88
A S A L Y TICAL EDI TI0,V
Vol. 2,
KO.1
To determine the dissociation constant of the unknown
Literature Cited
acid it is therefore only necessary to find from a titration the half-neutralization point, then to prepare a half-neutralized solution and measure its pH. V h e n a Douglas fir extract was treated in this way the value of KOwas 1.23 X Since K , for pure acetic acid is 1.74 X this is taken as elidence that the volatile acid present in wood is acetic with considerable admixture of acidic substancps of lower dissociation constants.
(1) Burns a n d Campbell, Trans. A m . Electrochem. SOL.,86, 271 (1929). (2) Burns a n d Freed, J . A m , I n s t . Elec. Eng., 47, 576 (1928). (3) Clark, "Determination of Hydrogen Ions," p. 25, \Villiams and Jvilkins, 1926.
:4) Clarke and IX'ooten, J . Phys. Chem., 33, 1468 (1929). 8:s) Cross and Tollens, J . Land;&.,59, 185 (1911). (6) Klasson, Z . angew. Ciiem., 22, 1205 (1909); 23, 125 (1910). ( 7 ) Schorger, "Chemistry of Cellulose a n d Wood," pp 441, 415, 446, 461, IIcGraw-Hill, 1926. ( 8 ) Zacharias, Papier-Pabl'., 10 (1912).
Determination of the Vapor Pressures of Naphthas' J. C. Stauffer, J . K. Roberts, and W . G. Whitman STASDARDOIL
COXPANY (INDIANA),
\~HITINC,I N D
This paper describes a method for the determination HE object of this invesits early use. Kilde. Alead, of the vapor pressure of naphthas. Measured amounts and Coleman (5) use this tigation was to develop of dry air are bubbled through a sample and the inmethod for determining the a method for the rapid crease in vapor volume is measured. Saturation of the vapor pressure by measuring determination of the initial air with hydrocarbon vapors is obtained, as verified by the gas-air mixture resulting vapor pressure of naphthas checking the vapor pressures of pure organic liquids from bringing a measured and of the decrease in vapor of known vapor pressures. amount of air into equilibrium pressure caused by evaporaThe vapor pressure is calculated from volume measx i t h a n a p h t h a . Several tion of the light ends during urements and the pressure existing in the system. increments of air are used, Tveathering. The apparatus The determined vapor pressures are plotted against the vapor pressures being developed to meet these rethe percentage evaporation obtained by measurement computed from this series and quirements is based upon vaof the liquid volumes, and the initial vapor pressure plotted against' the amount of porization of the light ends is estimated by extrapolation of the curve to zero per yapor taken off. The initial of the naphtha by a current vapor pressure is obtained by cent loss. of air or inert gas. The vapor pressures determined by this method extrapolation to zero vaporiThe T-arious methods einzation. Their p r o c e d u r e , check reasonably with those computed by means of ployed for determining vapor while possessing distinct adRaoult's law from the fractional analyses of the naphpressures have been revieived vantages in determining the thas. The method affords a convenient and valuable by Oberfell. Alden, and Hepp initial vapor pressure of a tool for the investigation of problems such as evapora(I), and by Pearce and Snow naphtha, is not adapted to tion losses during storage or transportation and the ( 2 ) . The method most genmeasuring the lolvering of performance of stabilizing and condensing equipment. erally used for naphthas is to vapor measure directly the nressuie - -pressure as appreciable in a closed system containing the naphtha saniple and x apor losses are incurrecl! and does not lend itself to a jirect dein equilibrium with it. I n its simplest forin the apparatus termination of the liquid 10~5es. The vapor pressures of liquid5 coiiaihng of niixturca of consists of tv-o closed end manometers into one of n hich a small gasoline sample is introduced. The difference in hydrocarbons such as are found in naphthas may be calcuheights of the two mercury columns gires the yapor pres- lated from fractional analyses by the ube of Ilaoult's law, sure of the sample. This type of apparatus has been modified assuming the law to hold for b u c l i mixtures. T h e change in in the various bomb methods in n-l-hich the pressure is read vapor pressure with evaporation loss may be calculated by directly by a gage attached to the vapor space of the bomb. eniploying subsequently the Rayleigli (4) equation. A vapor The chief error characteristic of this method is caused by pressure-evaporation loss curve for any given temperature the lowering of the vapor pressure of the liquid under test may thus be constructed. However, the accuracy of the due to the evaporation of light ends to fill the vapor space. analytical method of calculating the yapor pressure of naphThis procedure has been used primarily for determining thas from the fractional analyses of the light ends is limited vapor pressure of gasoline in connection with shipping regu- by all the inaccuracies of a fractional distillat'ion and by lations, and is not readily applicable to determining the the deviations from Raoult's law. The calculation of a lowering of vapor pressure as a result of e\ aporation losses. 1-apor pressure-evaporation loss curre from a fractional The other general method of measuring vapor pressures, analysis is a laborious procedure, and the value of such calcufirst employed by Regnault (a), has been used primarily lated curves is largely one of obtaining a theoretical check for the determination of vapor pressures of pure liquids and on determined curves. Yarious niethods have been proposed to estimate tlic c0111aqueous solutions. The method consists of passing a ineasured volume of dry air over the surface or through the liquid position (and from this the vapor pressure) from Engler and calculating the vapor pressure froin the volume increase. distillations of the sample, but these are obviously useless The amount of liquid evaporated is usually measured by for exact work. Other nipthods have been employed which absorption. Various modifications have been made since determine the pressure of naphthas, after they have been degassed. Khile such a procedure may be developed to a 1 Received September 18, 1929. Presented before t h e Divisionof high degree of accuracy, the results are not significant where Petroleum Chemistry at t h e 78th Meeting of t h e American Chemical the initial vapor pressure of the original stock is desired. Society, Minneapolis, Minn., September 9 t o 13, 1929.
