This method, which determines only free phenols, is appliciable only for such oils that are free from alcohols, aldehydes and ketones, to those oils that are mixtures of phenols and terpenes.1) On account of the great capacity of the amide to react with water, the oils must be, in addition, absolutely free from moisture.

The method of Verley and Bcelsing2) is based on the observation that the phenols react energetically in the cold with acetic acid anhydride provided pyridine be present. The acetic acid resulting combines directly with pyridine. In as much as pyridine is neutral towards phenolphthalein, the acetic acid not combined with phenol can be determined titrimetrically. For the assay about 3 g. of oil are heated for 15 minutes with 25 cc. of a mixture of 120 g. acetic acid anhydride and 880 g. pyridine in a 200 cc. flask without condenser on a water bath. Upon cooling an equal volume of water is added to convert any unchanged acetic acid anhydride into acetic acid, /. e., into pyridine acetate. The acetic acid not combined with phenol is then titrated back, using phenolphthalein as indicator. In like manner the acetic acid content of 25 cc. of the mixture is established. Subtraction reveals the amount of acetic acid combined with the phenol, whence the phenol content of the oil can be computed.

1) Report of Schimmel & Co. October 1904, 129. 2) Berl. Berichte 34 (1901), 3354.

Phenol present as ester, e. g., aceteugenol in oil of cloves, is not determined by this method. However, it should be noted that the oils to be examined should contain no alcohols since these react like the phenols.

According to the method proposed by Hesse,1) phenols and more particularly phenol carboxylic esters, such as salicylic acid, can be determined. The oil to be examined is dissolved as such, or after previous dilution with an indifferent solvent (benzyl alcohol) in three parts of anhydrous ether. To the cooled solution half-normal alcoholic potassium hydroxide is carefully added, when the potassium compound of the phenol or phenol-carboxylic ester separates in crystalline form. The crystals are filtered off, washed with anhydrous ether, and decomposed with carbonic acid. The amount of phenol or ester set free can be determined by weight. If, in the case of small amounts, the alkali formed be determined titrimetrically, a considerable excess of potassium hydroxide should be avoided, otherwise it may occur that solid alkali as well as potassium phenylate may separate out.

In addition to these general methods several special methods are known which may here be described.

For the determination of thymol and carvacrol in volatile oils Kremers and Schreiner2) have worked out a method. It is a modification of the method recommended by Messinger and Vortmann'5) and depends upon the property of these phenols to combine, in alkaline solution, with iodine to a red iodine thymol compound which is precipitated. The excess of iodine, after acidulation of the liquid, can be titrated back with sodium thiosulphate solution. Every molecule of thymol, respectively carvacrol, requires 4 atoms of iodine for precipitation.

1) Chem. Zeitschr. 2 (1903), 434.

2) Pharm. Review 14 (1896), 221.

3) Berl. Berichte 23 (1890), 2753.

The process for thymol, which differs slightly from that for carvacrol, is as follows:

5 cc. of the oil to be assayed are weighed, transferred to a glass-stoppered burette that is calibrated into 1/10 cc, and diluted with about an equal volume of petroleum ether. Five p. c. soda solution is added, the mixture well shaken and then allowed to separate. Complete separation having resulted, the alkaline solution is allowed to flow into a 100 cc. measuring flask and the shaking out with alkali solution repeated as often as a diminution of volume results.

The alkaline thymol solution is diluted with 5 p. c. sodium hydroxide solution to 100 cc. or, if necessary, to 200 cc.

To 10 cc. of this solution, transferred to a 500 cc. measuring flask, a slight excess of N/10 iodine solution is added causing the thymol to be precipitated as a dark brown iodine compound. In order to test whether sufficient iodine has been added, a few drops are transferred to a test tube and several drops of hydrochloric acid added. If a sufficient amount of iodine has been added, the solution is colored brown by free iodine, otherwise it becomes milky due to precipitated thymol. An excess of iodine having been added, the solution is acidified with dilute hydrochloric acid and diluted to 500 cc. In 100 cc. of the filtrate the excess of

N iodine is titrated by means of N/10 sodium thiosulphate solution.

The number of cc. of thiosulphate solution are deducted from the volume of iodine used and the resultant multiplied by five, this product indicating the total amount of iodine used by the thymol.

Each cc. of N/10 iodine solution consumed corresponds to

0,0037528 g. thymol.

