Fractionating Column for Use under Diminished Pressure M. T. BUSHAND A. M. SCHWARTZ, Cornell Uniuersiiy, Ithaca, N . Y.
A
LTHOUGH there are many columns described in the literature, there are none suitable for general use under diminished pressure, and the separation of liquids by fractional distillation in wcuo is regarded as an arduous task in many organic laboratories. It is the purpose of this paper to describe a complete fractionating apparatus which is efficient under a wide variety of conditions.
(50-cm.) column. The visibility afforded by the glass insulation allows control of the heating by means of thermometers which are tied to small glass hooks on the column. The column itself may be either the Vigreux or the filled type. The authors have had success with a 12-mm. tube filled with small jack-chain by hanging two or three lengths from a metal ring a t the top ( A ) . The mercury valve of the flow divider (W) has been simplified by surrounding the rubber tube with mercury thus making it leak-proof. The heavy-walled rubber tube J (3 by 6 mm.) is slipped over the valve stem, the steel collar K (1 cm. wide), fitted with the nut M , is slipped on, and the open end of the tube is plugged. The rubber-to-glass connections are sealed with shellac. The steel plate L (8 mm. wide) is put in place and the side-arm test tube adjusted as shown. The screw N , with the nut P adjusted, is then connected with the nut M through the side arm and the rubber tube 0. The valve is filled with mercury through the opening a t F and the tube GI and the test tube, HI is filled with mercury through the funnel &. With this arrangement the tube does not leak a t a pressure of 1 mm. It is well to scrub out the tube J before putting i t in place to facilitate the removal of adsorbed air. The most satisfactory receiver for cutting off fractions of distillate is a slight modification of the Bredt-Parlatosche “Star” ( I ) , in which the receivers are either bulbs or test tubes. This avoids contamination of the distillate by stoppers or stopcock grease. After the apparatus is once assembled it is not difficult to run a fractionation. The column is first warmed to about 5” below the initial boiling temperature, and the flask, carrying a very fine capillary ebullition tube, adjusted at the bottom. The rate of distillation is controlled by the tempera-
I90 PI70 c
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E150 3 U
El30
FIGURE 1. FRACTIONA~NG COLUMN AND MODIFIED FLOW DIVIDER
E
c”, 10
Perhaps the greatest difficulty in the operation of a still is to provide adequate insulation. When distilling at atmospheric pressure it is often possible to do this by wrapping the column with “85 per cent magnesia.” When operating under relatively low pressure, however, there is excessive condensation, and if the column is as much as 2 to 3 feet (50 to 76 cm.) long it is not possible to distil satisfactorily at temperatures above 80” to 100” C. with either magnesia or an evacuated jacket as insulation. This difficulty is removed by using a heating unit in conjunction with an evacuated jacket. I n the drawing, Figure 1, the Pyrex tube B (30 mm. in diameter) is secured within the evacuated jacket C (45 mm. internal diameter) by means of rubber stoppers. The heating is accomplished electrically with a helix of chromel wire on a narrow strip of asbestos paper,fastened a t the endswith copper wire. About fifty turns of chromel are convenient for a %foot
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cc. of Distillate
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FIGURE 2. TYPICAL FRACTIONATION CURVE
ture of the heating bath. It is necessary, of course, to heat the column more strongly as the temperature of the distillation rises. The amount of reflux is adjusted, and the fractions taken according to the readings on the thermometer. To indicate the efficiency which can be expected from this column, a typical fractionation curve is given in Figure 2. The mixture of esters obtained in the examination of a natural oil was fractionated at 8 mm. and the fractions cut as indicated. The methyl caprate and methyl laurate fractions were relatively pure, but the methyl oleate apparently contained some linoleic and linolenic esters, which explains the
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INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
continuous slope of that portion of the curve. The abrupt rise in temperature in the middle of the laurate fraction is due to an increase in the rate of distillation. The 60-cc. intermediate between the caprate and lauratefractionswas carefully refractionated into 22 cc. at 92" to 93" C., 10 cc. a t 93" to 115" C., and 23 cc. at 115" to 117" C., at a pressure of 5 mm. The apparatus can be used conveniently for amounts of material from 50 to 400 cc. having a boiling point up to about 200" C., a t pressures down to 1 mm. It is most conveniently assembled with the aid of standard ground-glass connections, as indicated in Figure 1. The size of the connection a t E is
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number 9.5, at F number 11, a t D number 15, and at the bottom of the column number 11. For temperatures above 150" C. it is well to insulate the flow divider with "85 per cent magnesia." LITEIL4TURE CITED
(1) Houben-Weyl, "Die Methoden der organischen Chemie," 2nd ed., Vol. I, p. 568, Georg Thieme, Leipzig, 1921. (2) Schwartz, A. M., and Bush, M. T., IXD.ENQ.CHEM.,Anal. Ed., 3, 138 (1931). RECEIVED July 6, 1931.
