A comparison of Figure 3, where t'lie temperatures defining the limits of miscibility are plotted against the numlier of grams of sulfur dioxide in 100 grams of mixt'ure, with Figure 2 . \There mols of sulfur dioxide in 100 mols of mixture are used) shows the difference in the slope of the curves.
The normal hydrocarbons butane, hexane, octane, decane, and dotriacontane were all synthesized in this laboratory. The first four were prepared from the necessary alcohols which had been obtained in as pure a form as possible. These alcohols were converted into the iodides, which \yere in turn treated with sodium. After several distillations their physical properties were measured, and found to agree reasonably well with the constants given in the Critical Tables. S o difficulty was experienced in preparing the dotriacontane by the method of Kraft ( 4 ) from Eastman's c. P. cetyl alcohol. It had quite a sharp melting point a t 70" C. The decane had been prepared several years previously by A. F. Gallaugher from exceptionally pure amyl alcohol obtained from Kahlbaum. Dodecane and tetradecane were purchased from the Eastman Kodak Company and beyond a distillation over sodium to remove water were not further purified. The sulfur dioxide used was the c. P. material put up by Baker in small iron tanks. It was passed through several wash bottles of sulfuric acid and then over phosphorus pentoxide to remove any moisture before being condensed.
Procedure
Determination of Miscible Points
The esperiniental method used was the plethostatic first devised by Alesejew ( 2 ) . By keeping the temperature of the bulbs at the freezing point of the sulfur dioxide before and uliile condensation was taking place, the amount of hydrocarbon lost when the air in the t'ubes was displaced was negligible. A special apparatus was required for filling the tubes with butane and sulfur dioxide. Two 5-liter flasks, d and B in Figure 1, served as reservoirs for the two gases. By means of the tube C they could be connected a t will to a manometer. The tubes to be filled with gases were connected to D. The whole system was evacuated as much as
The miscible points were determined in the usual way. Below 0" C. a bath of acetone was used, from 0" to 80" C., water, and above that, petrolatum. Considerable difficulty was encountered in getting the miscible points when small amounts of hydrocarbon were present, and the temperature difference from the points a t which the two phases disappeared when the temperature of the bath was rising to the point of their reappearance when the temperature of the bath was falling was seldom less than 1" C. I n the neighborhood of the critical temperatures an accuracy of 0.2" C. was attainable. I n no case, however, was the bluish opalescence observed that is so common in the region of the C. S. T. This
I
Received Rovembar 29, 1930.
ofLsulfur d i o x i d e could be introduced, as the volume of the system was known. I n a similar manner the desired amounts of butane could be condensed in each tube. Immediately after the introduction of the butane the tubes were sealed off a t the constrictions. Materials
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Figure 1-Apparatus
for Filling Tubes with Sulfur Dioxide a n d Hydrocarbon
accounted somewhat for the difficulty in getting consistent results. It was also observed that certain mixtures had a miscible point almost a tenth of a degree higher in the petrolatum bath than in the water bath. This was due, no doubt, to the difference in refractive indices of water and petrolatum. The results are given in Tables I to VI. The miscible points of sulfur dioxide and octane were taken from the paper by Gallaugher and Seyer published in 1926 ( 7 ) . Discussion
Inasmuch as some of the hydrocarbons used may not have been so pure as desirable, owing to the nature of the synthesis, no greater claim to accuracy in connectionwith the C. S.T. than 0.5" C. is claimed. Table I-Miscible
Vol. 23, No. 3
