A RAPID-ACTING SEMIMICRO VAPOR DENSITY APPARATUS JOHN T. STOCK1 and M. A. FILL Nomood Technical College, London, England
T H E parts of the apparatus, which is of the Victor Meyer type, are supported on a 5/s-in. hardwood vertical panel, which is screwed to a baseplate of the same material. A side view of the apparatus is given a t (a) in Figure 1; the appearance of the front of the panel, which is faced with a light-colored plastic ("Formica"), is shown a t (b).
(o)side view; ( b ) front view of panel
A 3/16-in. asbestos cement shield A covers the back of the panel and is secured by screws passing through distance pieces BB. The air space thus formed greatly reduces heat transfer from the hot "vaporizing side" t o the cool "measuring side." Flask hotplate C is of heavy-gage aluminum sheet and is bolted to the shield. A 2l/=-in. hole cut in the hotplate allows direct access of 1 Present address: Minn.
University of Minnesota, Minneapolis,
the flame to the oneliter heavy-duty conical flask D. A similar narrower support carries Bunsen burner E, which is removed from its base and secured by the baseretaining screw. Serving as a boiler and steam jacket, the flask has a 3-mm. escape hole F blown in the upper part of the wall. If extended runs are envisaged or the use of jacketing substances other than water is required, F is replaced by a vertical tube for the attachment of a reflux condenser. Vaporizing chamber G, which is surmounted by 3-mm. bore stopcock TI, is shown on a larger scale a t (a) in Figure 2. The alternative simpler but not so satisfactory version shown a t (b) may be improvised from a boiling tube.= Either form of vaporizing chamber is supported in the flask by means of a snugly bored cork (for chamher (a), the cork is split diametrically) and carries cap H. The latter is a 5 X 6/s-in. test tube having a small hole blown in the wall. The 1.5-mm.-bore exit tube is joined to the 1.5-mm.bore T-piece I (Figure I), the side arm of which passes through the panel. All such joints are glass-to-glass within tightly fitting sleeves of rubber tubing. Stopcock T2, vhich has the same bore as the T-piece, carries a rubber bulb J . A similar stopcock Tajoins the lower arm of the T-piece to measuring pipet K. An ordinary 10-ml. graduated pipet with 0.1-ml. subdivisions serves excellently, the mouthpiece being bent as shown. Flow rate must he rapid, so that an ideal pipet is one withdrawn from normal service because of a broken jet. Rubber tubing joins the lower end of the pipet to leveling vessel L, which slides vertically in "Terry" spring clips MM. Suitably placed but smaller spring clips are used to support the measuring assembly on the face of the panel, while a large clip mounted on a length of brass screw-rod projecting from the shield is used to secure the flask. Stop N, cut from rubber tubing, prevents the leveling vessel from being pushed down too far. The stop also anchors the thread supporting thermometer 0, which dips into colored water contained in the leveling vessel. The liquid to be examined is weighed into a thinwalled tube that is closed at one end and is about 40 mm. long and 1.5 mm. in bore. The same tube may be used repeatedly, although its weight should be checked from time to time. To prevent chipping when dropped into the apparatus, the open end should be lightly fire~olished. If several tubes are made. thev mav be dif2 STOCK, J. T.,AND C. HEITLER,School Science Rev., 35, 39 (1953).
DECEMBER, 19% Rssulta Obtained with Chloroform (m. w. 119.4) Run no.