T
January 15, 1930
IiI;D USTRIAL An'D ENGINEERIA'G CHEMISTRY
Since none of the various methods used for the direct determination of vapor pressure were readily applicable for measuring the vapor pressure with evaporation loss, the apparatus described below was developed. Apparatus
89
of air from A to stopcock 4. Water is admitted to the lowest graduation. After allowing the sample and the air to come to the temperature of the jacket water, initial readings of the liquid volume in F, the air-measuring buret A , and of manometer C are taken. The buret G or H has been ?et a t zero. A constant flow of water to A is provided by the constanthead device, B , which is supplied with water from the thermostat. All glass tubing, A to G, is capillary in order to reduce to a minimum the unjacketed portion of the apparatus.
The apparatus is shown in Figure 1. Successive quantities of air, measured in the buret A , are bubbled through the naphtha sample contained in F , and the total gaseous volumes which result are measured in a second buret, G or H . The decreases in liquid volume are read directly in F . The vapor pressure of the gasoline for any one measurement is equal to the increase in volume of air divided by the total volume of air plus iaporized gasoline times the pressure on the gas-measuring device-i. e., atmospheric pressure. Since the computed vapor pressure is the average of a pressure TI hich varies as evaporation proceeds, it is plotted against the average loss ahich has been -sustained. This average- loss is taken as the mean of the total percentage losses a t the beginning and end of the measurement. A curve drawn through the plotted points is extrapolated to zero per cent evaporated to obtain the initial vapor pressure of the sample. Supersaturation or undersaturation of the naphtha sample with respect to air at the temperature of the vapor pressure determination will cause the first point to be inconsistent with the other portion of the curve. Hence the first few increments of air are small ( 5 or 6 cc.) and the first point is disregarded in plotting the curve. L%IR-BUBBLISGD E V I C E - T ~ ~device S consists of a water-jacketed buret, F , of approximately 100 cc. capacity, the ungraduated portion of which is thl attached a t its top to a graduated 5-cc. pipet d c which can be read directly to 0.05 cc. and estiFigure 1-Vapor-Pressure Apparatus mated to 0.01 cc. The buret is equipped with a stopcock a t the bottom to admit the previously chilled GAS-MEASURING DEncE-This consists of two h r e t s , sp.mple of naphtha, forced in by water displacement. The G and H , connected in parallel to the gas exit h i e . G is an first 150-cc. portion of the naphtha is allowed to overflow ordinary 50-cc. buret calibrated to read to 0.1 cc. and is at stopcock 7 in order to obtain a uniform sample which has used to measure the first series of small air-gas increments. not lost light ends by evaporation. A fine capillary tube I n order to furnish a larger air-gas measuring collector, the through which the air is admitted to the bottom of the column buret H is provided. The capacity of this buret may be of sample extends from the top to the bottom of the buret. any volume between 200 and 400 cc. and is calibrated t o F . The buret is calibrated and the volume of gasoline is read to an accuracy of 0.5 cc. Ethylene glycol is used as read with this capillary full of air to the extreme tip. The leveling liquid in both of these mixed-gas collectors, making it submerged portion of the capillary tube itself has a displace- unnecessary to correct the volume for the vapor pressure ment volume of only 0.1 cc. Since the air for bubbling is of a leveling liquid. Calcium chloride drying tubes at the measured over water in buret A , humidifier D is used to in- top of the lereling bottles prevent the absorption of water sure saturation of the air pulled into A . The measured air vapor by the ethylene glycol. is then dried by calcium chloride in tube E before it comes A constant flow of water through the jacket3 is provided into contact with the naphtha in F . The humidifier is im- by recirculation through a thermostat which provides water mersed in the thermostat which supplies water a t a constant with a maximum variation of 0.1"F. (0.06' C.) Determitemperature for circulation through the jackets. nations are ordinarily made a t 75" F. (24" C.) although AIR-MEBSURIKGDEvrcE-The water-jacketed