9.1 × 0.154 × 100
Per cent. of geraniol=————————=70.2
2.3825 - (9.1 × 0.042)

The percentage of combined alcohols can be calculated from the amount of ester found, and by subtracting this from the percentage of total alcohols, that of the free alcohols is obtained.

In the example quoted, the ester corresponds to 17.6 per cent. geraniol, and this, deducted from the total alcohols, gives 52.6 per cent. free alcohols, calculated as geraniol.

This process gives accurate results with geraniol, borneol, and menthol, but with linalol and terpineol the figures obtained are only comparative, a considerable quantity of these alcohols being decomposed during the acetylation. The aldehyde citronellal is converted by acetic anhydride into isopulegol acetate, so that this is also included in the determination of graniol in citronella oil.

Phenols.—These bodies are soluble in alkalies, and may be estimated by measuring 5 c.c. or 10 c.c. of the oil into a Hirschsohn flask (a flask of about 100 c.c. capacity with a long narrow neck holding 10 c.c., graduated in tenths of a c.c.), adding 25 c.c. of a 5 per cent. aqueous caustic potash solution, and warming in the water-bath, then adding another 25 c.c., and after one hour in the water-bath filling the flask with the potash solution until the unabsorbed oil rises into the neck of the flask, the volume of this oil being read off when it has cooled down to the temperature of the laboratory. From the volume of oil dissolved the percentage of phenols is readily calculated.

Aldehydes.—In the estimation of these substances, use is made of their property of combining with sodium bisulphite to form compounds soluble in hot water. From 5-10 c.c. of the oil is measured into a Hirschsohn flask, about 30 c.c. of a hot saturated solution of sodium bisulphite added, and the flask immersed in a boiling water bath, and thoroughly shaken at frequent intervals. Further quantities of the bisulphite solution are gradually added, until, after about one hour, the unabsorbed oil rises into the neck of the flask, where, after cooling, its volume is read off, and the percentage of absorbed oil, or aldehydes, calculated.

In the case of lemon oil, where the proportion of aldehydes, though of great importance, is relatively very small, it is necessary to first concentrate the aldehydes before determining them. For this purpose, 100 c.c. of the oil is placed in a Ladenburg fractional distillation flask, and 90 c.c. distilled off under a pressure of not more than 40 mm., and the residue steam distilled. The oil so obtained is separated from the condensed water, measured, dried, and 5 c.c. assayed for aldehydes either by the process already described, or by the following process devised by Burgess (Analyst, 1904, 78):—

Five c.c. of the oil are placed in the Hirschsohn flask, about 20 c.c. of a saturated solution of neutral sodium sulphite added, together with a few drops of rosolic acid solution as indicator, and the flask placed in a boiling water-bath and continually agitated. The contents of the flask soon become red owing to the liberation of free alkali by the combination of the aldehyde with part of the sodium sulphite, and this coloration is just discharged by the addition of sufficient 10 per cent. acetic-acid solution. The flask is again placed in the water-bath, the shaking continued, and any further alkali liberated neutralised by more acetic acid, the process being continued in this way until no further red colour is produced. The flask is then filled with the sodium sulphite solution, the volume of the cooled unabsorbed oil read off, and the percentage of aldehydes calculated as before.

Solidifying Point, or Congealing Point.—This is of some importance in the examination of anise and fennel oils, and is also useful in the examination of otto of rose. A suitable apparatus may be made by obtaining three test tubes, of different sizes, which will fit one inside the other, and fixing them together in this way through corks. The innermost tube is then filled with the oil, and a sensitive thermometer, similar to that described under the Titre test for fats, suspended with its bulb completely immersed in the oil. With anise and fennel, the oil is cooled down with constant stirring until it just starts crystallising, when the stirring is interrupted, and the maximum temperature to which the mercury rises noted. This is the solidifying point.