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vided with a jacket through which water could be circulated, was also tried. This requires some means of circulating water at constant temperature. No advantage is gained over the use of the simple design in a water bath. The accuracy attainable with the apparatus is subject to the same criticism as that of any method of sedimentation analysis. The shape of the particle and the lack of complete dispersion, partial flocculation, etc., have been discussed a t length by other authors. It is felt, however, that the advantages over the microscopic or air-elutriation methods, as pointed out by Borchers and May ( I ) , are suqcient to justify the use of sedimentation analysis, especially for technical purposes. The precision obtainable with this instrument is dependent upon the accuracy with which the tangents t o the sedimentation curve can be drawn. Actual
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readings taken on the manometer during carefully conducted check runs do not vary more than 2 per cent.
Literature Cited (1) Borchers, F., and May, E., Mitt. Biol. Reichansf Land u. Forstw., 50, 5-55, especially 17 (1935). (2) Crowther, E. M., J. Soc. Chem. Ind., 46, 105T (1927). (3) Fisher, R. A., and O d h , S.,Proc. Rou. Soc. Edinburgh, 36, 219
(1916); 44, 98 (1924). (4) Gessner, H., Kolloid-Z., 38, 115-23 (1926). ENG.CHEW,16, 928 (1924). (5) Kelly, W. J., IXD.
(6) OdBn, S., Kolloid-Z., 18, 33-48 (1916). (7) O d h , S., in Alexander’s “Colloid Chemistry,” Vol. I, 58, New York, Chemical Catalog Co., 1926. (8) Puri, A. N., Soil Science, 34, 115 (1932). (9) Wiegner, G . , Landto. Vers. Sta., 91, 41 (1918).
Chapter
RECEIVED May 27, 1936.
A Variable Vapor-Volume Barometric Type of Vapor Pressure Apparatus EVERETT M. BARBER
AND
A. V. RITCHIE, The Texas Company, New York, N. Y.
Design of Apparatus
to the atmosphere at the top. The tube, B, is sealed off at the top with a large carefully ground three-way stopcock and has alternately long narrow sections 30 cm. long and 1 cm. in diameter to obtain accurate readings of the liquid volume, and bulbous sections 15 cm. long and 3 cm. in diameter to accommodate a relatively large vapor volume. The open tube, the buret, and the leveling bottle, which constitute the essential parts of the vapor pressure measuring system, are filled with mercury, and in order to obtain temperature control are immersed in water bath a t 37.8’ C. (100’ F.). The bath is thermostatically controlled and has a temperature gradient of 0 . 0 5 O C. (0.1’ F.) from top to bottom and a variation in the average of about 0.1” C. (0.2” F.). Readings are made by means of a cathetometer which clamps to the tie rod at the front of the apparatus and is adjusted between the meniscus level and fixed stations on the tube and buret by means of a 40-thread micrometer screw. Successive trial measurements between two fixed points indicate that the settings of the cathetometer hair lines can be reliably reproduced to about 10.004 ml. in sections 1, 3, and 5. A bright light behind the apparatus silhouettes the meniscus levels and greatly facilitates the setting of the cathetometer cross hairs.
The apparatus, which consists essentially of two glass tubes joined a t the bottom to a leveling bottle, is illustrated in Figures 1 and 2. The long glass tube, A , is 220 cm. long, has a scale graduated in millimeters over 200 cm. of its length, and is open
Preliminary to running each test, the mercury level is dropped below the air injector, D, and with the stopcock opened to the
IS
MANY problems involving the use of gasolines, naphthas, and similar volatile products it is desirable t o know the vapor pressure of these products over the initial range from zero to some intermediate per cent evaporated. In connection with several problems requiring this type of information, a variable vapor-volume barometric type of vapor pressure apparatus, which gives somewhat more detailed and accurate information than the usual Reid vapor pressure test and yet is sufficiently rapid in operation to allow its use for semi-routine testing, has been used by the writers with considerable success. It is the purpose of this paper to describe briefly the design and operation of this apparatus and to indicate the manner in which the results obtained from it may be applied to several typical problems, particularly to the measurement of evaporation losses and the vapor-locking characteristics of gasolines.
Operation of Apparatus
TABLEI. RESULTS OF REPRESEXTATWE TEST Mercurya Level Mm.
Nm.
iSamule 131.8) , .
Mercury Levelb Corrected Mm. 351 390 501 704
AP 1Mm.
