In testing for rosin as an adulterant in oils, the following pertinent properties of this substance should be borne in mind. It is a nonvolatile material, and consequently may be concentrated in the residue by distillation of the oil under vacuum or at atmospheric pressure; it is found also in the evaporation residue. Rosin consists primarily of complex acids and, therefore, will increase the acid number of an oil or of the evaporation residue if such residue normally consists of solid esters or paraffins; this is specifically of importance in the case of citrus oils. Rosin is soluble in most organic solvents, including petroleum ether, benzene, and xylene; since cinnamic aldehyde (the main constituent of cassia oil) is practically insoluble in petroleum ether, this permits a convenient separation of added rosin for this oil, and is the basis of "The United States Pharmacopoeia" test described below. Rosin gives a dark green copper salt when treated with cupric acetate ; this salt is sufficiently soluble in petroleum ether to impart to this solvent a green color. Rosin is a relatively high melting solid, normally a hard, noncrystalline material which fractures readily; hence the consistency of the evaporation residue is frequently altered if rosin is present.

a. Detection of Rosin in Balsams and Gums.

Procedure:193 Place in a small mortar 1 g. of the substance, powdered or crushed if necessary, and add 10 cc. of purified petroleum ether. Triturate well for 1 or 2 min. Filter into a test tube and add to the nitrate 10 cc. of a freshly prepared aqueous solution of cupric acetate (1 g. in 200 cc.). Shake well and allow the liquids to separate. The petroleum ether layer should not show a green color.

b. Detection of Rosin in Cassia Oils.

Procedure I:184 Shake about 2 cc. of the oil in a test tube with 10 cc. of petroleum ether. Permit the liquids to separate and decant the benzene layer into a second test tube. Add an equal volume of cupric acetate solution (1 in 1000); a green color indicates the presence of rosin in the oil.
It is well to carry out simultaneously a test with an oil known to be free of rosin, to act as a blank. Unfortunately, tests based upon color reactions have not proved too reliable in mixtures as complex as essential oils; nevertheless, this test will give an indication of the presence or absence of rosin.
Procedure II:185 About 50 g. of the oil, accurately weighed, are distilled from a tared distilling flask over an open flame. Continue the distillation until decomposition is evidenced by the formation of white fumes within the flask; this usuallyoccurs at a temperature of about 280o. Cool the flask and weigh; calculate the percentage of residue.
This test will reveal adulteration with nonvolatile material such as rosin, if large amounts have been added. Normal oils show a distillation residue of 6 to 8 per cent, or at most 10 per cent, according to Gildemeister and Hoffmann.186 Furthermore, the residue should be tacky, but not hard and brittle. According to Allen,187 formerly of Hongkong, the residue should not be higher than 5 per cent for a pure oil. Treff188 has pointed out that distillation should be carried out rapidly, since the amount of residue obtained is greatly dependent upon the rate of distillation.
Procedure III: Determine the acid number of the oil in the usual manner. If the oil is pure and has been properly stored, the acid number should not be greater than 15.

c. Detection of Rosin in Orange Oils.

Procedure: Determine the evaporation residue in the usual manner. In the case of pure oils this residue upon cooling should be soft and waxy, not hard, brittle or tacky. The acid number of the residue should lie between 11 and 28, the ester number between 118 and 157.189
182 Schimmel & Co., April (1904), 29; April (1910), 32; April (1911), 47.
183 United States Pharmacopoeia," Thirteenth Revision, 688.
184 "The United States Pharmacopoeia," Thirteenth Revision, 132.
185 Ber. Schimmel & Co., October (1889), 15. Gildemeister and Hoffmann, "Die atherischen Ole," 3d Ed., Vol. II, 631.
186 "Die atherischen Ole," 3d Ed., Vol. II, 631.
187 D. Allen, private communication.
188 Z. angew. Chem. 39 (1926), 1308.
189 Gildemeister and Hoffmann, "Die atherischen Ole” 3d Ed., Vol. Ill, 79. These data apply to Italian orange oils. However, the values for oils from other origins do not appear to differ materially from these limits.

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