air-measur- any temperature between 40' and 120" F. (4" arid 49" C.) ing device, A , consists of a buret of 100 cc. capacity graduated can be maintained. The rate of bubbling air through the in 0.1 cc. increments. Air is drawn into the buret through sample has been varied from 8 cc. to 12 cc. per minute with the humidfier D by displacing water. I n order to secure no apparent effect on the degree of qaturation of the air maximum capacity from buret A , the following procedure obtained in F . is employed: With the buret full of humidified air, water Il'ith this apparatus operating on naphthas of 400-mm. is allowed to drop into A , with stopcock 4 closed, until about initial pressure or higher and carried to a 3 per cent volume 40 inches (102 cm.) of water pressure have developed. Stop- loss, three samples can be run and the results computed cock 3 is then opened to the humidifier, isolating compressed in 8 hours. Samples of lower vapor pressure require a someair in E, after closing stopcock 2. The humidifier is of suffi- what longer time. cient capacity to take u p the excess air pressure without Experimental Results having the water forced out. Additional humidified air is admitted to the buret by venting the water at stopcock 1, The accuracy of the results obtained with this apparatus which is then closed, and stopcock 3 is set for the free passage depends primarily upon the accuracy with which the air,
.
AS.4 L Y TICAL EDZ TZOS
90
vapor, and liquid volumes can be measured, and the degree of saturation of the air with hydrocarbon vapors. That essentially complete saturation of the air is reached is evidenced by the fact that the known vapor pressures of pure liquids, such as ether, chloroform, or carbon disulfide, are checked within 0.5 per cent. Accuracy in volume measurements is secured by close control of temperature and by suitable design of the measuring equipment. The reproducibility of the vapor-pressure curves within 5 mm. verifies the fact that close readings are obtained with this apparatus.
Vol. 2, No. 1
The decrease of vapor pressure with evaporation loss is calculated from the fractional analysis by use of a special form of the Rayleigh equation. It can be shown that for any two components of a multicomponent liquid mixture, if these follow Raoult's law, -dA -dB -
A
3
where A equals mols of component A in the liquid, B equals mols of B in the liquid, and a equals the relative volatilityi. e., the ratio of the vapor pressure of component A in the pure state to that of component B in the pure state. Integrating this equation between limits, we have A B log f = log IAz Bz By calling one of the components .4, the decrease in mols of each of the other components may be calculated corresponding to any decrease in mols of A . The vapor pressure of the naphtha remaining may then be calculated and the volume per cent loss computed from molecular Tveights and densities. By carrying this process through for several different loset-, a curve of vapor pressure T-ersus eJ-aporation loss may be constructed which is based solely upon a fractional analysis of the original naphtha and the ule of Raoult's and Rayleigh's laws. A series of such calculated points i:, shown in Figure 2 . I n Table 111 the calculated points are compared with the corresponding values taken from the determined curve of Figure 2. The discrepancy between the measured and computed pressures is small. (Y
P E R C E N T LOSS
I n order to demonstrate the method of calculating the vapor-pressure curve from the data taken, Table I is included for the first few points used in plotting the curve in Figure 2 . The sample used was a re-run pressure light naphtha. The inspection results are given in Table IV. The gas volumes are corrected to a dry state at 60" F. (15.6" C.) and 760 mm. pressure. Table I-Re-Run Pressure Naphtha. Barometer, 746.0 m m . Temperature of naphtha, 75' F. (23.9' C . ) : initial volume of naphtha, 07.1 cc. VOLUME VOLuxB AIR XET VAPOR TOTAL AV. AIR VAPOR VOLUME PRESEVAPOEYAPO(COR.) (COR.) VAPOR SURE RATED RATED CC. CC. Cc. Jim. H g Per cent Per cenl 0.031 14.3 542 0.063 19.7 19.1 13.6 532 0.134 0.09s 0.165 13.4 532 0.196 5.4 18.8 9.7 32.8 23.1 525 0.285 0,242 0.335 22.0 5'21 0.381 9.5 31.5 19.8 510 0.463 0.482 9.2 29.0 0.505 18.2 502 0,546 8.9 27.1 0.587 16.9 498 0.628 8.4 25.3
of Calculated a n d Observed Vapor Pressure, CALCDVAPOR 0 8 s VAPOR EV~PORATION PRESSURE PRESSWRS Per cenl M rn .Ii m .
