8. BOILING RANGE
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
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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.
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.
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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