10. FLASH POINT
The flash point may prove useful in the
evaluation of an essential oil. Unfortunately insufficient data exist to use this
property as a criterion of quality for normal, unadulterated oils. However, the
flash point has value as an indication of adulteration: additions of adulterants
such as alcohol and low boiling mineral spirits will greatly lower the flash point.
Occasionally it is necessary to determine the
flash point of a synthetic, solvent, or a mixture because of shipping regulations.
Several types of instruments are available
for the determination; e.g., the Pensky-Martin closed tester,50 the Tag
closed tester,61 the Cleaveland52 and the Tag open cup testers.
The Tag open cup tester is simple, inexpensive, and entirely satisfactory for use
in the essential oil industry. The procedure described below is intended primarily
for this instrument (see Diagram 4.6).
Procedure:
Fill the metal bath with water of about 60o F. (15.6oC.) temperature,54
leaving room for displacement by the glass oil cup which is placed in the water
bath. Suspend the thermometer in a vertical position so that the bottom of the bulb
is about 1/4 in. from the bottom of the glass cup and so that the back of the glass
cup. Fill the glass cup with the oil to be tested in such a manner that the top
of the meniscus is exactly at the filling line at room temperature (i.e., 1/4 in.
from the upper edge of the cup). Be sure that there is no oil on the outside of
the cup or on its upper level edge ; use soft paper to clean the cup in preference
to a cloth. Remove any air bubbles from the
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50 A description of the instrument and a detailed procedure for its use
may be found in A.S.T.M. Designation D93-42.
51 A.S.T.M. Designation D5(>-36.
52 A.S.T.M. Designation D02-33.
53 This procedure is based on the directions for using the Tag Open Fire
Tester supplied by the Tagliabue Manufacturing Co., Brooklyn, N. Y.
54 It is customary in the United States to report flash points in degrees
Fahrenheit. the thermometer is suspended
half way between the center and
surface
of the oil. Adjust the horizontal flashing taper guide wire in place. The instrument
should stand level and should be protected from drafts. It is desirable that
the room be darkened sufficiently so that the flash may be readily discernible.
Avoid
breathing over the surface of the oil. Heat the water bath with a small burner so
that it will raise the temperature of the oil at a rate not faster than 2o
F. (1.1o C.) per min. without removing the burner during the whole operation.
Adjust the test flame on the flashing taper so that it is the same size as the metal
bead mounted on the instrument. Apply this test flame to the oil at 5o
F. (2.8o C.) intervals: hold the flashing taper in a horizontal position
and draw it across the guide wire quickly and without pause from left to right.
(The time of passage of the test flame across the cup should be approximately 1
sec.) The first or initial flash55 is called the "flash point."
Continue heating and testing the oil until the surface ignites and continues to
burn until quickly blown out with a mouth-open breath. This burning point temperature
is called the "fire test" or "fire point." Repeat the determination
and try for a flash at the proper trial temperatures indicated in Table 4.8.
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58 The true initial flash should not be confused with a bluish halo that sometimes surround the test flame
TABLE 4.8. TRIAL TEMPERATURE
TABLE FOR FLASH POINTS
(All Temperatures in o
F.)
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