ANALYTICAL CHEMISTRY
1214 during the distillation is burned to refresh the tip. Capillary B (Figure 3) is flush Kith the bottom of flask a; a Porous glass Protruding tip, shown by the enlarged figure, c, is obtained.
It is desirable to carry out distillation by heating over a free flame rather than in a bath, as heating in a bath does not always give satisfactory results. Therefore distillation flask b should be made of hard glass such as borosilicate glass. When the flask is of glass with a large coefficient of expansion, the protruding tip may break off or the flask develop a crack during distillation. This distillation flask provides an excellent compact ebullition mechanism. The bottom of the tube will always be heated above the boiling point of the liquid in the flask, so that vapor bubbles evolved form the ebullition nucleus. This is also true in the case of low pressure. Therefore reduced pressure distillation with this flask can be carried out without help of a capillary. For example, an ether solution may be distilled off at atmospheric pressure and the ether residue distilled immediately at a reduced pressure, without changing flasks or introducing a capillary tube. This protruding glass tube utilizes the vapor itself as the ebullition nucleus and does not require air bubbles for its continued operation. Therefore it is also possible to distill in a high vacuum.
Method of Degassing liquids Amos S. Newton, Radiation laboratory, University of California, Berkeley, Calif.
u on the radiolysis of alcohols and ethers using Ivery low energy input, and therefore with a product yield of only a few micromoles, traces of air in the liquids markedly inEXPERIMEXTS
fluenced the results. The quantitative recovery of very small amounts of gaseous products, especially the higher boiling hydrocarbons such aa propane and the butanes, was difficult. In order to simplify and speed both the original deaerating and product recovery, a method has been devised whereby the liquid can be refluxed under vacuum while pumping on it either with a regular vacuum pump for deaerating or with a Toepler pump for collecting dissolved gases.
degassed liquid of lower boiling point and refluxing this mixture under vacuum. This has been illustrated with benzene. EXPERIMENTAL
Degassing Liquids. The apparatus used for refluxing liquids under vacuum is shown in Figure 1. I t was set up with the receiving flasks sealed in place on the line. The liquid to be degassed was introduced into flask F , sealed off with the usual precautions, and refluxed under vacuum using a dry ice-trichloroethylene slurry in the condenser and a liquid nitrogen trap at T. Precooling flask F is an aid in avoiding bumping during the initial phases of the evacuation. Pumping was continued until the pressure in the connecting vacuum system was down to less than about 1 micron (as measured by a vacuum thermocouple gage). The pressure in the vacuum system beyond the nitrogen trap showed no change when stopcock A was turned off for 10 or 15 minutes and then opened. For 250 ml. of liquid, starting a t atmospheric pressure, this process required from 1 to 2 hours. The initial phases of the degassing can be further speeded by stirring the liquid in flask F with a magnetic stirre1 Stopcock B was then turned off, the dry ice removed from the condenser with a long-handled spoon and a precooled syringe, and the liquid distilled into the final receivers with dry ice cooling. F is cooled with dry ice and B is opened while each receiver is being sealed off. The system is not limited to dry ice cooling; any cooling agent that gives a temperature above the melting point of the solvent, yet low enough to reduce the vapor pressure to a reasonable value, may be used. A fairly high vapor pressure can be tolerated if the size of the liquid nitrogen trap is increased and some loss of material can be tolerated. -4 carbon disulfide slush bath (melting point -110” C.) has been used successfullv in degassing diethyl ether
TO TOEPLER PUMP AND GAS BURET
” C
TO VACUUMA-&
BREAK SCALE, I N C H E S
I N DEN TAT IONS
Figure 2 O 2 - 2 SCALE, I N C H E S
Figure 1
Recovery of Gases from Liquids. The system for the quantitative recovery of dissolved gases from liquids is essentially the same, but if gases as high boiling as butanes are to be recovered, an additional step is npceesary. The apparatus is shown in Figure 2.
In essence, the apparatus is a low-temperature reflux condenser, in which the liquid can be refluxed a t a temperature a t which it8 vapor pressure is negligible ( < 1 mm.). The method for deaerating is limited to materials that have low vapor pressures at temperatures above their melting points, thereby excluding such compounds as JVater, benzene, and glacial acetic acid. I n principle, dissolved gases can be determined in these high-melting liquids by forming a low-melting eut,ectic with a previously
The gas sample was sealed to the side arm of flask F (irradiated samples are contained in flasks with break and contain highboiling “polymers” which are later recovered from residual material in flask F , hence the vertical placement of the sample flask for complete drainage into flask F , and the system evacuated. The condenser was then filled with dry ice and trichloroethylene slush, flask F r a s cooled nith dry ice, stopcock D was closed. and the samule was introduced to F . The glass-wool plug served as a shock a6eorber for the iron hammer on Tts return.
1
1
ii
V O L U M E 28, NO. 7, J U L Y 1 9 5 6
121s
Liquid nitrogen traps were placed around A and B, the dry ice bath was removed from F , and the noncondensahle gases were collected by the Toeder p u m n while all condensable eases. together with a small i m o u i t of solvent, were collected in tra, A . When no further gas can be collected through the liquid nitrogen traps, any higher boiling gas can be fractionated from the small amount of material collected in A . The collection of material from F into A should be continued until the residue in F will reflux indefinitely with stopcock D closed. Further removals may be done intermittently while the next step is in progress. The distillate in A may he cut into as many fractions as desired by distillation from A to B , with liauid nitrogen p u m ~ i n ea t C (9).