-Chamber Ob8emed m. m. (X)
~
Av. (a)
(a)-Chamber Deviation Observed ( X - i)m. w. ( X )
~
~
119.8
Standard deviation =
(b)Deviation ( X - 3)
~
120.2
4
Z(X
-
=
1
Chamber (a), 0.82 Chamber (b), 2.01
ferentiated by sealing a tiny bead of colored glass to the closed end of each. PROCEDURE (1) Introduce water (distilled, to prevent scale formation on evaporation) into the flaskuntil the levelis about 5 mm. below the bottom of the vaporizing chamber. A wash bottle, the coarse jet of which is inserted through hole F, is convenient for this operation. Open stopcocks Ttand TI hut close T1. Heat the Bask and adjust the flame height to maintain steady, rather rapid boiling. Move the leveling vessel to bring the meniscus in the pipet to the uppermost graduation. (2) Warm a weighed capillary over a small flame and a t once immerse the open end in the sample to be examined. On cooling, a short column of liquid will be drawn in. Using sharp wrist action, shake the column down to the closed end. Cautiously reheat until this liquid just boils; a t once reimmerse the open end, and thus obtain EL longer column. Wipe the exterior of the tuhe snd reweigh. The amount of sample taken should, on vaporization, displace between 7 and 9 ml. of air, an measured under the prevding conditions. For a. liquid having a molecular weight of 100 the weight required is about 30 milligrams. Far very rapid but less precise working the tuhe is calibrated with mercury, when the weight of sample may be calcula.ted from the column length, provided that the density is known.3 (3) After the water in the Rask has been boiling for at least 10 minutes, close TI. If no change in the pipet reading has oocurred after one minute, remove cap H and drop the loaded sample tube, mouth downwards, into the way of T,. Slmly rotate the latter to the open position, when the sample tuhe will fall into the vapodsing chamber. Close T, as soon as the tuhe drops. A downward movement of the meniscus in the pipet should be observed almost a t once; then steadily lower the leveling vessel to maintain atmospheric presmre within the apparatus. The rapid movement of the meniscus usually Ceases within about 10 seconds after introduction of the sample. Exactly equalize levels, close T8,and read off the volume of displaced sir. Also note the thermometer reading and the barometric height, so that the molecular weight may be calculated in the usual way.' Normally, all the readings are completed within one minute of release of the sample tuhe. (4) Replace cap H and make sure that the bore of the d u e "DAIR, A,, School Science Rev., 30,240 (1949). 'See, for example, DANIELS,F., J. H. MATAEWS,J. W. WILLIAMS,P. BENDER,G. W. MURPHY,AND R. A. ALBERTY, "Experimental Physical Chemistry," McGraw-Hill Book Co., Inc., New York, 1949, p. 13.
(0)
final form; ( b ) improvised form; (el cap, ahowing eample tube after
eiection from apmmtua. of TIis in line with the way in the barrel. Open TIand squeese the hulh. The empty sample tube will ahoat upwards into the cap and come to rest as shown a t (c) in Figure2. While heating, slowly compress fully and release the hulh six times; this will expel the vaporized sample. Remove the cap, take out the sample tuhe, and make replicate runs as required. About two minutes should he allowed between expulsion of vapor and the iutroduction of a new eample.
RESULTS First experience of the apparatus was gained using the boiling-tube vaporization chamber, Figure 2(b). Results of numerous experiments led to the conclusion that the least satisfactory feature was the delay in establishing temperature equilibrium, or in re-establishing the latter after the introduction of a sample. The improved vaporization chamber has a thin-walled bulb and a greater surface-to-volume ratio than the boiling tube. Further, the tube down which the sample tuhe is dropped
0
20
10
30
Time, min. rim-
3.
Blank Run r i t h Find Form of v.poririn.
Ch.mb.
698
is itself directly exposed to the steam. The thermal performance of the improved chamber was tested by bringing the meniscus to the zero mark of the pipet, closing TIwith the apparatus still a t room temperature, and then observing the rate of expansion of the entrapped air on heating the water in the flask. The result of a typical run, shown in Figure 3, indicates that expansion is complete within 10 minutes after boiling begins. At points XI and X2the expulsion procedure given in (4) was carried out, resulting in a slight, nonpersistent "creep" of the meniscus. It is therefore concluded that equilibrium is virtually re-established in a time comparable with that required for the manipula-
JOURNAL OF CHEMICAL EDUCATION
tion of the various stopcocks, etc. Similar trials with the boiling-tube vaporization chamber showed that noticeable expansion of air continued for several minutes after commencement of boiling, and that slight "creep" persisted even after 30 minutes. Using pure chloroform as a test substance, the results shown in the table were obtained. Although the runs were performed in groups, the experiments with both forms of apparatus were spread over several days. The results show the decided superiority of the later form of vaporizing chamber. The authors thank Mr. R. Michell, B.Sc., for experimental assistance.