Table 111-Comparison
+
g;;
The initial vapor pressure of a naphtha may be calculated from a fractional analysis by means of Raoult's law. Table I1 shows the molal and volume analyses of the light naphtha for which the vapor pressure curve is given in Figure 2 . The calculated vapor pressure in this case was 537 mm. as compared with 540 mm. measured. It is unusual for such close checks to be obtained, the calculated vapor pressures usually being 5 to 15 per cent lower than the measured vapor pressures. This may be due to deviations from Raoult's law, to errors in analysis, to the presence of methane, or to errors in assigning the true molecular weight and vapor pressure to the residue from the fractional analysis.
537, a 518.2 493.4 445.5 357.7
0 0.2704 0.7156 1.575 3.672
540 520
490 44 1
352
Applicability of Method
There are several uses for the data obtained in this type of apparatus, in addition to the usual applications of vaporpressure determinations. One of these is the estimation of losses occurring during storage or transportation. The vapor pressure-evaporation curve of gasoline is determined before a storage period ancl compared with a curve of the gasoline after storage. The loss sustained during storage is given by the percentage evaporation of the original gasoline necessary to produce coincidence of the two curves. Especially in cases where the vapor pressure of the gasoline is high, appreciable differences in vapor pressures are found 3.-
r
.J
S T O R A G E E V A P O R A T I O N LO5
.E% n
c 400 iL.
R c
E
3w
2 Table II- -Fractional Analysis of R e - R u n Pressure Pu'aphtha MOL VOLUME P o b AT 75' F. P o l e AT 75' F?. HYDROCARBONPER CEFT PER CEWT ( 2 4 O C.) (240 C . ! Ethane Propylene
Propane
Isobutane Butane Isopentane Pentane Bottoms5
0.093 1.360 1.370 1,809 7,590 5.689 5.689 76,400
___
0.05 0.80 0.84 1.30 5.43 4.70 4.66 82.22
__
100.000 100.00 Mol. wt., 112; sp. gr., 0.7479. b Pw = vapor pressure of components. e Po+ = partial pressure of components.
Mm.
Mm.
30,200 8,560 6,950 2,570 1,760 660 500 68
28 1
116,s 95.2 46.5 132.8 37.5
28.5
52.0 -_
Vapor pressure 537.1
w
P
m 0.
3 200
05
15
IO
PERCENT
20
LOSS
with only slight volume losses. The vapor-pressure curves should be made a t the average temperature a t which the evaporation loss takes place, as the shape of the curve changes somewhat with temperature. Such an application is shown in Figure 3 for a commercial gasoline. The sample represented by curve A , Figure 3 , was taken from a n unprotected storage tank in June, 1929,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
January 15, 1930
when the average temperature of the tank was 68’ F. (20’ C.). The sample represented by curve B was taken from the same full tank 17 days later when the average tank temperature was 73” F. (23’ C.). Both samples were taken from the 20-foot (6-meter) level, the depth of liquid in the tank being 40 feet (12 meters). Curve B will approach approximate coincidence with curve A if it is shifted to the right a distance corresponding to 0.25 per cent loss. Since the curves are not exactly parallel, the loss indication is somewhat variable, but if the portions of the curves near the origin, where the results are most subject to error, are disregarded, the variation of indicated loss is not appreciable. The Engler distillations of the tm-o samples are given in Table
Vapor-pressure determinations made in conjunction with fractional analyses greatly increase tJhe value of the latter. T a b l e IV-Inspection Analyses of S a m p l e s GASOLINE A RE-RUNPRESSURE COMMERCIAL &-APHTHA 6/18/29 7/5/29 . . , , 64.5’ 58.80 A. P. I . gravity. . , 58.6’ 1nitialb.p . . . . . . . . . 88°F.(31.10C.) 96°F.(35.60C.) 9’3’F. (37.2OC.) Per cenl Per (:enf Per cent F. C. 158 70 14.0 11.0 11.0 221 105 38.0 31.5 31.0 72.0 284 140 54 0 80.0 62 0 302 I50 94.3 83 0 356 180 .. 88 0 374 190 92 0 392 200 4:0 3 0 Loss
.
