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the application of heat distill together in a fixed proportion, and the boiling point will remain unchanged as long as the two constituents of the mixture are present. This boiling will take place at a temperature below the boiling point of either the oil or the water. This lowering of boiling point is characteristic of a mixture of two mutually insoluble liquids, and because of this fact an oil with a high boiling point may be distilled with water at a temperature below the boiling point of the latter. The same conditions exist and the same results follow when saturated steam is blown into the oil. In practice the steam distillation is carried on either by mixing the water with the oil in the still and heating the still externally from beneath, or by blowing steam into the oil from a boiler; the latter process is easier to regulate. If, however, nearly equal quantities of two oils which are freely miscible with one another and have different boiling points be mixed and distilled with steam, the proportion of oil and water constantly changes and the distillation temperature of such a mixture does not remain constant, but will vary between the approximate temperature at which the separate oils distill with
The separation of light and heavy oils by steam distillation offers the same difficulties
tioned in the discussion of FIG. 1.-Percentage weight-steam temperature curve for a
direct heat fractionation. To remove these difficulties the principles of dephlegmation can be applied to steam distillation in the same manner as to ordinary distillation, but the still head must be of special construction.
mixture of two oils distilled in the ordinary manner.
(3) In scientific work of this character it is often very difficult, even with the data in hand, to interpret the results quickly and accurately. This is due in part to the fact that the eye can not readily grasp the meaning of a mass of figures. They are too detailed, and too much calculation is necessary in order to make comparisons. Something less complicated would greatly aid matters; a picture, as it were,
represents the exact conditions.
If in the case of an ordinary distillation without a well made still head the maximum boiling points of the various fractions be plotted on the horizontal axis, and the corresponding percentages by weight of distillate on the vertical axis, a curve which approximates figure 1 will
be obtained, which shows graphically what has happened during distillation. This curve shows that with the increase in the total per
If, however, the same mixture is distilled through a dephlegmator, the percentage weight-boiling temperature curve will take a form similar to that shown in figure 2. In this figure the curve shows that during the first part of the distillation from a to b the temperature remained nearly constant, from b to c the temperature rose rapidly, while from c to d the temperature was again practically constant. and remained so until the end of the process.
FIG. 2.-Percentage weight-steam temperature curve for a mixture of two oils distilled through a dephlegmator.
fore, that a curve which is nearly vertical indicates that the distillate during that portion of the distillation consists of a nearly pure substance, while a curve which rises slowly toward the right or is nearly horizontal indicates that the distillate is composed of two or more substances. A comparison of figure 1 with figure 2 shows clearly the more complete fractionation due to dephlegmation, and also shows that when dephlegmation is used the shape of the curve gives a more accurate picture of the composition of the mixture distilled.
It is evident, there
FIG. 3.-Percentage weight-steam temperature curve for a nearly pure oil mixed with small quantities of higher boiling substances distilled through a dephlegmator.
Figure 3 is the type of curve obtained when the oil distilled consists chiefly of one substance with a relatively small admixture of higher boiling substances.
(4) Any physical property, such as specific gravity or index of refraction, may be used in the place of boiling temperature in these curves; and the more physical properties determined, the greater will be the reliability of the final interpretation. In the analyses made by the Forest Service, the color, odor, and maximum boiling temperature of all fractions were noticed, and specific gravity and index of refraction were determined.
THE NEW METHOD OF ANALYSIS.
The complete method as finally modified consists, then, in distilling the sample with steam through a dephlegmator and collecting the dis
tillate in several fractions. The color, odor, and boiling temperature of each of these fractions are noted, and the weight, specific gravity, and index of refraction of each determined. From these data, tables are made and curves are plotted. The interpretation and comparison of the curves furnish information concerning the composition of the samples as shown by the physical properties of the leading constituents, and also render the similarity or dissimilarity of various oils quickly evident.
The apparatus, shown in figure 4, is supported by the ordinary laboratory stands and clamps, which are omitted in the drawing. A
44780-Cir. 152-08- -2
is the flask which holds the turpentine to be analyzed. It is supported in a manner to permit its removal without disturbing the rest of the apparatus. F is the steam boiler, which is provided with a glass safety tube H, and in which the steam for the distillation is generated. The delivery tube through which the steam is conveyed passes through the f cork at the mouth of the flask A and extends nearly to the bottom.
The temperature of the escaping vapors is read by means of the thermometer C, which is graduated to tenths of a degree centigrade. G is a separatory funnel arranged to permit water to drop slowly into the top of the dephlegmator, B. D is the condenser. E is a cylinder of 100 c. c. capacity for collecting the distillate. About fifteen of these cylinders are ordinarily used in each analysis. Ordinary cork stoppers are used to make the connections.
The still head is the most important part of the apparatus, since here the fractionation is carried on. Several forms were tried, but the one shown in the figure proved to be the most satisfactory. Figure 5 is a working drawing of the device. The foundation is an ordinary soft glass tube, 11-inch bore, provided at regular intervals with constrictions of different diameters, the smallest being at the bottom and the largest at the top. Traps were made by inserting into these constrictions small glass U tubes of different lengths and supporting them by brass wire-gauze collars. These tubes are inserted in order to facilitate the reflow of the condensed vapors to the flask below. They are made in the form of traps so that the uprising vapors will be forced through the meshes FIG. 5.—Working drawing of dephleg- and between the coils of the gauze col
lars instead of through the tube itself. The bulbs at the upper ends of the U tubes prevent them from falling, and the bends are made in order to hold a small amount of liquid, and thus act as a trap to prevent the escape of vapors through the tubes instead of through the gauze. Each trap was one-fourth inch shorter than the one below it, since it was found advisable to have successively heavier water seals from top to bottom. Each constriction was made smaller than the one above it, with the exception of the two at the top, in order to allow the collars to be easily placed in position when the dephlegmator is being constructed. A strip of 40-mesh brass gauze a three-eighths of an inch wide was carefully wrapped about each tube until the size of the collar was sufficient to fit its particular constriction snugly. The U was then dropped carefully into the large tube and gently forced into position with a glass rod. A snug but not tight fit is desired to prevent the tubes from blowing out of position while the apparatus is in operation and at the same time to permit of easy withdrawal for cleaning. To facilitate removal, a loop of copper wire is fastened to the top of the tube into which a hook can be inserted. In this way each section can be readily removed. It is a matter of some skill to set up the dephlegmator, but when the apparatus is once in position it is ready for a long series of determinations. Cleaning, in the case of turpentine distillations, can be done by ordinary solvents, such as alcohol and ether, without taking the apparatus apart.
Before use each time it is necessary to see that the bent tubes inside the dephlegmator are filled with water. This is done by pouring water into the top and allowing it slowly to run out.
The operation of the apparatus is simple. The boiler is about. half filled with water, and the separatory funnel G is entirely filled. The flask A, previously dried and weighed, is placed on a balance capable of weighing accurately to 10 mg., and exactly 500 grams of the sample of turpentine is poured in. The flask is then replaced in the position shown in the diagram. A sample of the original oil should be reserved for comparison with the several fractions of the distillate. The burner under the boiler is then lighted, and when the steam is freely escaping through the valve J, the valve is closed to allow the vapor to pass into A and the burner turned down to a point determined by experience. It is necessary to have the distillation carried on at a slow and uniform rate. Two drops of distillate per second is the rate that has been found to give the best results. In order to keep this rate constant all heat conditions must be carefully regulated. The flame under A should be small and low to prevent excessive con
"Platinum could, of course, be used to distill liquids that would attack brass.