Figure 2 contains the curves obtained for the various systems when miscible temperature is plotted against the mol fraction x 100 of the sulfur dioxide. It will be seen that the curves are all distorted to the right, owing to the fact that the molecular weight of the hydrocarbon is much higher than that of the sulfur dioxide. I n the case of butane the curve has very nearly the conventional shape. Here the molecular weights of the components are almost equal. Thus the distorted shape of the dotriacontane curve is due entirely to the high molecular weight of the hydrocarbon (450). When the miscible temperatures are plotted against per cent by weight of the sulfur dioxide, a curve is obtained which is symmetrical and quite similar to the classic example of phenol and water. Rothmund in 1898 (5) discussed in some detail the most appropriate method of plotting the results and decided on the per cent-by-weight method on account of the uncertainty connected with molecular weight of many liquids, especially water. Thus a rather unwarranted similarity is obtained in the curves of such systems having only one (an upper) consolute temperature. Surface-tension measurements indicate that association is nil or negligible in both hydrocarbons and sulfur dioxide. Consequently, Rothmund's objection to molal concentration falls down in this case. The system of curves brings out a number of interesting facts. Thus it is immediately apparent that below the C. S. T. the solubility of the sulfur dioxide in the hydrocarbon is greater a t the same temperature than of the hydrocarbon in the sulfur dioxide. It is also seen that the solubility of the hydrocarbons decreases with increasing molecular weight. This fact is further brought out in Table VII. Table IV-Miscible DODECANE Grams 1.9477 1.0582 0.6656 0.7308 0.8428 0.911 1.061 0,8189 0,3556 0,4966 0.389 0.122
Temperatures of Dodecane a n d Sulfur Dioxide SULFUR MOLFRACTION MISCIBLE DIOXIDE so2 x 100 TEMPERATURE Grams 0 c. .. 0.253 . 1.0 25.7 0.2616 39.2 14.5 0.2970 31.1 54.3 0,2804 36.0 50.6 0.5850 40.3 65 0,8206 41.6 70.5 1.2054 44.0 75.2 1.2756 45.3 80.4 0.9719 87.8 47.3 2.1279 46.4 91.8 2.7217 41.8 95 2.114 30.2 97.8
Temperatures of B u t a n e and Sulfur Dioxide SULFUR MOLFRACTION MISCIBLE BUTANE' DIOXIDE" SO2 X 100 TEMPERATURE Grams Gram O c. 0.0895 9 17 64 0.8045 0.2545 --i7.2 0.2310 -16.0 0.4320 -15.0 0.4880 - 6.8 0.7120 - 5.1 0.7275 4.7 Table V-Miscible Temperatures of Tetradecane a n d Sulfur Dioxide 0.9030 - 8 SULFUR MOLFRACTION MISCIBLE 1.0S4 26 TETRADECANE DIOXIDE SO1 X 100 TEXPERATURB 0 Calculated from pressure change. Grams Grams c. 1,164 0.041 2.6'3 .9.8 Table 11-Miscible Temperatures of Hexane a n d Sulfur Dioxide 1.0866 0 2105 16.9 37.4 SULFUR MOLFRACTION MISCIBLE 1.4280 0.341 26.0 42.5 HEXANE DIOXIDE SO2 X 100 TEMPERATURE 1,0280 0.3900 33.7 54 1.1580 0.5696 40.5 60.4 Grams Grams = c. 0.7929 53.4 0 7180 77.4 17.9 53 1.414 55,4 0.7736 85 27 5 - 22 2.044 55.5 86 1.05 - 17 30.0 1.515 55.3 0.5300 89.9 8 54.0 2.508 55.1 91 0.779 9.9 58.0 4.264 53.7 94 3 0.7985 10.6 63.5 4.122 52.7 94.8 0.6845 10 65.0 2.9440 44.7 97.7 0.2260 10.2 73.7 3.7100 21.6 9 9 . 4 0,0762 85.5 9.8 11.5 4.0850 99.6 0.0478 6.5 90 a Freezing point. 3.8 91.7 92.5 2.5 - 10 96.5 Table VI-Miscible Temperatures of Dotriacontane a n d Sulfur - 20 97 5 Dioxide SULFUR MOLFRACTION FREEZING Table III- -Miscible Tempera tures of Decane a n d Sulfur Dioxide SOP x 100 POINTS DOTRIACONTANE DIOXIDE SULFUR MOLFRACTION MISCIBLE 0 c. Gram Grams DIOXIDE TEMPERATURE DECANE so2 x 100 60.1 82.9 0.2887 0.2004 Grams Grams c. 63.1 77.8 0.2828 0.2413 13.3 -23 0.1088 1.5750 64.3 56.4 0,2274 0.0337 29.8 0 1806 0.0 0,9428 64.5 56.7 0,384 0.066 0.2530 35.0 7.5 1,0590 MISCIBLE 0.3151 14.1 40.0 1.0520 TEMPBRATUR E 26 0 50.6 0.7892 1.6960 90.9 79 29 0 54.6 1.5750 0.8559 94.6 101 73 8 1.188 37.0 0.9345 94.7 104 37.3 81.3 1.3588 0.6919 97.4 110 1.2400 37.2 82.7 0.5760 98.3 108 2.5120 34 4 93 0,4264 98.8 103.3 33.8 93.5 0.2290 1.4820 1.5864 99.1 32 8 103.5 94.09 0.2220 1.6600 0.4 99.8 75 99 0.0349
