In the case of isolates and synthetics, the determination of the boiling range is an important criterion of purity.
Procedure: Use the apparatus shown in Diagram 4.4. The bulb of the distilling flask should have a capacity of 50 cc. The neck of the flask above the side arm should be as short as possible. The bottom of the flask rests in a circular opening, 2.5 cm. in diameter, cut in a square piece of asbestos board having a thickness of about 3 mm.; this perforation should be slightly beveled on the upper edge to make it fit closely to the surface of the flask.47 A wrapping of asbestos paper reaching to a point about 1 cm. above the side arm should be used to prevent condensation
due to drafts.
Introduce 25 cc. of the sample into the flask by means of a pipette. Add a small clay chip. Insert the thermometer along the central axis of the flask with its bulb slightly below the side tube; attach a light auxiliary thermometer to the main thermometer to correct for stem exposure, the bulb of this second thermometer being placed half way from the cork to the top of the mercury column at the expected reading. (A shortstemmed thermometer of the Anschiitz type having the proper

47 This is to prevent upward leakage of hot gases from the flame and subsequen superheating.

range may be used ; this will require no correction for stem exposure.) Distill at a uniform rate of about 0.5 cc. per min. until the level of the liquid remaining in the flask falls to the level of the asbestos diaphragm. 
Apparatus for the determination of boiling range

 DIAGRAM 4.4. Apparatus for the determination of boiling range.
Since some time will elapse before the thermometer can acquire the temperature of the vapor, little significance can be attached to readings taken before the end of the first minute after the fall of the first drop of distillate from the side tube. Any readings taken after the liquid falls below the level of the asbestos board will be greatly influenced by superheating. In the case of pure compounds that boil without decomposition, the difference between the first and last significant readings should not amount to more than 1o.
The stem exposure correction may be found by the following formula:
Correction = 0.000154 N(t1 – t2)
where : N = number of degrees of emergent stem ;
            t1 observed temperature of main thermometer;
            t2 = temperature of auxiliary thermometer.
This correction is to be added.
To reduce boiling points taken under pressures between 720 mm. and 780 mm. to their approximate values at 760 mm., apply a correction of 0.1o for every 2.7 mm. difference ; the correction is to be added if the observed pressure is below 760 mm. and to be subtracted if above 760 mm.
The percentage of an essential oil which distills below a given temperature is frequently of importance in evaluating the oil ; also, the percentage which distills between certain limits. However, it must be remembered that when fractionating an oil, the quantitative results of different observers will vary greatly; this is due to differences in the types of distilling flasks and condensers employed, to the distillation rates, and to the degree of superheating of the vapors.
Examination of the various fractions is of great importance; the determination of physical and chemical properties of these fractions and a study of the odor is frequently very revealing. Furthermore, suspected adulterants may be tested for chemically, and if present identified by derivatives. 
Apparatus for the determination of boiling rango

 DIAGRAM 4.5. Apparatus for the determination of boiling rango.
Only through experience will the chemist know whether or not it is better to distill at atmospheric pressure or under vacuum. In general, for the collection of first fractions it is better to distill at atmospheric pressure. Usually it is more advantageous to separate fractions according to the temperature, measuring the volumes collected; occasionally it is desirable to collect definite amounts, noting the temperatures at which these fractions are obtained.
For fractionations at normal pressure the following technique will generally give satisfactory results. The procedure as described is intended primarily for the distillation of turpentine oil and for the removal and collection of the first. 10 precent of citrus oils.

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