V O L U M E 19, NO. 2, F E B R U A R Y 1 9 4 7 degassed by thoroughly flaming the glass. Mercury is then distilled into the apparatus through tube a until the liquid just enters the upper reservoirs. Tube a is then sealed off and partially der d n-pentane is admitted to the lower reservoirs through .C. 1 (Figure 1) until the interface between the mercury and pentane stands halfway up the vertical sides of the reservoir. At this point the mercury half fills the upper reservoirs with the meniscus on the vertical sides. A bubble of air is next introduced into the capillary tube by opening S.C. 2, and applying gentle suction to the pentane reservoir with S.C. 1 turned in such a position that air is drawn into and partially fills the bore of S.C. 1. S.C. 1 is then turned to connect the pentane reservoir with the capillary, thus forcing the air bubble into the capillary tube by the flow of pentane. The bubble position is adjusted by either adding or withdrawing a small amount of pentane from the manometer reservoirs through S.C. l. The pressure on the pentane is slightly less than atmospheric, so that flow will normally take place into the manometer when S.C. 1 is opened. Gentle suction on the pentane reservoir is then sufficient to remove pentane from the system when necessary. Some degassing of the pentane usually occurs after the manometer is first filled. This ceases, however, after standing under vacuum for several hours, and the air bubbles resulting from this degassing must be removed through S.C. 1. Both S.C. 1 and S.C. 2 in contact with pentane are lubricated with hydrocarbon-insoluble starch-glycerol grease. The mercury cutoff between the upper reservoirs serves two purposes: i t acts as a safety valve t o prevent fluid from being blown into the reference reservoir should high pressure accidentally be admitted to the manometer, and it eliminates the need for a stopcock a t this point with the attendant difficulties of leakage and absorption-desorption of gas. CALIBRATION CHECK AND OPERATION
Because of the uncertainty involved in accurately determining the effective diameters of the reservoirs and capillary tube, it was considered desirable to check the calculated sensitivity against a McLeod gage. The manometer and McLeod gage were attached to a vacuum system and evacuated to 0.01 micron. The reference reservoir was shut off and the scale zero set to coincide with one end of the bubble. A controlled amount of air was then let into the system,
135
so that the resulting pressure came between 10 and 80 microns. The bubble travel was read from the scale after an equilibrium period of approximately 30 seconds had elapsed. The gas was then trapped in the compression bulb of the McLeod gage and the pressure read several times until checks were obtained within 0.1 micron, The average of 40 pressure readings made in this manner indicated a sensitivity of 1015. Compared to the calculated sensitivity of 1056, it seems that this represents satisfactory agrecment, in view of the uncertainties involved in measuring the capillary and reservoir diameters and in determining the constant of the McLeod gage. Using the measured average sensitivity of 1015, the manometer pressure was found to agree with the McLeod gage pressure to better than 1 micron in approximately 85% of the measurements. Table I shows these data and the agreement obtained. The differences found seem to be independent of pressure, and in the 20- to 50-micron range amount to an average error of 1%. I n most cases a slight zero shift occurs during a pressure reading. Normally this shift will amount to only a few tenths of a micron and may be neglected, but where rapid temperature fluctuations are occurring larger shifts have been found. I n these cases an arithmetic average of two zeros, one taken just before and another taken immediately following a reading, has been found to result in satisfactory accuracy. Where the manometer is to be used as a continuous indicating instrument, it is believed that proper thennostating will eliminate this zero shift. LITERATURE CITED
(1) Henry, A , , Compt. rend., 15, 1078-88 (1912). (2) Mellen, G.S., Electronics, 19,142(1946). (3) Reilly and Rae, “Physico-ChemicalMethods”,Vol. 1, New York, D. Van Nostrand Co.,1939. (4) Roberts, B. J. P., PTOC. Roy. Soc., A78,410(1906). (6) Young, W. S., and Taylor, R. C., ANAL.CHEY.19, 136 (1947).