Vapord Pressure
Barometer Mm. 756 756 756 756 756 756 756 756
Mm.
Vapor Volume Observede Correctedl
cc .
cc.
Liquid0 Volume
Evaporatedh
MI.
%
V/Ll
5.6 10 I008 0.31 157 7.1 0.56 599 358 508 10.000 0.39 9.1 594 162 9.1 0.72 397 552 4 6 . 2 800 9 2 . 5 1 462 294 7 6 . 0 4.73 502 795 120,7 68.7 9.665 3.72 7.10 320 436 711 1024 7 2 . 8 9 . 6 4 3 3 . 9 4 1 2 8 . 2 325 806 431 7 .56 813 1131 96.8 9.500 5.24 10.20 178.5 344 917 412 918 1261 1 2 2 . 4 2 4 0 . 3 9 . 3 7 3 6 . 6 4 1134 1 3 .00 369 387 1141 1503 1152 124.0 13.29 241.6 9.364 6.72 371 385 1159 1523 a Values given as mm. of mercury, are obtained by direct observation of mercury levels in tubes. When approximately 10-cc. sample is used, subtract 7 mm. in sections I, 3, and 5; 1 mm. i n b Columh 2 corrected for height of column of gasoline. sections 2 and 4. 3; or column 1 - 2 corrected. c Difference in mercury levels: column 1 d Vapor pressure = barometric pressure (column 6 ) - A P (column 4 ) . e Volume of vapor above gasoline. VP f Corrected to 760 mm. = observed vapor volume X 76037.75’ , C. (100’ F,). g Volume of liquid gasoline =, volume between meniscus levels volume included in meniscus (1,8; Figure 3). Original volume, 10,045 ml. initial volume - liquld volume x 100. h Per cent evaporated = initial volume i Vapor-to-liquid volume ratio vapor volume (oolumn 8 ) + liquid volume (column 9). At 37.75O C . (loo0 F.),760 mm.
-
+
-
I
NOVEMBER 15, 1936
ANALYTICAL EDITION
atmosphere, air is blown through the buret to rid it of any vapors from the previous test. The mercury is then raised to the top of the buret, the stopcock closed, and the leveling bottle dropped until a vacuum of almost one atmosphere can be sustained. This procedure appears to be effective in freeing both the buret and the mercury from gases and volatile impurities. Following these preliminaries, the mercury level is again raised and the stopcock opened to the sample bottle. The required volume of sample is then displaced into the buret by gradually lowering the mercury level and simultaneously pushing the sample by a slight pressure head from the water bottle (Figure 2). When the sample has been obtained in this manner, the stopcock is sealed off and the sample is allowed to stand for several minutes to reach the temperature of the bath. After this condition is attained the mercury level is dropped until some of the sample has evaporated, The barometric pressure, the vapor and liquid volumes, and the mercury levels me then recorded and the vaporization is repeated in successive increments to the bottom of the buret. Readings are, of course, taken after each increment of vaporization. After each increment of vaporization a period of 10 to 12 minutes was allowed before readings were taken, so as to permit a close approach to equilibrium between the liquid and the vapor. It is believed that this period was sufficient to ensure approximately equilibrium vaporization, since allowing a period of as much as 6 hours in several cases produced an additional change of only I to 2 mm. in the vapor pressure. W h e n very accurate liquidvolume measurements a r e required, as in measuring small evaporation losses (0.5 per cent and less) it is n e c e s s a r y t o read t h e h e i g h t of the meniscus and apply a correction for the volume of l i q u i d that is contained in it (7, 8). This correction, shown i n Figure 3 for t h e buret used in this work, n o r m a l l y ranges from 0.02 t o 0.05 ml. and introduces no complication, as it is simply an additive correction to he applied to the liquid volume measurement. Table I gives the o b s e r v a t i o n s and the computation of the results for a representative test on a gasoliiie sample, and Figure 4 is a plot of the results for t h i s s a m p l e test. All the calculations a r e s i m p l e arithmetic a n d c a n b e carried out b y t h e operator in the s h o r t p e r i o d s of waiting for equilibrium after each successive increment of vaporization.
473
S C H E M A T I C D I A G R A M OF VAPOR PRESS UR E APPARATUS De K h o t i n s y T h e r m o - Reqhater 5tirrinq
Open Mercury
Column
A
-
burette
B
Overflow/ TPyrex Tube
Ref lector 7 Plate
FIGURE2
Using this apparatus and procedure a n operator can, barring accidents, run about six samples in an 8-hour day. I n running samples where evaporation losses are very small (0.5 per cent or less) and very accurate curves are required, four samples a day are about average. I n other cases less care is required and six or eight samples are usually handled.
