36
ANALYTICAL EDITION
the solution, and when the ball passes through the coils a change in the note is observed due t o the change in frequency of oscillations. Since the coils are identical, the time elapsing between the change in hum a t the upper and lower coils is the time required by the ball to pass through the distance being measured.
Vol. 1, No. 1
Both cuprammonium solutions of cellulose and nitrocellulose solutions in various solvents have been run by the method with satisfactory results. The following table gives an idea of the accuracy of the method when used to measure viscosities of cellulose in cuprammonium solutions and using steel balls of various sizes:
Conclusions
The apparatus has several limitations. If the solution is too viscous, the change in note may be so gradual as to be inaudible to any but the trained observer. The apparatus works best with steel balls above 1/8 inch (3 mm.) in diameter. Smaller balls do not exert enough influence on the coils t o change the note sufficiently for accurate audible detection.
I/S-INCHBALL Seconds 14.0 14.6 14.6 14.8 14.6 14.0
8/16-INCA BALL Seconds 8.0 8.0 8.0 8.0 Duplicate soln. 8.2 8.4 8.0 8.2
I/I-INCEI BALL Seconds 5.8 6.8 5.8
Improved Apparatus for Vapor Pressure Determinations’ 0. A. Pickett HERCULES EXPERIMENTAL STATION, HERCULES POWDER Co., KENVIL, N. J.
T
HOSE who have made any large number of accurate have proved so satisfactory as t o accuracy and economy of vapor pressure determinations fully appreciate the time, labor, and materials that they are thought worthy of faults of most all methods, both static and dynamic, being passed on to other possible users. which have been published. The static methods are unApparatus desirable either because of inherent sources of inaccuracy or because of the excessive time necessary to get results. Some Figure 1 illustrates the apparatus described by Ramsay of the dynamic methods offer possibilities of high accuracy, and Young, while Figure 2 shows the present writer’s modibut they are time-consuming, and those that offer speed do fication of the part ACE, Figure 1. The main reason for so a t the sacrifice of ac- modifying Ramsay and Young’s apparatus was to prevent c u r a c y . A good r6- distillation of the sample under test from A into C, where it sum6 of vapor pressure would condense with a consequent frequent interruption of methods published up tests. With the apparatus modified as in Figure 2, all disto 1926 is g i v e n b y tillation from A was condensed and returned a t a slow, R e i 11y , R a e , a n d uniform rate, such as would never exert any disturbing effect WI Wheeler.2 on the vaDor-liauid eauilibrium relationshitx existinn in ~~-A During the past year proper. Figure 3 is a diagram of the apparatus as used, while Figure Ramsay and Young’s vapor pressure method3 4 is a photograph of an actual set-up, although in use the has been modified in this laboratory to give a procedure combining high relative accuracy with much saving of time. The immediate objective was a rapid means of o b t a i n i n g Figure 1-Ramsay and Young’s vapor pressure-tem* Original Apparatus for Vapor Pres- perature Curve data for sure Determinations pinene, dipentene, terpinolene, fenchyl alcohol, a-terpineol, and other terpenes and terpene alcohols. It has been found possible to determine as many as fifty to seventy-five points along the course of any curve from 0.1 to 760 mm. pressure in 2 to 4 hours for temperatures not higher than 250’ C. The curves obtained had, Figure 2-Modification of Essential Part of Ramsay and an accuracy well within 0.5 per cent of the true values for Young’s Vapor Preseure Apparatus the materials unditr test. The results are reDorted in anprecision bridge was set to the right of the manometer inother paper to appear in IWDUSTRIAL AND ENGINEERING stead of between it and the burner as shown. CHEMISTRY. A , a, Figure 3, is the vapor pressure apparatus proper. The advantages of the modified method, for general use, B is a 3-hole rubber stopper. The platinum resistance 1 Received October 11, 1928. the thermometer, c, is inserted through One Of these 2 Reilly, Rae, Wheeler, “Physico-Chemical Methods,” p. 437 (1926). mercury thermometer, D , through the second, while the stem 8 J. Chem. SOC.,41, 45 (1885).
