SOLUBILITY-DETERMINATION OF PHYSICAL PROPERTIES

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.
TABLE 4.7. PREPARATION OF DILUTE ALCOHOLS 
PREPARATION OF DILUTE ALCOHOLS

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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