Preliminary Tests with Apparatus Among the first tests made with this apparatus was a series of preliminary runs t o determine (1) the agreement of measured vapor pressure values with accepted standard values for a series of pure compounds, and (2) the accuracy with which the values on one product could be reproduced in successive tests. VAPORPRESSURES OF PURECOMPOUXDG. Table I1 gives the vapor pressure values of cyclohexane, benzene, chloroform, TABLE 11. VAPORPRESSURES OF PURECOMPOUNDS Material Tested Cyclohexane
FIGURE1. APPARATUS FOR MEASUREMENT OF EVAPORATION LOSSES
Benzene Chloroform Carbon disulfide Water
Vapor Pressure
Vapor Pressure Given by International Critical Tables Preparation Mm. 166 c. P. material redistilled and middle cut taken 100.10 173 (6) Same 100.10 336 Same 574 100 Same 4 8 . 9 Distilled 100
Test Apparatus Temperature Mm. C. ' F. 170 37.75 100 by
172 340 574 60
37.83 37.83 37.75 37.75
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FIGURE4. RESULTSOF SAMPLE TEST,TABLEI
FIGURE3. CORRECTION FOR VOLUME OF MENISCUS For use in sections 1 3 and 5 of vapor pressure apparatus. Both correctiods &e added to gasoline volume.
carbon disulfide, and water, determined a t a temperature of 37.8" C. (100" F.) in this apparatus. For comparison, the "standard values" for the vapor pressure of these products, obtained by interpolation in the vapor-pressure tables in the International Critical Tables (S), are also given. It will be noted that 2 mm. is the greatest discrepancy between the experimental and accepted values. I n making these tests, the procedure of running a curve of vapor pressure versus vapor-to-liquid volume ratio and per cent evaporated was followed. Despite the care taken in purifying these materials, a slightly high value of the vapor pressure was noted with very small amounts evaporated in the case of all the materials except water. However, as the amount evaporated was increased, the vapor pressure values quickly approached the steady values reported in Table 11. As these high initial vapor pressures were considered to indicate the presence of small quantities of volatile impurities and as it was necessary to carry the evaporation furthest in the case of the least pure material, it is suggested that this apparatus might offer a quick and simple means of estimating the amount of volatile impurities in supposedly pure compounds. ACCURACY AND REPRODUCIBILITY OF TEST. A series of curves of vapor pressure versus v a p o r - t o - l i q u i d volume ratio and per cent evaporated were run all on the same gasoline sample in order to obtain a reasonably reliable figure by which to judge the reproducibility of subsequent tests. I n the first attempt to run this series of tests the following interesting though accidental results were obtained: A 500-ml. sample of the gasoline for test was poured into a sample bottle which was subsequently connected to the buret as shown in Figure 2; and when successive portions of this sample were run into the buret and tested, curves 2,3, and 4 of Figure 5 were obtained. As these tests showed a, very disappointing spread, the operation of the next test was watched carefully to determine if possible the reason for this variation. While watching this test it was noticed that after the sample was displaced into the buret, the water bottle was placed base up (inverted posi-
tion of that shown in Figure 2) on aledge slightly below the sample bottle. This tended to siphon some of the water from the bottom of the sample bottle back into the water bottle and allowed vapor to form to a depth of about 3 cni. a t the top of the sample bottle. After this test, from which curve 5 of Figure 5 was determined, a new sample was taken and curve 1 was determined. In taking this second sample the gasoline was displaced from the main storage into the sample bottle and thence into the vapor pressure apparatus in such a way that no vapor formed at any time during the transfer. The difference between curves 1 and 2 (corresponding to about 1 per cent loss) was attributed to the displacement loss of pouring the first sample into the sample bottle and the difference in curves 2, 3, 4, and 5 to the small vapor loss that occurred each time the water bottle was inverted. These accidental results serve to illustrate the importance of careful sampling and handling of volatile materials and also to indicate the sensitivity of the apparatus. As a result of these tests all samples were handled by water displacement and for this purpose two-hole rubber stoppers with two glass tubes in them are sealed into pint wide-mouth bottles. One glass tube extends just through the stopper flush with its inside edge and the other extends almost to the bottom of the bottle. The bottle is filled with water and the sample is displaced into it a t the time of sampling and out of it into the buret a t the t h e of test. Curve 1 of Figure 5 was subsequently verified by several additional runs in which the samples were all handled by the method just described and, as it was impossible readily t o
FIGURE5
NOVEMBER 15, 1936
ANALYTICAL EDITION
475
distinguish the curves for the separate runs when the plots of them weIe superimposed, the following method was used for the comparison of the results: The equation of the curve determined by each individual run was developed by the method of averages (4) for all points above 1 per cent evaporated and the separate equations were solved to determine the vapor pressure a t a common per cent evaporated. The results of these tests and calculations indicated a probable (50 per cent) error of about 1 mm. of mercury and a maximum range of about 3 mm. of mercury. This method of comparing the results was felt to be the most justifiable one, since in all cases the average curve through the points is the result that is of interest.
