INDUSTRIAL AND ENGINEERING CHEMISTRY
420
Vol. 20. No. 4
A Micro-Fractionating Column for Analytical Purposes’ C. M. Cooper and E. V. Fasce DEPARTMENT OF CHEXICAL ENGINEERING, MASSACHUSETTS INSTITUTE
N T H E course of experimental work the need for an
I
analytical fractionation method applicable to liquid volumes of 10 cc. or less is frequently felt. Dufton,Z Clarke and R a h r ~Davis,‘ ,~ Widmer, Peters,6and others have developed successful laboratory fractionation apparatus for samples of 100 cc. or more, but no apparatus for practical laboratory use with small sample volumes has been described. The present paper concerns a method, developed and used by this laboratory, which meets these requirements with considerable success, affecting satisfactory separation between liquids of boiling points 10” C. apart when using a 10-cc. sample. Analytical fractionation equipment should accomplish at least two ends: (1) effect a quantitative separation of the various components, and (2) leave behind as “hold up” in the apparatus an amount of material small compared to the sample size. Rapidity and simplicity of operation are also desirable. To meet the second requirement of small “hold up” the volume of vapor space must be made a minimum; and a packed column is out of the question on account of the large amount of liquid held on the packing material by capillary forces. What might be termed a “wetted surface” type of apparatus seems to be the best answer. Description
Figure 1 gives an idea of the arrangement and the imp o r t a n t dimensions of the equipment employed. The acCOOLING tive part of the column consists WATER of a 7-mm. inside diameter Pyrex tube 36 cm. long, constricted by projections formed when small N-Heat
losses through the col-
u m n wall cause condensation and return liquid t o the still, the amount returned varying with the boiling points of the liquids employed as well as with their latent heats. Flooding occurs when high boiling liquids or liquids of low latent heats of vaporization are dealt with, and in any case operation is made difficult and efficiency is decreased. To obviate this di5culty, Davis4 provided a jacket for the column having an annular space through which heated air could be blown. T h e oir entered a t the still temperature, lost heat through the metallic jacket, and left the top of the column at the boiling point of the prod uct. T h e electrical method described in tMs article cannot provide the temperature gradient obtained wlth the Davis still, but when used in conjunction with a vacuum jacket such refinement was believed to be unnecessary. 1 2
I 4
Received October 20, 1927. J . Sac. Chcm. Ind., 86, 45 (1919). I n d . Eng. Chcm., 16, 349 (1923). M. I. T. Undergraduate Thesis,
1923. 6 8
Helv. Chim. Acta, 7, 59 (1924). I n d . Eng. Chem., 18, 69 (1926).
CO
OF
TECHNOLOGY, CAMBRIDGE, MASS.
spots on the tube wall are heated and then pushed in with a needle. This tube is vacuum-jacketed, the jacket being covered with asbestos tape around which No. 30 chrome1 wire is wound. To prevent excessive condensation in the column when handling materials boiling above 120° C. it has been found necessary to maintain the heater at a temperature not more than 20” C. below the boiling point of the material inside. A condenser, A , cooled by water for liquids boiling below 100” C. and by a stream of air for materials of higher boiling points, returns reflux to the column. Nole-The smaller the diameter of the column the greater the importance of careful insulation against, or of provision for balancing, heat lossw. This is evident from the fact t h a t the wall area increases directly with t h e diameter while the vapor space increases with the square of the diameter. Thus, with a given surrounding air temperature a larger proportion of ascending vapor will be condensed in a column of small diameter than i o one of a large diameter.
