Refractive Index.—This figure is occasionally useful, and is best determined with an Abbé refractometer, at 20° C.
Solubility in Alcohol.—This is found by running alcohol of the requisite strength from a burette into a measured volume of the oil with constant agitation, until the oil forms a clear solution with the alcohol. Having noted the quantity of alcohol added, it is well to run in a small further quantity of alcohol, and observe whether any opalescence or cloudiness appears.
Acid, ester, and saponification values are determined exactly as described under fats and oils. Instead of expressing the result as saponification value or number, the percentage of ester, calculated in the form of the most important ester present, may be obtained by multiplying the number of c.c. of N/1 alkali absorbed in the saponification by the molecular weight of the ester. Thus, to find the percentage as linalyl acetate, the number of c.c. absorbed would be multiplied by 0.196 and by 100, and divided by the weight of oil taken.
Alcohols.—For the estimation of these, if the oil contains much ester it must first be saponified with alcoholic potash, to liberate the combined alcohols, and after neutralising the excess of alkali with acid, the oil is washed into a separating funnel with water, separated, dried with anhydrous sodium sulphate, and is then ready for the alcohol determination.
If there is only a small quantity of ester present, this preliminary saponification is unnecessary.
The alcohols are estimated by conversion into their acetic esters, which are then saponified with standard alcoholic potash, thereby furnishing a measure of the amount of alcohol esterified.
Ten c.c. of the oil is placed in a flask with an equal volume of acetic anhydride, and 2 grammes of anhydrous sodium acetate, and gently boiled for an hour to an hour and a half. After cooling, water is added, and the contents of the flask heated on the water-bath for fifteen to thirty minutes, after which they are cooled, transferred to a separating funnel, and washed with a brine solution until the washings cease to give an acid reaction with litmus paper. The oil is now dried with anhydrous sodium sulphate, filtered, and 1-2 grammes weighed into a flask and saponified with alcoholic potash as in the determination of ester or saponification value.
The calculation is a little complicated, but an example may perhaps serve to make it clear.
A geranium oil containing 26.9 per cent. of ester, calculated as geranyl tiglate, was acetylated, after saponification, to liberate the combined geraniol, and 2.3825 grammes of the acetylated oil required 9.1 c.c. of N/1 alkali for its saponification.
Now every 196 grammes of geranyl acetate present in the acetylated oil correspond to 154 grammes of geraniol, so that for every 196 grammes of ester now present in the oil, 42 grammes have been added to its weight, and it is therefore necessary to make a deduction from the weight of oil taken for the final saponification to allow for this, and since each c.c. of N/1 alkali absorbed corresponds to 0.196 gramme of geranyl acetate, the amount to be deducted is found by multiplying the number of c.c. absorbed by 0.042 gramme, the formula for the estimation of total alcohols thus becoming in the example given:—