5. SOLUBILITY
a. Solubility in Alcohol.
Since most essential oils are only slightly soluble
in water and are miscible with absolute alcohol, it is possible to determine the
number of volumes of dilute alcohol required for the complete solubility of one
volume of oil. The determination of such a solubility is a convenient and rapid
aid in the evaluation of quality of an oil. In general, oils rich in oxygenated
constituents31 are more readily soluble in dilute alcohol than oils rich in terpenes.
Adulteration with relatively insoluble material
will often greatly affect the solubility. Sometimes an actual separation of the
adulterant may be observed. For example, adulteration of citronella oils (which
are normally soluble in 80 per cent alcohol) with relatively large amounts of petroleum
fractions will result in a poor solubility for the oil in 80 per cent alcohol and
an actual separation of oily droplets of the adulterant. However, certain oils will
show a normal separation in dilute alcohol. Expressed orange oil, for example, will
separate natural waxes in 90 per cent alcohol. In alcohol of lower strength such
an oil will separate a terpene fraction in addition to the waxes. Use of this
fact sometimes is made in the preparation of terpeneless and sesquiterpeneless oils,
concentrates and extracts.
The solubility of an oil may change with age.
Polymerization is usually accompanied with a decrease in solubility; i.e., a stronger
alcohol may be required to yield a clear solution. Such polymerization may be
very rapid if the oil contains large amounts of easily resinified terpenes e.g.,
jumper berry oil, bay oil. Improper storage may hasten polymerization; factors such
as light, air, heat, and the presence of water, usually exert an unfavorable influence.
Occasionally the solubility of an oil improves upon aging e.g., oil of anise.32
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31 However, the oxygenated constituents belonging to the sesquiterpene
series are relatively insoluble; e.g., cedrol, santalol. Several other
exceptions are also encountered; e.g., safrole, anethole.
Alcohols of the following strengths are
customarily used in determining solubilities of essential oils :
50%-60%-70%-80%-90%-95% and occasionally 65% and
75%.
These are volume percentages at 15.56/15.56. In
preparing dilute alcohols it is convenient to weigh the alcohol (95 per cent by
volume) and the distilled water to give the proper volume percentage. Preparation
in this manner is independent of temperature. The strength of the alcohol
should be checked by determining the specific gravity at 15.56o/15.56.o
Final adjustments may be made if necessary.
Procedure:
Introduce exactly
1 cc. of the oil into a 10 cc. glass-stoppered cylinder (calibrated
to 0.1 cc.),
and add slowly, in small portions, alcohol of proper strength. Shake the cylinder thoroughly after each addition. When a clear solution is first obtained, record the strength
and the number of volumes of alcohol required. Continue the additions of alcohol until 10 cc. has been added. If opalescence or cloudiness occurs during these subsequent additions of alcohol, record the point at
which
this phenomenon
occurs. In the event that a clear solution is not obtained at any point during the addition of the alcohol, repeat the determination, using an alcohol of higher strength.
Since the solubility is dependent upon the
temperature, all determination should be made at 20O. It should be noted,
however, that "The Unite States Pharmacopoeia"33 and
"The National Formulary"34 specify an officia temperature of
25O for solubilities; "The British Pharmacopoeia,"36 a ternperature
of 15.5o. The proper temperature may be maintained by frequent immersion
of the cylinder in a water bath previously adjusted to the desired temperature.
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32 This is due to the presence of the difficultly soluble anethole, which
yields upon oxida tion the readily soluble anisic aldehyde.
33 Thirteenth Revision, 8.
34 Eighth Edition, 10.
35 (1932), 9.
If an oil is not clearly soluble in the dilute alcohols, it is advisable
to describe more fully the appearance of the solubility test.
The following terms, which are relative and entirely empirical, are used
in the laboratories of Fritzsche Brothers, Inc., to describe the appearance of the
solution :
Clearly soluble
Opalescent
Slightly hazy
Slightly turbid
Hazy Turbid
Slightly opalescent
Cloudy
A further term occasionally used is "fluorescent." In the case
of turbidity or cloudiness, record any separation of wax or oil that occurs, as
well as the period of time required for such separation.
If an oil is soluble in a number of volumes of alcohol which is not a multiple
of 1/2, report the solubility as being between the closest such limits.