Molecular Weight Determination with a Vacuum Micromanometer W. S . YOUNG
AND
R. C. TAYLOR, The Atlantic Rejining Company, Philadelphia, Pa.
A method for determining molecular weights of volatile liquids has been developed which is considerably more rapid than the conventional cryoscopic method. Results on a number of organic compounds in the vapor pressure range from 730 to 0.3 mm. of mercury at room temperature indicate that an average accuracy of approximately *2% may be realized in an elapsed time of 3 to 4 minutes per determination. Although the method was developed primarily for molecular weight determinations of hydrocarbon mixtures occurring in the gasoline boiling range, it is applicable to a variety of liquid substances. Liquids whose molecular weights cannot be determined by this method are those with vapor pressures below 0.1 mm. of mercury, which react with mercury, or which show association or dissociation in the vapor state. Acetic acid gave results which indicated considerable vapor phase association, as was expected (2). No other compounds tested showed such a phenomenon.
T
H E method is essentially a modification of the Gay-Lussac or Hoffman vapor density method (S),consisting of means for vaporizing a small accurately measured quantity of liquid into an evacuated vessel and measuring the resultant pressure. I n contrast to (S), however, operations are carried out a t room temperature and pressures are measured a t less than I mm. of mercury absolute. Results are calculated by means of the gas law rearranged to
M
= gRl’/PV. In practice the weight of liquid vaporized is calculated from the density and a measured liquid volume. Pressure is measured by means of a vacuum micromanometer reading pressures of 0.1 mm. with an accuracy of =t1%. APPARATUS
Figure 1 shows the general arrangement of apparatus.
136
ANALYTICAL CHEMISTRY
A 2-liter flask, A , is attached to the vacuum micromanometer,
C (6),and to a mercury-sealed sintered disk, B, which is porous
t o liquids and gases in general but nonporous to mercury (1). Suitable disks are available commercially in the form of "sealing tubes" with 10-mm. diameter disks of F porosity. This volume is then connected through a solenoid-operated mercury cutoff , D,to a suitable vacuum system consisting of the usual diffusion and mechanical pump combination capable of rapid evacuation to 0.1 micron or less. E
VA%i PUMP
SINTERED DISK 10 MH.014MI F POROSITV
MERCURV CUT-OFF
micromanometer, contact between the disk and the buret is broken and a final buret reading taken a t the top of the liquid column with the tip still immersed in the mercury seal of B. Inasmuch as the reading a t the bottom of the liquid column is a known constant of the buret, the amount of liquid added may be readily determined. Although a small amount of air is introduced with the liquid by this procedure, a consideration of the relative volumes involved shows that a negligible error results. After equilibrium has been attained, which ordinarily takes less than 1 minute, the pressure increase due to the vaporized liquid is read on the micromanometer. The system is then pumped out and is ready for the next sample. This pump-out does not have to be complete, as it has been shown (6)that a residual pressure of 10 micronsintroduces an error of only 0.013% in the pressure difference measured. Compounds having vapor pressures of 1 mm. and lower a t room temperature require several minutes to vaporize completely from the sintered disk. For this reason it is advisable to aid the vaporization of such compounds by heating the disk to 70" to 80' C. with a small flame. Calculation of molecular weight is simplified by using the equation
M = -TidKT rt
1 where
T~
(1)
= amount of liquid introduced, units of microburet
graduations micromanometer scale reading resulting from the vaporization of the liquid d = density of liquid a t the temperature of the microburet T = absolute temperature K = apparatus constant defined as
f
~2
3--u-
=
where kl = microburet constant, ml. of liquid delivered per graduation kz = micromanometer constant, microns pressure per scale division V = volume of apparatus into which liquid vaporizes, CC. .. cc. mm. R = universal gas constant, O
c.