The amount thus sent back is measured by observing the drops leaving the condenser tip AI, and is controlled by varying the rate of flow of cooling material as observed by means of the dripper, J. A Fisher organic thermometer, B , indicates the temperature of the vapor leaving as product. The glass jacket, C, is necessary to obtain accurate temperature readings on such materials as petroleum hydrocarbons which possess a relatively small latent heat of vaporization. The product is condensed in D and drops from the dripper E into the 5-cc. water-jacketed buret, F. The dripper gives means of observing the rate of product formation, and also Dermits better accuracy in determ i n i n g the volume bf liquid, since it is not necessary to wet the buret walls before the product begins to collect a t the bottom. Permanent gases escape by the exit line, G. By attaching a vacuum pump here fractionation under reduced pressure is’ possible. Finally the whole column is supported in a box m e d with kieselguhr, indicated in the drawing by dotted lines, and provided with windows at,points (1) and (2) to allow the operation to be observed. A small electric light mounted in the box back of the column and opposite the window, 1, facilitates the measurement of the reED flux. Operation
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F i g u r e 1-Micro-Fractionating
Apparatus
A small quantity of sample, 10 cc. or less, is placed in the 25-cc. round-bottom Pyrex flask, H , and attached to the column by means of a cork stopper. A stream of water sufficient to condense all condensable vapor coming to the column head is passed through the condenser, A , and
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1928
the s a m p l e g e n t l y boiled by means of a micro Bunsen burner p l a y i n g on the flask just below the l i q u i d level. This produces a smooth ebullition without noticeable bumping. After the column is well warmed up and the non-c o nd e n s a b l e gases are driven out, the flame is adjusted to give the proper vapor velocity up the column as shown by the number of drops of reflux per minute observed to fall from the tip of the con5 0 ~ IO 20 30 do VOLUME PER CENT DISTILLED OVER denser, A . By manipFigure 2 u l a t i n g the stopcock, L, the rate of cooling water flow is decreased till the correct product rate is indicated by dripper E. During the distillation the rate of product formation and the amount of reflux are kept constant by manipulating the stopcock, L, and the Bunsen burner, 0. Best results have been observed with a reflux rate of from 20 to 40 and a product rate of from 2 to 10 drops of liquid per minute, slower distillation being necessary for separation of materials whose boiling points lie closer together than 10" C. Under these conditions the iilm of refluxed liquid on the column wall is scarcely observable. The material remaining in the column after distillation amounts to about 0.4 cc. The time required for
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fractionation of a IO-cc. I M s a m p l e v a r i e s from about 30 minutes for a IP0 simple binary mixture of a b o u t 15' C. be'lo tween t h e boiling g p o i n t s , to about 90 IM) minutes for more com- 5 plicated mixtures. 5 eo Results
The curves of Figure 2 are the r e s u l t s of fractionations of a 36 volume per cent solut i o n of m e t h a n o l in water. From these results it is evident that
:
MIXTURE OF ACETONEAND METHANOL
Y
5 t
80
CONSTANT BOILING MIXTURE O F E T H A N O L - - 1 4 . 3
70
e
'
IO 2
METHANOL ............. 20 6
60
50
o.
~o
1o
,oo ~
Apparatus for Measuring the Hydrogen-Ion Concentration of the Soil's' R. H. Bray DrvrsIoN
OF
SOIL FSRTILITY, DEPARTMENT OF AGRONOMY, UNIVERSITY OF ILLIKOIS. URBANA, ILL.
I
N MEASURING the hydrogen-ion concentration of the soil, many different types of hydrogen electrodes and electrode vessels have been used with more or less success since Gillespie3 and Sharp and Hoagland4 published consistent results on soils. The maintenance of a suspension of the soil particles has been the principle back of the more recent types of apparatus, since it has been found that when a soil settles the supernatant liquid has a different p H from that of the soil suspension. Platinum- or palladium-plated electrodes have been used, with the usual difficulty that the moving soil particles wear the electrodes bright. Crowther6 replated his electrodes every day but mentioned that they would have lasted much longer. He used a thin film of palladium. Electrode and Electrode Vessel
A simple hydrogen electrode, based on a combination of ideas used in other electrode^,^^^ has been devised which gives Received November 14, 1927. with approval of the Director of the Illinois Agricultural Experiment Station. 8 J . Wush. Acad. Sci., 6, 7 (1916). 4 J . Agr. Research, 7, 123 (1916). 6 J . Agr. Sci., 15, 201 (1925). I
* Published
quick and accurate results. (Figure 1) The apparatus is used with a Leeds and Northrup type K potentiometer, Weston standard cell, and Leeds and Northrup enclosed lamp and scale galvanometer, No. 2420c. The electrode vessel is a Gooch funnel, A , with the stem bent up and a perforated porcelain plate, B , sealed into the bottom. The hydrogen enters the tube, bubbles up through the plate to the electrode, C, and escapes from a small hole in the stopper, D, which also serves as an entrance for the agar-KC1 tube, E. The electrode is a piece of approximately 20-gage platinum wire, 10 cm. long, sealed into a glass tube a t one end, and bent into the shape of a spiral cone. It is held in place by the stopper. The platinum wire is plated with palladium black. Flat electrodes plated with palladium have been tried with this type of vessel, but do not give such good results as the spiral cones. The agar-KC1 tube consists of a glass tube, 0.7 to 0.8 cm. in outside diameter, bent into a half-circle a t one end and drawn out with a very gradual taper at the other. It is filled with agar gel saturated with potassium chloride. The bent end dips into the saturated potassium chloride solution, 6
7
Simms, J. A m . Chem. SOL.,40, 2504 (1923). Sideris, Science, 62, 331 (1925).
