The optical rotation is such a characteristic property of most of the volatile oils that its determination should never be omitted in an examination. Especially adapted for this purpose is the half-shadow polariscope according to Lippich2) with polarisor in two parts. The best division of the circle for the purpose in question is that into degrees and minutes in such a way that the numbers on each side of the zero point run up to 180°. If the dark color of the oil does not allow of making the observation in a 100 mm. tube, which is the one usually employed, one of 50 or even 20 mm. may be used and the values obtained are multiplied by 2 and 5 respectively. If the color permits, inactive substances are observed in 200 mm. tubes in order that slight deviations may be detected. When no special mention is made of the temperature, room temperature is to be understood. In general it is not necessary, although desirable, to make the observation at a fixed temperature, as the natural variations in the rotation of an oil are usually greater than the differences due to a variation in temperature of several degrees. Exceptions to this are the oils of lemon and orange, the rotation of which is relatively strongly influenced by even small changes in temperature. It is necessary, in order to get comparable figures, to determine the rotation of these two oils at +20° or else to reduce the result to this temperature by calculation. The details of this will be found in the description of these oils in the special part.
1) Pharmaceutical Archives 4 (1901), 165. Comp. also Report of Schimmel & Co. April 1906, 71.
2) With regard to the manipulation of the polariscope the reader is referred to the wellknown work by H. Landolt, Das optische Drehungsvermdgen organischer Substanzen, II. ed., Braunschweig, 1898.
In the following, uD is the observed angle of rotation in a 100 mm. tube with sodium light, and [a]D is the specific rotation as calculated by the formula
[a]D=a/1.d where a is the observed angle of rotation, / the length of the tube in decimeters and d the specific gravity of the liquid.
Solid substances are dissolved in an optically inactive liquid. The computation is effected acording to one of two formulas, depending as to how the concentration is expressed: either by c, i. e. the number of grams of substance in 100 cc. of solution, or by percentage, /'. e. the number of grams of active substance in 100 g. of solution. In the latter instance the specific gravity (d) of the solution must be known. If the solution deviates the ray of polarized light a degrees the specific angle of rotation follows from the following equation:
The angle of rotation thus ascertained is not strictly constant for most substances, but varies with the nature of the solvent, the concentration, and the temperature.1) Consequently it is necessary to record the conditions observed with the specific angle of rotation, e.g. [a]D20° + 10° in a 43,5 p. c. alcoholic solution.