Figure 1. Molecular Weight Apparatus
A microburet suitable for measuring the liquid sample is prepared ( 4 ) from thermometer tubing selected to deliver 1.5 to 2.0 X lO-4ml. per cm. having 20 to 30 graduations per cm. of length. One end is ground to a truncated cone to assure good contact between the capillary and the sintered disk. PROCEDURE
K is most easily determined experimentally, however, utilizing Equation 1 and measurements on a compound of accurately known molecular weight and density. For example: %Heptane (20.0 units) is vaporized through B from the microburet, causing ZL pressure increase corresponding t o 123.0 mm. of bubble travel on the micromanometer scale. Room temperature near the apparatus is 25" C., and the density of n-heptane a t this temperature is 0.680. Then from Equation 1,
The apparatus is first evacuated to a pressure of 1 micron or 20.0 X 0.680 X K X 298 less with mercury cutoff D' open. D' is then closed and the sys100 = 123.0 tem checked for leaks by cutting off the pumps a t D and observing any pressure increase on the micromanometer. If there is no measurable increase in 5 minutes, this portion of the apparatus is ready for use. A small change in index bubble position during this Table I. Molecular Weight Determinations period may be due to zero shift caused by room No. of Molecular Weight Standard temperature fluctuations. This zero shift will DeterminaTheoreError, Deviaordinarily be so slow that no appreciable error % tions Compound tions Found tical results during the period of a pressure measure+1.4 72.0 1.1 6 73.0 Isopentane ment. +0.1 72.1 72.0 0.9 6 n-Pentane -0.5 0.7 85.6 86.0 4 n-Hexane The microburet is then fitted a t the top with a +0.2 100.0 1.9 100.2 50 n-Heptane rubber medicine dropper bulb and the sample 114.0 +0.9 2.0 115.0 30 Iso-octane whose molecular weight is to be determined is -0.8 142.0 2.6 140.8 3 n-Decane -1.7 2.0 168.0 165.1 4 Triisobutylene drawn up into the bore. The rubber bulb is re$1.2 104.0 1.5 105.2 4 Styrene moved and the liquid !i the bore is caused to run 78.0 1.3 0.0 78.0 26 Benaene back from the conical tip by inverting the buret. 1.8 -0.4 92.0 91.6 9 Toluene 1.6 -1.1 106.0 104.9 4 +Xylene After the outside of the buret tip has been wiped 3.7 -2.0 162.0 158.7 9 Diisopropylbenzene dry, the length of liquid column in the bore is 0.9 +l.9 32.0 32.6 8 Methyl alcohol measured in terms of buret graduations. The +1.1 46.0 0.8 46.5 17 Ethyl alcohol +0.3 1.8 74.0 74.2 8 sec-Butyl alcohol micromanometer zero is then read and the buret -2.3 3.1 102,o 99.7 4 Isohexyl alcohol inclined toward the vertical, so that liquid slowly +0.7 0.2 72.0 72.5 4 n-Butyraldehyde approaches the tip. When the liquid has reached -0.7 0.3 88.0 87.4 4 Ethyl acetate +0.4 0.2 74.0 74.3 4 Diethyl ether a point within 1 mm. of the tip it is quickly im-2.1 0.9 58.0 56.8 7 Acetone mersed in the mercury seal of sintered disk B. On -0.4 0.8 73.0 72.7 4 n-Butyl amine touching the disk with the tip, liquid will be drawn -1.3 2.9 92.0 90.8 6 Ethyl sulfide +3.4 ... 122,o 2 126.2 Ethyl disulfide out of the buret and vaporized into the evacuated +O. 5 0.5 18.0 18.1 5 Water volume. When sufficient liquid has been vaporized to give approximately full-scale deflection of the
Standard Deviations, % 1.5 1.3 0.8 1.9 1.7 1.8 1.2 1.4 1.7 2.0 1.5 2.3 2.8 1.7 2.4 3.1 0.3 0.3 0.2 1.6 1.1 3.2
i:B
V O L U M E 19, NO. 2, F E B R U A R Y 1 9 4 7 100
where from which
=
K
M of n-heptane =
3.03
An unknown sample is then run with which 21.0 units of liquid cause a pressure increase corresponding to 125.0 mm. of bubble travel a t 26 O C . The density of the liquid is 0.720 and its molecular weight is calculated from Equation 1:
M =
21.0 X 0.720 X 3.03 X 299 = 125.0
Table I shows typical results on a variety of compounds of It appears that good accuracy is maintained until C.P. quality. . the vapor pressure-of the substance falls below approximately 0.2 through the disk mm. of mercury. At this and in spite Of heating is not complete* This becomes very sets a practical upper limit for molecular weights of hydrocarbons