Application to Typical Problems ESTIMATION OF EVAPORATION LOSSES. Stauffer, Whitman, and Roberts (9) have suggested that evaporation losses can be estimated accurately by determining a curve of vapor pressure versus per cent evaporated on samples of the product both before and after the occurrence of the evaporation loss. I n this method the loss is measured as the difference in per cent evaporated a t a value of the vapor pressure common to both the weathered and unweathered samples. This method has been substantiated by Chenicek and Whitman (2) who made a comparison of losses measured by careful gaging and parallel measurements on the apparatus suggested by Stauffer, Whitman, and Roberts. As it was proposed to use the present apparatus for the same purpose, a series of tests was made to verify its applicability to this sort of measurement and to learn the accuracy with which losses could be determined. I n order to do this, losses of known magnitude were produced by weathering samples of several commercial grades of gasoline. The original and the weathered samples were run in the vapor pressure apparatus and the losses estimated from the vapor pressure-per cent evaporated curves. At the start of these tests the apparatus and a definite plan of procedure were turned over to an experienced operator who carried out the entire series of tests. No check runs were made and the loss data are those obtained as the result of a single carefully made test. Figure 6 shows the known losses plotted against the values estimated from the vapor pressure measurements. The solid 45" line indicates the locus of points for perfect agreement between the known and experimentally determined evaporation lobses. The black dots on either side of the line represent the experimental points and indicate their approach to perfect agreement with the known losses. The points in Figure 7 show the actual per cent error of the estimated loss plotted against the known loss. The solid line gives the probable per cent error (based on the average deviation of the points in Figure 6, regardless of sign) that might be expected in measuring losses of various magnitudes by this method. VAPOR-LOCKING CHARACTERISTICS OF GASOLINES. I n recent publications on the subject of vapor lock (1, 5 ) it has been shown that a gasoline in the fuel system of a car will cause vapor lock if, at the existing fuel system temperature, it forms more vapor than the fuel system can handle without disturbing the flow of liquid gasoline enough to interrupt normal engine operation. The available data further indicate that many cars can handle relatively large quantities of vapor, from V / L = 10 to V / L = 50 and greater (where V / L is the volume of vapor divided by the volume of liquid, both measured a t the existing temperature and pressure) being the range of values reported for current model cars. To tie in with this conception of the problem, the vaporlocking characteristics of the gasoline are expressed by a curve showing the quantity of vapor formed plotted as a
FIGURE6 . AGREEMENT BETWEEN KNOWNLOSSESAND MEASUREMENTS BY VAPORPRESSURE METHOD
function of the boiling temperature, at the pressure existing in the fuel system. Bridgeman (1) has shown how such a curve can be established by running vapor pressure tests a t various vapor-to-liquid volume ratios in a bomb similar to that used in the Reid vapor pressure test. I n this method the vapor pressure measurements were converted to boiling temperature and plotted against vapor-to-liquid volume ratio. The variable vapor-volume apparatus offers a very convenient means by which to obtain the same sort of information. Figure 8 gives the vapor-locking characteristics of six representative gasolines that have been evaluated in this manner. The application of these data to corresponding data on car vapor-locking characteristics brings up an interesting point. Take, for example, the case of a popular 1934 model car having a vapor-handling capacity of V / L = 34 and fuel system temperature of 45 O C. (1 13 O F.) under certain operating conditions. When this car is operated under those conditions on a fuel having the vapor-locking characteristics shown
FIGURE7. ERRORS IN LOSSESMEASURED BY VAPORPRESSURE METHOD
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per cent appear to be quite normal. In addition to the volume loss which tends to decrease the over-all fuel economy, an octane depreciation of 2 to 3 units and a general decrease in volatility also result from these losses.
