After cooling, the flask with the residue is weighed. In the case of good oils, the latter is thick, of tough consistence and amounts to from 6 to 8, or at most 10 p. c. of the original oil. The residue of oils adulterated with colophony is hard, vitreous and brittle and is correspondingly larger.
Instead of weighing the oil, 50 cc. can be measured in a pipette and the distillate can be collected in a graduated cylinder. The amount of residue can be ascertained with sufficient accuracy by deducting the number of cc. of distillate from 50.
The presence of petroleum can be shown in the distillate. In the case of pure oils this forms a clear solution with 70 and 80 p.c. alcohol. If petroleum be present, solution is not complete2). The liquid is milky at first but becomes clear on standing. The petroleum floating on the surface can be decanted and recognized by its behavior toward sulphuric and nitric acids. (Comp. Vol. I, p. 614).
According to H. Gilbert3) the test for colophony can be carried out without much loss of substance by evaporating a few grams of oil on a watchglass in a drying oven at a temperature of from 110 to 120° until of constant weight and computing the amount of residue.
1) Schimmel's Bericht October 1889, 15.
2) As previously mentioned, cassia oil which contains petroleum as well as colophony, is soluble in 80 p.c. alcohol, whereas the petroleum added to pure cassia oil is separated in the form of oily drops by 80 p.c. alcohol.
3) Chem. Ztg. 13 (1889), 1406.
Also, according to Gilbert, the determination of the acid value yields results that can be utilized for the detection of colophony. An oil with an evaporation-residue of 6 p. c. yielded an acid value of 13. After 20 p. c. of colophony (A. V. 150) had been dissolved in the oil the acid value had risen to 40. A cassia oil with 28 p. c. residue had an acid value of 47.
Inasmuch as an alcoholic solution of lead acetate yields a precipitate with solutions of colophony, Hirschsohn1) has proposed this reagent for the detection of rosin in cassia oil. For this purpose a solution of 1 vol. of cassia oil in 3 vol. of 70 p.c. alcohol is prepared. To it a freshly prepared solution of lead acetate in 70 p.c. alcohol, saturated at room temperature, is added drop by drop. If a precipitate results, the oil has been adulterated with colophony. In this manner as little as 5 p.c. of colophony can be detected.
Aldehyde Determination. Inasmuch as the value of cassia oil depends on its cinnamic aldehyde content, the quantitative determination of this substance is of the greatest importance. The directions for carrying out the bisulphite method, as suggested by Schimmel & Co.2) and as now generally practised in commercial transactions3), are given in vol. I, p. 582.
Stricktly speaking this method yields percentages by volume, not weight percentages. Inasmuch, however, as the specific gravities of the aldehyde and the non-aldehyde constituents of the oil are almost the same, the percentage by weight and volume are practically the same.
The chemical changes that come into consideration in the aldehyde test can be expressed by the following equations4):
1) Pharm. Zeitschr. f. Russl. 29 (1890), 255. 2) Report of Schimmel & Co. October 1890, 15.
3) In an account of his trip to the cassia district, O. Struckmeyer states that he found the implements recommended for the aldehyde determination by Schimmel & Co. in the interior of China, viz., on a cassia plantation in the neighborhood of Lotingchow. Report of Schimmel & Co. October 1890, 20.
4) F. Heusler, Berl. Berichte 24 (1891), 1805; Schimmel's Bericht April 1890, 13.
First, a non-soluble addition product of 1 mol. cinnamic aldehyde and 1 mol. sodium bisulphite is formed.
C8H6CH:Ch Cho + NaHs03 = C6H8CH:CHCHO.NaHSO3
Cinnamic aldehyde Sodium bisulphite Cinnamic aldehyde sodium sulphite.
This addition product decomposes on boiling with water, and from two molecules there is formed one molecule each of cinnamic aldehyde and of sulphocinnamic aldehyde sodium sulphite.
2 C6 H5•CH : CH•Cho•Na Hso3
= C6H5CH:CH.Cho + C6H5Ch2.CH(SO3Na)CHONaHSO3
Cinnamic aldehyde Sulphocinnamic aldehyde sulphite of sodium.
Hence, in order to convert the entire aldehyde into the water-soluble compound an excess of sodium acid sulphite (2 mol.) must be used.
The salts of the sulphocinnamic aldehyde sulphonic acid are readily soluble in water and are stable when boiled. They are decomposed only when subjected to destructive distillation or when heated with caustic soda solution or sulphuric acid.
Suitable for the determination of the cinnamic aldehyde is also the sulphite method described in Vol. I, p. 584.
The cinnamic aldehyde content of good cassia oils amounts to at least 80 p.c, but is rarely greater than 90 p. c. However, oils have been obtained from China which contained no more than 35 to 50 p. c. aldehyde and yet no adulteration could be detected. According to statements made by the Chinese, these oils had been obtained from young leaves, a claim the correctness of which, naturally, cannot be verified.
Of such oils as contain less than 40 p.c. of cinnamic aldehyde only 5 cc. should be taken for an assay, otherwise a flask should be used the neck of which has a capacity of more than 10 cc. and the calibration of which ranges from 0 to 10.
Oils obtained from the several parts of the cassia shrub.
All parts of the cassia shrub yield oils with a high, aldehyde content1). The differences in specific gravity, though but small, indicate that the non-aldehyde constituents vary somewhat. For the production of cassia oil, as already indicated, the leaves and branches alone come into consideration, since the bark and the so-called flowers are too expensive, whereas of flower stems no sufficient quantities can be collected. It is not impossible, however, that occasionally all these parts are utilized.