From the amount of thymol found in the alkaline solution the percentage of phenol in the original oil can readily be computed.

The reaction can be expressed by means of the following equation:

C10H140 + 41+ 2NaOH = C10H12I20 + 2NaI + 2H20.

In the assay of carvacrol a slight modification becomes necessary because the carvacrol iodide separates milky. In order to cause the formation of a precipitate the mixture is shaken thoroughly after the iodine has been added and then filtered. It is only then that the solution is acidified with hydrochloric acid. The process after that is the same as for thymol. The computation is likewise the same.

A method for the assay of eugenol in caraway oil has been suggested by Thorns.1) The method is based on the separation of eugenol as benzoyl eugenol. It enjoys the advantage that a melting point determination reveals identity as well as purity of the separated phenol. Somewhat later Thorns2) modified this method by removing the sesquiterpenes, which interfere at times, before adding the benzoyl chloride. He enlarged upon it by taking into consideration the eugenol acetate occurring in the oil.

Assay of total eugenol. 5 g. of oil of cloves, contained in a 150 cc. beaker, are saponified with 20 g. of 15 p. c. caustic soda solution by heating on a water bath for half an hour. When still warm the contents of the beaker are transferred to a separating funnel with a short tube. After complete separation of the two layers, the solution of eugenol sodium is transferred back to the beaker. The sesquiterpenes remaining in the separating funnel are washed twice, each time with 5 cc. of 15 p. c. caustic soda solution and the lye in each case added to the eugenol sodium solution. 6 g. of benzoyl chloride are now added and the mixture shaken thoroughly until a uniform mixture has been obtained. In a few minutes the ester formation has taken place with appreciable rise of temperature. Any excess of benzoyl chloride is destroyed by brief heating on the water bath. After cooling 50 cc. of water are added, the mixture heated until the crystalline ester has been liquified, and again allowed to cool. The supernatant, clear liquid is removed by filtration, allowing the crystalline cake to remain in the beaker. Again 50 cc. of water are added, heated on a water bath until the ester has melted, cooled and filtered. Finally, the ester is washed a third time in like manner with 50 cc. of water. The excess of caustic soda and sodium salts are then regarded as having been removed.

1) Berichte d. deutsch. pharm Ges. 1 (1891), 278. 2) Arch, der Pharm. 241 (1903), 592.

Any crystalline particles that have been cought in the filter are retransferred to the beaker. The benzoyl eugenol, while still moist, is dissolved in 25 cc. of alcohol (90 p. c. by weight) with the aid of the heat of the water bath and gentle agitation. Even after the beaker has been removed from the water bath, the shaking is continued until the benzoyl eugenol has crystallized out. This takes place within several minutes. The temperature of the mixture is lowered to 17°, the precipitate collected on a filter 9 cm. in diameter, and the filtrate collected in a graduated cylinder. About 20 cc. of filtrate are thus obtained. The alcoholic solution that has been retained by the crystalline magma in the filter is displaced by 90 p. c. (by weight) alcohol until the filtrate amounts to 25 ccm. The moist filter and precipitate are then transferred to a weighing dish (the latter together with the filter dried at 101° having been weighed previously), and dried at 101° until of constant weight. In as much as 25 cc. of 90 p. c. alcohol dissolve 0,55 g. of pure benzoyl eugenol at 17°, this amount should be added to the amount weighed.

If a stands for the amount of benzoic ester found, b for the amount of clove oil employed (about 5 g.), and if 25 cc. of alcoholic solution of ester are removed by filtration in accordance with the above directions, then the percentage of eugenol in clove oil is indicated by the formula

Phenol Determination 231

This formula results from the two equations:

(Benzoyl eugenol) (Eugenol)

268 : 164 = (a + 0,55): the amount of eugenol found.

Phenol Determination 232

Assay of free eugenol. 5 g. of clove oil dissolved in 20 g. ether are quickly shaken out in a separating funnel with 20 g. of 15 p. c. caustic soda solution. The eugenol sodium solution is transferred to a beaker and the ethereal solution of sesquiterpenes is washed twice with 5 g. each of caustic soda solution of like strength. The united alkaline solutions are heated on a water bath for the purpose of driving off the ether and the benzoylation carried out in the manner described above.

Thus the free eugenol as well as that present as ester can be determined quantitatively. Thorn's method can be applied equally well to other eugenol-containing oils provided they contain no free alcohols.