a
0
~~
January 15, 1929
INDUXTRIAL A N D ENGINEERING CHEMISTRY
of the separatory funnel, E , passes through the third. The mercury thermometer is used to make rough temperature readings, the final accurate readings being taken with the platinum resistance thermometer by means of a Leeds and Northrup “8067 Precision Temperature Bridge” (Figure 4). Each thermometer is fitted with a cotton wick, F , such as is used on the wet bulb of a psychrometer thermometer. The
37
temperature-control vapor bath, N , developed by Ryan and Lantz4 is used. Such constant-boiling liquids as acetone, benzene, toluene, xylene, and aniline, or mixtures of any two for intermediate points, are used for controlling the temperature in A. When a higher or lower boiling liquid is wanted, N is emptied through 0 into a suction flask, and if the liquid being removed is still hot, pulling a current of air up through 0 will volatilize all residual liquid from the walls that might contaminate and change the boiling point of the next liquid added. Manipulation Details
Figure 3-Detailed
Sketch of Modified Vapor Pressure Apparatus
wicks have decided advantages over the swabs of cotton wool specified by Ramsay and Young as a medium for supporting liquid in equilibrium with vapor around the bulb. During a run their lower, free ends dip into liquid sample, G, under test, while their upper ends are bound snugly around the thermometer bulbs. They are less bulky in place than cotton wool and allow the operator to be surer that both liquid and vapor are present in sufficient quantities a t all times to allow opportunity for equilibrium conditions. In case tests are to be made at temperatures higher than the cotton wicks w ill stand, or on liquids which attack cotton, improvised wicks of asbestos or glass wool can be substituted and held in place by asbestos cord or resistant metal gauze or wire. The surface of the sample G in A should be far enough below the thermometer bulbs to prevent any splash of liquid from the surface of G from coming into contact with the bulbs and possibly upsetting the pressure-temperature equilibrium surrounding them through superheating. The separatory funnel E is used for introducing the sample after all connections are made and the system has been tested for leaks. Possible leakage around or through the stopper B is eliminated by painting all its outside contact surfaces with a paint made by intimate grinding in a mortar of 1 gram of gum arabic, 0.2 gram of aluminum dust, 1 cc. of water glass (ordinary grade), and 10 cc. of water. Leakage a t H is avoided by use of acid-cured natural gum tubing with all outer surfaces coated with films of castor oil. These joints, H , could all be made glass seals, but with them as shown, taking down and re-assembling the apparatus is greatly facilitated. The system is evacuated through the cocks I and J by means of a vacuum pump connected a t K , and pressure readings are made by means of manometer M , which is a fulllength closed type, making barometric correctione unneoessary. When tests are to be made above room temperature a
The apparatus is assembled as shown in Figure 3. Then with stopcocks I and J open and L closed, the vacuum pump, connected a t K , is started, and while it is running rubber stopper B is painted well with the gum arabic paint described above. When the manometer shows no further reduction in pressure, J is closed, the pump shut off, and L opened. If no appreciable leakage occurs over a 10-minute period, the sample is introduced through separatory funnel E , care being taken t o wet the wicks, F , thoroughly and not allow the surface of the liquid sample to approach too near bulbs of thermometers, as cautioned previously. The pump is again started and the system evacuated until all dissolved gases are removed from sample. This can be easily judged by the operator after a little experience. With all gases out the operator is ready to begin taking pressuretemperature readings. With sample G showing no ebullition, the pump is started and the vacuum turned on until positive, but not too vigorous, boiling begins. J is then closed, and as soon as mercury thermometer D remains constant in reading the resistant
Figure 4-Set-Up of Apparatus for Vapor Pressure Determinations
bridge is set for platinum resistance thermometer reading and a pressure reading made as soon thereafter as possible. After a moment a second and similar set of readings is taken, and then a third set. If all is well each succeeding reading in any set may be slightly higher than the last. Again the system is evacuated and another series of three readings for temperature and pressure is taken. With exgerience the operator soon learns to tell by inspection if his figures are checking satisfactorily, because out 4
Rynn and I,antz, IND.ENG.CHEM.,20, 40 (1928).
* Vol. 1, No. 1
ANALYTICAL EDITION
38
of his three sets of readings there should be satisfactory overlapping checks. With experience and everything going smoothly all three sets of readings are not necessary for assurance of good checks. Satisfactory results having been obtained for the temperat u r e u n d e r test, the l i q u i d P i n N i s removed, after removing the flame, the next desired bathliquid poured in through n, and temperature and pressure tests made as before. As the higher boiling liquid, P, i i heated up, a small amount of air is let in through L and J to adjust the pressure within A so that too vigorous boiling of sample G will not occur. The presence of small amounts of air a t J has no influence on the vap o r pressure-temperature relationships inside A so long as the thermometer bulbs are Figure &Relative Accuracy of Modifled A paratus when Used t o Detc mine surrounded by only the (8urve 1) Vapor Pressure of Water and (Curve 2) Vapor Pressure of a Saturated liquid and vapor phases Sodium Chloride Solution of the material under test. Temperatures higher or lower are adjusted by proper temperature baths in N .