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would amount to only a fraction of one degree. There might be, in keeping with Le Chatelier's principle, either a positive or negative deviation. Figure 4 shows the relationship between molecular weight and critical solution temperature. No simple equation was found to fit this curve. If the curve is extended to the 501right until it intersects the temperature axis -04 a t 155.5' C., the critical temperature of suli fur d i o x i d e , it will be seen that the molecular weight of a hydrocarbon h a v i n g a C. S. T. a t this temperature will be about 1000. Above this temperature, inasmuch as a slight displacement of the critical temperature of the sulfur dioxide would occur owing to the presence of hydrocarbon vapor, only one phase will appear. the hydrocarbon. It is thus shown that molecular weight or its reciprocal, molecular volume, is a factor in determining the C . S. T. point and must be considered in any attempt to deduce a general 0 IO 20 30 40 50 60 70 80 90 100 Grmr of Sulphur Diorlde in lo0 Grmr ofM,xture equation defining the limits of miscibility in Figure 3 any binary system. The results of the variCritical Solution Temperatures of Sulfur Dioxide a n d Normal Paraffin ous measurements indicate that the C. S. T. 1-Butane and sulfur dioxide 5-Dodecaoe and sulfur dioxide of any system must be a f u n c t i o n of the 2-Hexane and sulfur dioxide 6-Tetradecaoe and sulfur dioxide 3-Octane and sulfur dioxide 7-Dotriacontane and sulfur dioxide molecular weights of the components, their 4-Decane and sulfur dioxide critical temDeratures and chemical natures. The diagrams make it o k i o u s that the amount of hydroThe C. S. T. 1-alues given in column 2 were taken from the curves and represent the points where the tangent to the carbon soluble in sulfur dioxide a t its boiling point, -10' C., curve is parallel with the composition axis. From these is very small, and certainly negligible in any refining operapoints the composition of the mixtures could be determined tion where the molecular weights of the hydrocarbons treated from the graph and are given in column 2. The figures in are above 100. blthough as yet only the systems of paraffin column 3 were taken from the Critical Tablef. The last hydrocarbons have been investigated extensively, it is reasoncolumn gives in an approximate manner the number of sulfur able to assume that the C. S. T. of the saturated cyclic hydrodioxide molecules to one of hydrocarbon in solution a t the carbons will lie quite close to that of the straight-chain parC. S. T. pi
Table W - V a p o r Pressure a n d Solubility of Hydrocarbons i n Sulfur Dicxide a t Critical Solution Temperature VAPOR
PRESGRE OF SYSTEM
C. S.T.
c. -4.7 10.2 26.9 37.3 47.3 55.5 110.0
PURE
,502 AT
C. S. T.
Aim. 1.25 2.26 4.07 5.61 7.53 9.38 34.0
MOL NO. 0 F s 0 1 FRACTION hlOLECULES OF SO, A T TO ONE OF C. S T. HYDROCARBON
0.70 0.72 0.76 0.83 0.86
0.89 0 974
2.3 2.6 3.1 4.9
6.1
8.1 37
-10
Taking the two mtreme cases butane and dotriacontane, it is seen that they require 2.3 and 37 niolecules of sulfur dioxide, respectively, to remain in solution a t the C. 8. T. There is, of course the effect of pressure to be taken into consideration. Whereas the relative volatility of the hydrocarbons in the mixture is unknown, it is nevertheless safe to assume. owing to their small vapor pressures a t the temperatures encountered, that the total pressure is for all practical purposes given by the partial pressure of the sulfur dioxide, with the exception of the butane system. Here the total pressure is so small a t the critical temperature that it can have no bearing on the point in question. Kowalsky in 1894 (3) investigated the influence of pressure on solubility and the C.S.T. He found that pressures of several hundred atmospheres were required to bring about changes in the C. S. T. commensurable with the accuracy of measurement in the present case. Thus in the case of dotriacontane, where the pressure is 34 atmospheres, the effect on the C. S. T.
*
50
150 200 250 300 350 400 450 5M, 550 600 650 700 730 Moleculor Weiqhf o f Hydrocarbons
IO0
Figure 4-Critical
Solution Temperatures of Sulfur Dioxide a n d Normal Paraffin
affins with the same number of carbon atoms. Thus it has already been shown by Dunbar and Seyer (6) that cyclohexane and sulfur dioxide are completely miscible in all proportions above 13.5" C., which is only slightly higher than that of hexane. Literature Cited (1) Abaschew and Alesejew, J . Russ. Phys. Chem. SOC. (1876 t o 1885). (2) Alesejew, Wied. A n n . , 28, 305 (1886). (3) Kowalsky, Compl. vend., 119, 512 (1894). (4) Kraft, Ber., 19, 2, 2218. (5) Rothmund, Z. p h y s i k . Chem., 26, 433 (1898). (6) Seyer and Dunbar, Trans. Roy. SOC.Can., 16, 307 (1922). (7) Seyer and Gallaugher, I b i d . , 20, 343 (1926).
