12. DETERMINATION OF STEAROPTENE CONTENT OF ROSE OILS
Oil of rose contains as a natural constituent
a mixture of solid paraffinic hydrocarbons known collectively as "stearoptene."
The highly purified stearoptene is odorless and hence contributes little to the
odor value of the oil. However, for many years the quality of rose oils was judged
superficially by the "melting point" of the oil; oils with high
"melting points" were assumed to be unadulterated. As a consequence there
arose the practice of adding spermaceti, tristearin, high melting paraffins, and
guaiac wood oil as adulterants.
"The United States Pharmacopoeia"163
requires a certain minimum content of stearoptene and describes a limiting test
for its determination.
Procedure I: Introduce 1 cc. of oil of rose
into a 25 cc. glassstoppered, graduated cylinder and add 1 cc. of chloroform: a
clear solution should result. Then add 19 cc. of 90% alcohol (by volume): crystals
of stearoptene should crystallize out of the solution within 24 hr., the temperature
being maintained at 25o.
A rough indication of the amount of stearoptene
present in the oil can be obtained by this modified official test. Oils with high
stearoptene contents will deposit an abundant amount of crystalline material immediately
; oils with low stearoptene contents will sometimes separate only one or two well-formed
crystals after standing 24 hr. ; some oils will show no separation of crystals whatsoever.
The appearance of the crystals is also important; only through experience will an
essential oil chemist be able to draw conclusions as to possible adulteration from
the appearance of the separated material.
This test will also indicate whether or not the
oil has been properly dried; a cloudy solution in one volume of chloroform is usually
indicative of the presence of water in the oil.
For the determination of the amount of
stearoptene, the oil is usually dissolved in dilute alcohol and chilled; the relatively
insoluble paraffins separate out and can be filtered off and weighed. It is customary
to use 75 per cent alcohol164 for this determination, although certain
investigators have recommended the use of 85 per cent alcohol166 or acetone.166
Procedure
II: Dissolve 5 g. of the oil
in 50 cc. of 75% alcohol (by volume) with the aid of gentle heat if necessary. Cool
the solution in an ice bath at for 2 hr. and filter off the separated stearoptene
with suction, using a well cooled Blichner funnel. Wash the stearoptene with a 50
cc. portion of 75% alcohol cooled to 5o. Remove as much of the alcohol
as possible by suction, and then transfer the cake of stearoptene to a tared evaporating
dish. Break up the cake with a spatula and dry in a desiccator for 24 hr. Weigh,
and calculate the percentage of stearoptene present in the original oil.
To be assured of the absence of adulterants, it
is necessary to examine the separated stearoptene.
The naturally occurring paraffinic hydrocarbons
in rose stearoptene consist of at least two components167 having
melting points of 22o and 41o.168 The mixture separated
from rose oil should melt between 32o and 37o; usually at
about 33-34o.l69 Additions of spermaceti, guaiac wood oil,
and many readily available solid paraffins will raise the melting point.
Spermaceti, tristearin, or other fatty acid
esters may be detected by an abnormally high ester number of the separated
stearoptene. Occasionally it is possible to isolate the fatty acids from the saponified
material.
Guaiac wood oil consists mainly of the alcohol,
guaiol; its presence will be revealed by a high ester number after acetylation of
the separated stearoptene.
High melting paraffins are very difficult to detect
when used as adulterants for rose oils. The appearance of the stearoptene may reveal
their presence; a peculiar granular structure is frequently indicative of such additions.
The appearance of the crystals which separate in the test described under
Procedure I is sometimes helpful in this connection.
The congealing point of the rose oil itself is
also indicative of the amount of stearoptene present in the oil. The congealing
point of rose oil 170 has been defined as that temperature at which the
first crystals appear when the oil is subjected to slow cooling. (This is quite
different from the true congealing point of oils such as anise. See
"Congealing Point” p. 253.) Determine the "congealing point" of the
oil by the following technique:
Procedure III: Place 10 cc. of the oil in a test
tube having a diameter of 15 mm.; suspend a thermometer in the oil in such a way
that it touches neither the sides nor the bottom; warm the contents of the tube
to about 5 above the point of saturation; stir well; then permit the oil to
cool slowly until the first crystals appear; read the temperature. Repeat the
determination.
As a general rule, good Bulgarian oils produced
by the usual methods171 show a congealing point of 18o to
23o.l72
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162 Dry the sulfur dioxide by
bubbling through concentrated sulfuric acid. Commercial sulfur dioxide of refrigeration
grade is sufficiently pure for the preparation of the reagent.
163 Thirteenth Revision,
456.
164 Ber. Schimmel & Co.,
April (1889), 37.
165 Burgess, "Chemistry
of Essential Oils and Artificial Perfumes" (Parry), D. Van Nostrand Co., Inc.,
New York, 1921, 402.
166 Jeancard and Satie, Bull.
soc. chim. [3] 31, (1904), 934.
167 FlUckiger, "Pharmakognosie,"
3d Ed., 170.
168 Gildemeister and
Hoffmann, "Die atherischen Ole," 3d Ed., Vol. I, 302.
169 Parry, "Chemistry of
Essential Oils and Artificial Perfumes," D. Van Nostrand Co., New York (1921),
402.
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