For example, if 2.7 volumes of 70 per cent alcohol
were required to obtain a clear solution, and the solution remained clear upon further
additions of 70 per cent alcohol until a total of 10 volumes had been added, the
solubility would be recorded as :
"Clearly soluble in 2.5 to 3 volumes of 70
per cent alcohol and more, up to 10 volumes."
The behavior of the oil is best described by the
following typical notations:
1. Clearly soluble in volumes of per cent alcohol
and more, up to 10 volumes.
2. Clearly soluble in volumes of per cent alcohol;
opalescent with more, up to 10 volumes.
3. Clearly soluble in volumes of per cent alcohol;
opalescent to turbid with more, up to 10 volumes. No separation observed after 24
hr.
4. Clearly soluble in volumes of per cent alcohol
and more, up to volumes ; opalescent in volumes and more, up to 10 volumes.
5. Hazy in volumes of per cent alcohol ; cloudy
with more, up to 10 volumes. Oily separation observed after hr.
6. Clearly soluble up to 10 volumes of per
cent alcohol.
b. Solubility
in Nonalcoholic Media.
Several solubility tests have been introduced
for the rapid evaluation of oils. The following have proven valuable.
I. Carbon Disulfide Solubility for the Presence of Water36
Oils rich in oxygenated constituents
frequently contain dissolved water. This is particularly true in the case of oils
containing large amounts of phenolic bodies e.g., oil of bay. Such oils fail to
give a clear solution when diluted with an equal volume of carbon disulfide or chloroform.
This is the basis of a rapid test to ascertain whether or not an oil has been sufficiently
dried.
II. Potassium Hydroxide Solubility for Phenol
Containing Oils. Phenolic isolates and synthetics
as well as oils consisting almost exclusively of phenolic bodies may be
evaluated rapidly by dissolving 2 cc. of the oil in 20 or 25 cc. of a 1 N aqueous
solution of potassium hydroxide37 in a 25 cc. glass-stoppered, graduated cylinder.
This test is particularly of value in the case of sweet birch and wintergreen oils.
(See "Detection of Adulterants," p. 331.) It is well to examine critically
the odor of the solution or any insoluble portion, whereby additions of
foreign, odor-bearing substances may be detected.
Upon prolonged standing, the alkaline
solution may saponify an ester group, if present. If the products of such a saponification
are soluble in the alkaline solution, no separation will be observed e.g., methyl
salicylate. If the products are not completely soluble, a separation may occur e.g.,
amyl salicylate.
Solutions of the alkali phenolates are
frequently good solvents for other compounds; thus terpeneless bay oils containing
about 90 per cent eugonol often form clear solutions with a 1 N potassium
hydroxide solution. In this connection see the discussion under "Phenol
Determination," p. 293.
III. Sodium Bisulfite Solubility for Aldehyde
Containing Oils. Oils (such as oil of bitter almond,
free from prussic acid), and synthetics (such as benzaldehyde, tolyl aldehyde, cinnamic
aldehyde, and anisic aldehyde) and isolates (such as citral) may reveal impurities
by their incomplete solution in dilute bisulfite solution. This test is usually
carried out in a 25 cc. glavssstoppered, graduated cylinder: shake 1 cc. of the
oil with 9 cc. of a freshly prepared saturated solution of sodium bisulfite and
then add 10 cc. of water with further shaking. The odor of the resulting
solution should be carefully examined. Because of the relative insolubility of certain
bisulfite addition compounds, no general procedure is satisfactory for all aldehydes.
Thus, some must be heated in a beaker of boiling water ; and some require a larger
amount of water to yield a clear solution. Each chemist soon develops his own techniques in testing these
aldehydes; hence, specialized
procedures have been omitted here.
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36 "The National Formulary,"
Sixth Edition, 272.
37 The potassium Hydroxide
Test Solution of "The United States Pharmacopoeia" (13th Rev., 842) may
be used; this is prepared by dissolving 6.5 g. of potassium hydroxide, A.R., in
sufficient water to yield 100 cc. of solution. Since the potassium phenolates are
more soluble than the corresponding sodium compounds, the use of potassium hydroxide
is to be recommended.
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