Conclusions
0
FIGURE 8. VAPORLOCKINGCHARACTERISTICS OF SIX REPRESENTATIVE GASOLINES DETERMINATIONS BY VAPOR PRESSURE METHOD in Figure 9, it is seen to be free from vapor lock because at 45” c. (113’ F.) less than the limiting quantity of vapor will be formed. However, by projecting the point V / L = 34 and T = 113” F. horizontally to the curve for the gasoline characteristics, it is seen that a quantity of vapor equivalent to V / L = 11.5 is being formed in the fuel system, and since it cannot possibly get through the carburetor jet into the manifold, it must be vented out of the carburetor. This represents a loss in liquid fuel supplied to the engine, equivalent to the amount required to form the quantity of vapor that is vented off from the fuel system, in this case about 6 per cent. That this sort of loss actually occurs in cars has been verified on several occasions when samples were bled from the carburetor bowls of cars while they were running, and the samples compared with the original gasoline supplied to the car. With current model cars operating under approximately normal conditions, losses of as great as 10.8 per cent have been detected and during warm weather losses of 0.5 to 5
The barometric type of v a POr pressure apparatus h a v i n g a v a r i a b l e vapor volume offers a rapid and /o ’ 2 0 so 3G I & accurate method by which VAPOR T G L I 7 U I D VOLUME RATiG t o d e t e r m i n e the initial vapor pressures of volatile materials. Data of the sort obtained on this apparatus are useful in e s t i m a t i n g evaporation losses and in measuring the vapor-locking charakeristics of gasolines. There are also several other problems, such as the estimation of volatile impurities, for which the apparatus may be useful.
Acknowledgment The writers take this opportunity to make acknowledgment
to C. E. Cummings, B. Hegeman, and Keil MacCoull who gave suggestions and advice which were of great assistance in the final development of the apparatus and method.
Literature Cited
,
(5) i6j (7) (8) (9)
Bridgeman, 0. C., S. A . E. Journal, 32, 157 (1933). Chenicek and Whitman, Oil Gas J., 29, 78 (1930). International Critical Tables, Vol. 3, p. 248 (1933). Lipka, “Graphical and Mechanical Computation,” New York, John Wiley & Sons, 1928. MacCoull and Barber. 8. A . E. Journal. 37. 237 (1935). Perry, “Chemical Engineers’ Handbook,” New York, McGrawHill Book Co., 1934. Porter, A. W., Phil. Mag., [7] 17, 711 (1934). Porter, A. W., Phil. Mag. Supplement, [7] 14, 694 (1932). Stauffer, Whitman, and Roberts, ISD. ESG. CHEM.,Anal. Ed., 2, 88-91 (1930)
RECEIVED April 27, 1936. Presented before the Divisions of Gas and Fuel Chemistry, Industrial and Engineering Chemistry, and Petroleum Cbemistry, Symposium on Motor Fuels, at the Qlst Meeting of the Amerioan Chemical Society, Kansas City, Mo., April 13 to 17, 1936.
Laboratory-Scale Ebullition Tube JOHN W. BOEGEL 399 Dupont St., Toronto, Ontario, Canada
A
FTER research work requiring boiling point determinations, for which the method of Siwoloboff was found most suitable, an adaptation of the ebullition tube used in this method proved of value in securing steady boiling of the liquid in a small-scale fractional distillation. The ebullition tube consists of a thin glass rod, long enough to be held upright by resting against the neck of the vessel in which it is used, To the lower end of the rod is sealed a short length (less than 1 em.) of open tube, the diameter of which should be as large as is convenient, 1-em. tubes proving generally useful. For rapid distillation several tubes may be used, while for best results a vacuum distillation requires two or more small-diameter tubes, giving several rapid streams of small bubbles.
This piece of apparatus has proved useful in minimizing bumping in liquids boiled in test tubes, and especially in vacuum distillations, replacing the capillary tube usually used. The capillary must be carefully made, is fragile, is often in need of adjustment, and possesses the further disadvantage of passing air through the hot liquid. After boiling and cooling, the liquid retreats into the open tube, finally filling it. On heating again, the liquid is likely to become superheated ‘until the tube suddenly resumes normal operation with a bad bump. This may be avoided by lifting, draining, and replacing the tube before the second boiling. RECEIVED August 31, 1936.