137 a t about 170, corresponding to a CIIH~sparaffi. Inasmuch, however, as the boiling points of such compounds are in the vicin-' ity of 200" C., most of the gasoline boiling range hydrocarbons within the "Ope Of the apparatus' I n this work no attempt Was made to thermostat the nor to shield any parts except the micromanometer from air currents, and room temperature near the apparatus was used in all calculations. LITERATURE CITED
(1) Farkas and Melville, "Experimental Methods in Gas Reactions", New York, Macmillan Co., 1939. (2) Glasstone, "Textbook of Physical Chemistry", p. 311, New York, D. Van Nostrand Go., 1940. (3) Reilly and Rae, "Physico-Chemical Methods", vole 11, New York, D. Van Nostrand Co., 1939. ( 4 ) Taylor and Young, IND. ENG.CHEM.,ANAL.ED.,17,811 (1945). (6) Young and Taylor, ANAL.CHEU.,19, 133 (1947).
Microdetermination of Tetraethyllead in Gasoline B. E. GORDON AND R. A. BURDETT Wood River Research Laboratories, Shell Oil Co., Inc., Wood River, I l l .
A micromethod for the determination of tetraethyllead in gasolines is based upon decomposition with iodine, followed by the volumetric determination of precipitated lead chromate. The elapsed time is considerably shorter than that required by conventional methods, and the sample required is only 1 to 5 ml. The accuracy and reproducibility of method conform to A.S.T.M. specifications.
T
HE production of fuel for civilian and military consumption
has emphasized the need for a rapid, accurate method of determining the lead content of gasolines. Commercial blending operations can be greatly facilitated if data as to lead content are provided with a minimum of elapsed time. I n studies of engine manifold distribution and fuel decomposition, often only small quantities of gasoline are available for analysis. A method of analysis of such small quantities, having the accuracy of accepted macroanalytical procedures, would not only provide valuable data in these studies but expedite other investigations previously limited by the lack of an appropriate analytical method. HISTORICAL
Various methods have been proposed for the determination of tetraethyllead in gasolines. Some of these have been based on an acid extraction of the lead (2, 3, 7,8,11,12), others on the decomposition of the tetraethyllead by a halogenating agent (4, 6, 9, 13). Lykken el al. (IO) recently conducted a thorough investigation of several existing methods and made suitable recommendations regarding the attainment of accuracy and precision. Aborn and Brown (1) and Clark (6) developed rapid instrumental methods for determining lead in gasolines.
911 the methods re\+ewed required from 50- to 200-ml. samples and most of them were time-consuming. The instrumental methods lacked the accuracy and precision desirable in this analysis, and some of the chemical methods had to be modified when applied to gasolines rich in aromatics or olefins. In none of the methods suggested does the application of microanalytical technique with its accompanying advantages seem to have been investigated. The ease of manipulation, simplicity of equipment, and accompanying rapidity of a suitable micromethod indicate the advantages if applied to determinations of tetraethyllead. I n addition, the small samples necessary for analysis eliminate the difficulty attendant on the handling of large quantities of difficultly oxidizable organic residues. PRINCIPLE OF METHOD
The method is based on the decomposition of the tetraethyllead with iodine, evaporation of the volatile constituents, destruction of the organic residue with mixed sulfuric, nitric, and perchloric acids, and subsequent volumetric microdetermination of precipitated lead chromate. A simple antispatter device eliminates all loss due t o energetic reaction and ebullition, and the use of a platinum Monroe micro filter stick overcomes many disadvantages inherent in filtration. Once the gasoline is added to the flask, the entire analysis is conducted without transfer; thus a serious source of error is eliminated. APPARATUS
.,.a %I".