IV. Similar determinations have been made on samples of gasoline taken during transportation from storage to a filling station and thence to an automobile tank. Such surveys are highly valuable in determining a t which point in the operation the maximum loss is incurred and in estimating the probable effect of changes in the character of the naphtha.
91
Literature Cited (1) Oberfell, Alden, and Hepp, Nall. Pelroleurn S e w s , 20, KO.20, 57 (1928). (2) Pearce and Snow, J. Ph>’s Chem., 31, 231 (1927). (3) Regnault, Ann. chim. fihys., 3, KO. 15, 129 (1845). (4) Walker, Lewis, and McAdams, “Principles of Chemical Engineering,”
p . 598, McGraw-Hill,
1927.
( 5 ) Wilde, Mead, and Coleman, 012Gas J . . 27, No. 42, 102 (1929).
The Polarizing Microscope in Organic Chemistry‘ H. C. Benedict CHEMISTRY DEPARTMENT, NORTHWESTERN UNIVERSITY DENTALSCHOOL, CHICAGO, ILL.
I
N A recent report by a subcommittee of tlie Executive Committee of tlie Division of Chemistry and Chemical Technology of the Kational Research Council on the kind of education needed by a technical research chemist (Q), training in the fundamentals rather than specialization is stressed, However, the following statement is made: “An exception of the field of microchemistry should be made because the microscope has come to be so valuable a part of research laboratory equipment that every research chemist should be d l trained in its use.” Wright (Sd), Chamot (3), Chamot and Mason ( 4 ) , and Garner (8) emphasize the importance of the polarizing microscope in the identification of substances. Their studies have usually covered inorganic compounds, probably because more is known of the optical properties of these compounds and because a system for their identification can be worked out. Less is known about the optical properties of organic compounds, although more is appearing all the time in the literature] showing a spreading realization of the value of chemical microscopy as a time and labor saver (3). For instance, we find records of the optical properties of aldopentoses 124), of melezitose (27), of heptitols ( ~ 4 and of @-lactose (SO) by Wherry, of 13 sugars by Keenan ( l e ) , of tetramethylmannose by Green ( I I ) , and of a glucose derivative by Wolfrom (SI). The chemical laboratories of the American Medical Association] appreciating some of the advantages of a microscopical examination using polarized light, obtained the cooperation of Walcott in several studies of borocaine (.5), salyrgan (e), acriflavine ( 7 ) ] ephedrine (WI), and a-phenyl@-aminoethanolsulfate ( I O ) . Wherry has contributed papers on the optical properties of certain organic compounds (24) and of the calcium salts of maleic and fumaric acids 180). Wherry (26) and Keenan ( I ? ) have reported similar data on alkaloids. Amino acids have such indefinite melting points that the description of their optical properties by Keenan (15, 18) is particularly helpful. Data on the optical determination of the isomeric naphthalene sulfonic and 1 Received
October 4, 1929.
disulfonic acids have been presented by Ambler and Wherry ( I ) and by Hann and Keenan ( I S ) . A recent paper by Mason and co-workers (22) lists the optical properties of a monoarylguanidine. S o r is the work wholly confined to definite compounds. Grier (12) is able to identify rayons with a polarizing microscope, and this instrument is in continual use a t the Picatinny Arsenal (20) in the examination of nitrocelluloses. Keenan (14) has examined the d-globulin of sesame seed. Taylor and Sheard (23) have used such a microscope in a study of the calcification of tissues. A similar research on teeth was reported by Kitchin at the Chicago meeting of the American Association of Dental Schools on March 26, 1929. These examples, by no means complete, are enumerated to show the wide usefulness of the polarizing microscope. Anyone who has studied and used the method will be enthusiastic over its possibilities and could cite many examples of its helpfulness. The following are instances of its value as a “time and labor saver” in this laboratory. Amino Acids
, I n studying methods for preparing amino acids Keenan’s papers ( I C , 18) h a r e been most helpful. A drop of the reaction mixture can be smeared on a microscope slide and examined between crossed nicols to see whether or not any birefringent material is present. Usually a few other simple optical properties, such as extinction angle and sign of elongation, can be ascertained. These data alone are frequently sufficient and can be determined on the crude mixture. One can tell whether or not i t is worth while to go through a laborious isolation process. This is particularly valuable in testing short-cut methods. I n preparing alanine (2) it was possible to show that alanine was present in the reaction mixture without any attempt a t purification. The alanine was isolated by dissoh-ing the hydrochloride in alcohol and precipitating with aniline. Two obvious questions arose. (1) Was all of the alanine extracted from the sodium and ammonium chlorides by the alcohol? Examination of the
