Examination. The quality of citronella oil is best ascertained by the quantitative determination of the acetylizable constituents, viz. geraniol, nerol, citronellol, borneol and citronellal which, for this purpose, are designated by the collective name of total geraniol. The value of the oil increases with the content of these constituents. The designation "total geraniol" as applied to citronella oil is naturally not quite correct. Not only do other alcohols enter into the reaction, but the citronellal is thereby converted quantitatively into isopulegol acetate. Moreover, the original esters of the oil are included in the determination. Nevertheless this expression has become quite common and, for the sake of simplicity, may as well be retained in actual practice. The method of determination is given in detail in the first volume of this work (p. 578). Inasmuch, however, as comparable results of the so-called total geraniol depend so much on a rigid observation of details, the directions are herewith repeated.
A mixture of 10 cc. each of citronella oil and acetic acid anhydride, to which 2 g of fused sodium acetate and a few boiling chips have been added, are heated in an acetylation flask on a sandbath so as to maintain uniform boiling for 2 hours. Upon cooling some water is added to the contents of the flask and the latter heated in a water bath for 1/4 hour with occasional shaking so as to hydrate the excess of acetic acid anhydride. The oil is then separated by means of a separating funnel and washed with water, or better still with a solution of sodium chloride until the reaction is neutral. The acetylated oil is dried with anhydrous sodium sulphate, 1,5 to 2,0 g. of this dried oil are saponified with 20 cc. of half-normal potassium hydroxide, any traces of free acid having previously been carefully neutralized. The saponification should last at least one hour, the acetylation, however, 2 hours, for otherwise results are obtained that are from 1 to 5 p. c. too low or even more1). For similar reasons a longer period of acetylation should be avoided. This method has been tested by Schimmel & Co. in connection with mixtures of known geraniol and citronellal content.
1) Comp. Report of Schimmel & Co. October 1912, 47.
For the separate determination of the primary alcohols referred to here as geraniol Schimmel & Co.2) have worked out a method that is based on the different behavior of geraniol and citronellal toward phthalic acid anhydride. Whereas the former readily reacts with phthalic acid anhydride forming acid esters, citronellal does not undergo a change. The method is carried out in the following manner: In an acetylizing flask about 2 g. of phthalic acid anhydride, 2 g. of the oil to be examined and 2 cc. of benzene are heated for 2 hours in a water bath. The reaction mixture, having been allowed to cool, is shaken with 60 cc. of aqueous half-normal potassium hydroxide solution for 10 minutes. For this purpose the flask is closed with a glass stopper. At the end of 10 minutes all of the acid anhydride is converted into neutral potassium phthalate and the acid geraniol ester into its potassium salt. The excess of alkali is now titrated back with half-normal sulphuric acid. If from the amount of alkali equivalent to the amount of phthalic acid anhydride used, the amount of alkali actually used is deducted, the difference is the equivalent of the geraniol that has combined with phthalic acid. From this the percentage of geraniol can be computed. When applied by Schimmel & Co. to several Ceylon oils, this method yielded between 29,6 and 34,4 p. c. geraniol.
Inasmuch as citronella oil contains several alcohols in addition to geraniol, partly free partly as ester, which react when the oil is acetylated but not when it is treated with phthalic acid anhydride, it becomes apparent that the citronellal content is not equal to the difference between the geraniol content and the so-called total geraniol content, but must be determined by a special method. For this purpose several methods have been proposed. Of these the method of j. Dupont and L. Labaune1) should first be mentioned. It depends on the conversion of the citronellal into its oxime, the subsequent acetylation of the oil and the determination of the ester value of the acetylated oil. When the oximated oil is heated with acetic acid anhydride, the citronelloxime is converted into citronellonitrile. This is stable toward alkali even when heated, hence upon saponification the amount of ester only is determined. The method is as follows: 10 g of citronella oil are first shaken with a hydroxylamine solution2) for 2 hours at ordinary temperature (15 to 18°). The oximated oil is dried and then acetylated. The ester number of the acetylated oil reveals the content of alcohol C10H18O. By subtracting this from the so-called total geraniol content the amount of citronellal is ascertained. Inasmuch as the molecular weights of citronellal (154) and citronella nitrile (151) are in close approximation, the change which the citronellal has undergone can be ignored in the computation.
1) Report of Schimmel & Co. April 1910, 154. Comp. also A.W. K. de Jong, Andropogon Nardus Java. Teysmannia 1908, 1.
2) Report of Schimmel & Co. October 1899, 24. Comp. also ibidem October 1912, 43.
Applying this "method of oximating" to Ceylon citronella oil, Dupont and Labaune found 17,2 p. c. of citronellal, Schimmel & Co.3) found 6,5 and 6,7 p. c. in two oils.
For the citronellal assay, C. Kleber4) suggests a method originally applied to the citral assay in lemon oil, according to which the aldehyde is allowed to react with phenylhydrazine. He suggests the following details: 5 g. of Ceylon oil or 2 g. of Java oil are weighed accurately in a flask, 20 cc. of a freshly prepared 5 p. c. alcoholic solution of phenylhydrazine are added, the flask stoppered and set aside for about 1/2 hour at 35°. Then as much half-normal hydrogen chloride is added as is necessary for the neutralization of the phenylhydrazine solution used. This is previously ascertained with a blank. The mixture is transferred to a separating funnel, the flask rinsed with 20 cc. of water and the entire mixture shaken thoroughly. As soon as the two layers have separated, the lower layer is drawn off into an Erlenmeyer flask, the residue washed with 5 cc. of water and the total contents of the Erlenmeyer flask are titrated with half-normal sodium hydroxide. Ethyl orange is used as indicator, titration being stopped when the brownish color, which precedes the rose color, has been reached. Since each cc. of half-normal sodium hydroxide corresponds to 0,077 of citronellal, the citro-nellal content of the oil can be readily computed from the consumption of the soda solution.