Most essential oils when placed in a beam of polarized light possess the property of rotating the plane of polarization to the right (dextrorotatory), or to the left (hicvorotatory). The extent of the optical activity of an oil is determined by a polarimeter and is measured in degrees of rotation. Of the numerous types of polarimeters that are available, the most convenient for use with essential oils is probably the half-shadow instrument of the Lippich type.20
The angle of rotation is dependent upon the nature of the liquid, the length of the column through which the light passes, the wave length of the light used, and the temperature.
Both the degree of rotation and its direction are important as criteria of purity. In recording rotations it is customary to indicate the direction by the use of a plus sign (+) to indicate dextrorotation (rotation to the right, i.e., clockwise) or a minus sign ( ) to indicate laevorotation (rotation to the left, i.e., counterclockwise).
20 For a discussion of the theory involved, the reader is referred to H. Landolt: "The Optical Rotating Power of Organic Substances and Its Practical Application" (translated by J. H. Long), The Chemical Publishing Co., Boston, Pa. (1902). Landolt thoroughly covers the field of optical activity, including, inter alia, the causes of optical activity and inactivity, the theory and construction of the polarimeter, and the various types of instruments available.

Since the scale reading for an optically active liquid is directly proportional to the length of the transmitting column of liquid, it is necessary to use a standard tube, 100 mm. long. If for any reason a longer or shorter tube is used, the rotation should be calculated for a tube of 100 mm. and reported as such. Rotations for essential oils given in the literature may be assumed to be for this standard tube unless a different length is specified.
It has become customary in polarimetric work to use sodium light. A suitable source may be obtained by placing large crystals of sodium chloride upon the grid of a Meeker burner or by wrapping a piece of asbestos, previously saturated in a strong salt solution, around the conventional Bunsen burner. By far the most convenient and satisfactory method of maintaining a constant light source is the use of a sodium vapor lamp. Such lamps, designed especially for use with polarimeters, are available.
Although "The United States Pharmacopoeia" and "The National Formulary" specify 25 as the official temperature for all optical rotations, nevertheless, a standard temperature of 20 is usually adopted for essential oils reported in the literature. For most essential oils the change in optical rotation with temperature variations normally encountered in the laboratory is very small; hence, in routine analyses the readings are usually taken at room temperature. No corrections for temperature variations are made except in the case of citrus oils which contain large amounts of highly active terpenes. The corrections to be used, per degree centigrade, are :
Orange Oil............. 13.2'
Lemon Oil.............. 8.2'
Grapefruit Oil........... 13.2'

The proper correction is to be added if the reading is taken at a temperature higher than the desired temperature and, conversely, to be subtracted if the temperature of the reading is lower than the desired temperatu re.
In scientific work the temperature at which the rotation was determined should be specified. To adjust the temperature to standard, the polari meter tubes may be immersed in a constant temperature bath. Use may also be made of special water jacketed tubes. All determinations should be carried out in a dark room. Monochromatic sodium light should be employed.

a. Liquids.

The oil or liquid should be free from suspended material. Often oils are hazy owing to the presence of small amounts of water; such an oil should be dried with anhydrous sodium sulfate and filtered before a determination is attemoted.
Procedure: Place the 100 mm. polarimeter tube containing the oil or liquid under examination in the trough of the instrument between the polarizer and analyzer. Slowly turn the analyzer until both halves of the field, viewed through the telescope, show equal intensities of illumination. At the proper setting, a small rotation to the right or to the left will immediately cause a pronounced inequality in the intensities of illumination of the two halves of the field.
Determine the direction of rotation. If the analyzer was turned counterclockwise from the zero position to obtain the final reading, the rotation is laevo ( ); if clockwise, dextro
After the direction of rotation has been established, carefully readjust the analyzer until equal illumination of the two halves of the field is obtained. Adjust the eyepiece of the telescope to give a clear, sharp line between the two halves of the field. Determine the rotation by means of the protractor; read the degrees directly, and the minutes with the aid of either of the two fixed verniers; the movable magnifying glasses will aid in obtaining greater accuracy. A second reading should be taken; it should not differ by more than 5' from the previous reading.
Some oils are too dark in color for an accurate determination of the optical rotation when a 100 mm. tube is used. In such cases, a 50 mm. tube may be employed, or even a 25 mm. tube, if necessary. Since the rotation is reported for a 100 mm. tube, any experimental error will be multiplied by 2 for a 50 mm. tube, and by 4 for a 25 mm. tube. Conversely, if a clear, light colored oil is examined which is only slightly optically active, the use of a longer tube (200 mm.) may often prove of advantage; the value to be reported will be found by dividing the observed rotation by 2; any experimental error will also be halved.

b. Solids. 

The optical activity of a solid is best determined in solution and expressed as specific rotation. The following formulas may be used:

The optical activity of a solid is best determined in solution and expressed as specific rotation.

The optical activity of a solid is best determined in solution and expressed as specific rotation.

Formula (2) is more convenient, since it does not require the determination of the specific gravity of the solution.
The experimental value for the specific rotation of a solid is dependent upon the concentration of the solution and upon the particular solvent employed; therefore, the concentration and solvent used should be given when the specific rotation of a solid is reported. The rotation should be determined as soon as possible after the solution has been prepared, so that any change that might result from mutarotation will be minimized.
The use of specific rotation for a complex mixture such as an essential oil is not recommended. For the sake of completeness, the following formula is given:
[α]Dto = α/ld     (3)
Formula (3) applies to optically active liquids. The symbol [α]Dto is reserved exclusively for specific rotation; optical rotation determined in a 100 mm. tube is indicated by αDto ' , the brackets being omitted. If no temperature is given, it may be assumed that theoptical rotation was determined at room temperature.

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