Wet Microcombustion of Organic Compounds With Particular Reference to Cellulose Derivatives ANDREWCHALMERS, Du Pont Rayon Go., Buffalo, N . Y .
I
N A communication to be published in a later issue, the writer discusses the application of a strong oxidizing mixture upon cellulose compounds, and by using a specially designed apparatus, estimates the carbon content by the pressure exerted by the carbon dioxide formed. However, it is sometimes necessary to perform analyses using samples smaller than 0.10 gram, and as the sample size is reduced, the efficiency of the above method decreases. It was considered possible that by using a standard solution of the oxidizing reagent, a much smaller sample could be determined by using volumetric methods. Such a method has apparently been obtained. As an oxidizing reagent, a mixture of a concentrated aqueous solution of potassium dichromate and concentrated sulfuric acid was used. Thip mixture performed exceedingly well with most of the samples used, but some were found which resisted "wetting" to such an extent that it was necessary to boil the mixture for a relatively long time in order to consume the sample completely. Such a procedure did not commend itself to accuracy. A solution of chromic acid anhydride in c. P. phosphoric acid gave the necessary qualifications in that it consumed all the samples used, and since it contained no unnecessary water, steam was not released on subsequent heating. A special form of apparatus, shown in Figure 1, was designed for this method, the design given being the result of experimenting with many other forms of apparatus. The oxidizing reagent is prepared by dissolving one part, by weight of c. P. chromic acid in nine parts by weight of c. P. sirupy phosphoric acid. A more concentrated mixture may be used if desired. Approximately 15 cc. of the oxidizing reagent are run into the reaction flask, A , by means of a standard buret. It is suggested here that where a considerable number of analyses are made, a small buret capable of delivering about 15 cc. be constructed for use in this method. Twenty cubic centimeters of 0.05 N barium hydroxide solution are run into flask B from a constant-delivery buret. The sample (0.0050 gram or less), previously weighed in a piece of platinum foil, is introduced into the reaction flask. A very slow stream of dry air, which has been washed free of carbon dioxide, is conducted through the apparatus and a gentle heat is applied to flask A, using a microburner. When the reaction has ceased, the air should be continued to pass for a reasonable length of time. The apparatus is disconnected and the barium hydroxide solution is washed and precipitated into a beaker, a drop of phenolphthalein indicator
is added, and the excess hydroxide rapidly titrated with 0.05 N hydrochloric acid. The acid mixture in flask A is washed into a 250-cc. volumetric flask and made up to mark with distilled water. A 25-cc. portion is aliquoted and transferred to a beaker. To this 20 cc. of 20 per cent potassium iodide solution are added and the liberated iodine is titrated with 0.1 N sodium thiosulfate solution using starch solution as an internal indicator.
FIGURE1. SPECIAL APPARATUS FOR MICROCOMBUSTION
A blank determination is made upon the equivalent amounts of oxidizing reagent and barium hydroxide used. I n the latter, the blank determination would adjust a n error due to possible contamination by carbon dioxide during transferring of the solution. The actual titration subtracted from the blank titration gives the amount of reagent entering into the reaction. From each of the above results, the per cent carbon present in the sample is obtained, one depending upon the amount of carbon dioxide formed, and the other upon the amount of oxygen consumed. 1 cc. of 0.05 N NC2 = 0.0003 gram of carbon 1 cc. of 0.1 N NazSpOa= 0.0008 gram of oxygen