Pressure of Water a t O 0 C. Using Modified Method
Table I-Vapor
TEMPERATURE OBSERVED
LANDOLTBORNSTEIN
INTERN. BUREAU WEIGHTSAND MEASURES
c.
Mm. Hg
M m . Hg
M m . Hg
13.0 16.5 19.2 20.2 21.4 22.2 23.9 24.5 25.6 26.0 26.7 55.0 55.6 90.0 91.15 92.2 92.6
11.1 13.9 16.3 17.6 18.9 19.9 22.0 22.9 24.3 25.1 26.0 117.0 120.6 523,5 547.6 569.5 578.0
11.23 14.08 16.69 17.75 19.10 20.07 22.24 23.06 24.62 25.21 26.27 118.04 121.47 525.76 549.10 571.26 579.87
11.14 13.95 16.52 17.58 18.92 19.87 22.02 22.83 24.37 24.96 26.01 117.52 120.95 525.47 548.87 570.98 579.61
O
Table 11-Vapor Pressure Data on Saturated Sodium Chloride Solutions, as Obtained by Improved Apparatus (Pressure corrected for mercury at O o C.) TEMPERATBMPERATEMPERATURE
PRESSURE
OC.
M m . Hg
15.0 15.1 15.3 16.0 16.1 53.1 53.2 53.4 53.5 53.6
8.5 8.6 8.8 9.0 9.1 80.1 80.9 81.3 81.9 81.7
TURE
PRESSURE
C.
M m . Hg
53.7 53.8 53.9 55.4 55.5 55.6 84.8 85.0 85.1 85.1
82.4 82.8 83.3 89.9 90.1 90.4 319.0 322.1 323.1 323.5
TURE a
C.
PRESSURE
M m . Hg
85.3 85.5 85.5 85.6 85.7 85.7 85.8 86.9 86.0 86.1
Curve 2, Figure 5, was plotted from data from LandoltBornstein tables for saturated sodium chloride solutions. Against this curve are plotted, by circles, data obtained on saturated sodium chloride solutions using the apparatus described. These data show the relative accuracy attainable by the method described. Such accuracy can beattained by the average laboratory worker with ease and without sacrifice of speed. It is easily possible to get such data over a temperature range of 10" to 220" C. in a total time of from 2 to 4 hours, ifpressure readings to 0.1 mm. are sufficiently accurate for the desired purpose. Application of this apparatus can be extended. It can be used for determining the vapor pressure of solids in the same manner as described by Ramsay and Young for their original apparatus. Volatile corrosive materials might be tested by modifying the closure, B, of A in Figure 3.
Standardization
In order to test the accuracy and applicability of the method for the desired purpose, vapor pressure-temperature data were obtained using redistilled water. The data are given in Table I, and the closeness with which they fall on curve 1, Figure 5, plotted from Landolt-BiSrnstein data, shows the relative accuracy of the method, Greater accuracy could be attained easily by use of more refined pressure readings than used here, where 0.1 mm. was satisfactory. The method should also be as applicable for pressures lower than 0.1 mm. Another operator in this laboratory has shown the method to be highly satisfactory for determining vapor pressure data on saturated salt solutions.
Acknowledgment
The author acknowledgeshis indebtedness to J. M. Peterson for the data on saturated sodium chloride presented in Table 11, and to E. A. Lantz for part of that in Table I.
Simple Apparatus for Measuring Vapor Pressure of Volatile Liquids' Alfred W. Francis ARTHURD. LITTLE,INC.,
V
ARIOUS forms of apparatus have been used for determining the vapor pressure of gasoline. The serious errors inherent in many of these have been pointed out by Davis.* For this purpose an apparatus has been devised which measures the vapor pressure in a Torecelli vacimm,8 but the method of use is simplified greatly, The apparatus 1 Received
November 9. 1928.
* Dasis, IND. ENO.CHEM.,17, 1136 (1925). * Engler-Hofer, "Das Erdol," Vol. IV, p. 33 (1916).
CAXBRIDGE,
MASS.
consists merely of a small longatem dropping funnel supported preferably by a split ring and connected by a rubber pressure tube with a gas-analysis leveling bottle (bottom outlet) containing mercury. In practice, the funnel is first lowered until mercury flows into it, the stopcock is shut off, and the funnel is raised until mercury fells away from the stopcock. The height of the mercnry is read on a meter rod. This is repeated two or three times to obtain a constant reading, which is usually 3 or