tween distillations l;f flask F stops refluxing, the volatile material may be transferred to A while the material in B is being returned to A . A trace of solvent is always distilled to C in the transfer from A to B, if the vapor pressure of the solvent is a t all appreeiable a t the temperature of B. As a check on this procedure, 0.126 millimole of a mixture of isobutane, isobutene, propane, and propene (Phillips Pctroleum Co., research grade hydrocarbons) NZU added to 50 ml. of degassed isopro yl ether [freezing point -85.7'; literature ( l ) , -85.89"] b y h u i d nitrogen transfer from a gas buret. It Was degassed by the procedure outlined above (Figure 2 ) and the gases from the Toepler pump in gas buret. Over The added gas and the final recovered gas were analyzed using a Consolidated Engineering Corp. Model 21-103 mass spectrometer. The recovery is shown in Table I. ~
Table I. Recovery of Gases frum'Diisopropy1 Ether component Propene Pr0ps"e Isobutene Isobutane
Millimole Added. 0.0482 0.0320
o.oao9 0.0149 ~
Millimole Found"
A
Millimole +0.0003
0.0485
0.0328 0.0296 0.0156 ~
t-0.0003 -0.0013 +O ,0007
gen, and 14.5% oxygen. The high argon content proved the gas was dissolved in the benzene. No tests on recovery of added eases in benzene were made. ~
LITERATURE CITED
(1) ~ ~ ~ i R.~ R., b ~~ ~ ~h ih,R., ,, A,, ~
~ Bd ~. chm. . 41, 2876
(1949). (2) Sanderson, R. T., ''vacuum Manigulation of volatilecornpounds," PP. 8kL93, Wiley, New York, 1948.
WORBperformed undertheauspices of the U. 8. Atomio Energy Commission.
. ..
'
1.
Medica' Birminghr.
"..I
...LI-
1.
"
w;~
"..-.. _...- ...
"LULLLlLllrlYUULrVl
a-w-,,
LItLYT
.. .
. ..
IF~I....IFI~ Y , W
UL3.I
.~
YL.IL-IIve")
.
Y L ~ ~ C ~ ~ ~ W ~ ~
b"t:
,lrer.rar,ure
been reviewed by Conway ( 2 ) and by Kirk (8). In general, the better instruments of this kind perform well, hut their fragility, difficulties of maintenance, or high cost preclude their use in student or routine clinical laboratories. The writers have designed a buret, based on the principle of the differential screw. whioh can he huilt in any shoo eauioned with a " metal-working lathe and a milling attachment. It is simple, convenient, rugged, and inexpensive as regards cost of materials. The reagent being measured comes in contact with neither stop cock grease nor mercury, and the tobe can readily he rinsed when reagents are to be changed. Figure 1is an "e~ploded"view of the drive mechanism.
. . ..
~
Total 0.126 0.126 0 Recovered 6a8 also contained 0.0114 millimole of diisopropyl ether (8.3% of tots1 pas collected).
For comparison with one usual method of degassing (pumping a t low temperature followed by partial evaporation), 100 mi. of diisopropyl ether was satursted with methane a t room temperature then degassed by cooling with dry ice and pumping on it for 30 rhnutes. The ether N&S further divided into two parts by vacuum distillation of half of it into a second dry ioeeooled trap consisting of a U-trap in the vacuum line with a volume a t the bottom of the U for collecting the liquid. Pumping was continued during the entire transfer process. Both traps were then cooled with dry ice and sealed off. Dissolved gas in each fraction of the isopropyl ether was then determined by the procedure outlined. I n this ease, if the Toepler pump was to collect the gas, i t was expedient to add a small quantity of air (0.05 millimole) to the system. From the residual 50% of the ether, 0.0006 millimole of methane was reoovered, and from the distilled 50% of the ether 0.0003 millimole of methane was recovered. Application to High-Melting Solvent. Pure solvents which have high vapor pressures a t their melting points cannot he degassed by this procedure, hut, in principle, gases may be recovered from such solvents-e.g., benzene-by placing the solvent in flask F, in which there is another previously degassed solvent of lower boiling point which will form a low-melting mixture with the high-melting solvent in question.
As an example, benesene was distilled from sodium under an argon atmosphere, then degassed by repeated freezing and mel& ing under vacuum. It was frozen five times; after the second time no gas evolution wm noted during the freezing process. As a final step, about 20% of the material was vacuum-distilled and discarded. Same 50 ml. of the re8idud benzene was added to 50 ml. of degassed n-hexane in apparatus shown in Figure 2 and the noncandensable gases were collected. Dry ice cooling was used in the condenser. Some benzene precipitated in flask F during the operation, but the precipitated benzene was kept fairly well washed down from the condenser by the refluxing liquid. The 0.003 millimole of gas collected was 46% argon, 39.5% nitro-
. .
.
~
.
-.-
Figure 2.
Assembled ultramicroburet using differential screw mechanism
Parts A , B, and C me made from a single piece of 3/4-inohbrass rod, in which, before the three parts are cut apart, two slots, inch wide and ahout inch deep, are milled a t the ends of a diameter of the cross section. These receive guides D and E, which are made from '/rineh square steel key stock. A , B, and C are cut apart, and Cis drilled (while held in a callet) to hold the barrel of syringe G (a '/*-mI. tuberculin syringe). A is ~imilmly drilled and tapped for a '/,-inch screw, 20 threads per inch. B is turned down for part of its length and drilled (while held in a collet) with a tap drill for a */r-inch screw, 28 threads per inch. The hole is ta ped to a depth of a/4 inch fipm the left end of B, as shown, a n f t h e remainder of the hole IS dnlled to clear the m e w threads. A small recess, into which the head of the piston of G will fit, is then turned in the right end of B, and stirrup H (made of sheet brass) and two screws are provided to hold the piston to B. A and Care fastened between the guides by screm, as shown; B slides hetn-een the guides, Screw J is threaded on one end to fit A (20 threads er inch), and on the other end t o fit B (28 threads per inch). &ob K is provided to turn J .
