An important criterion of purity is the evaporation residue; i.e., the percentage of the oil which is not volatile at 100o.
A determination of the evaporation residue is of special value in the case of the citrus oils; a low value for an expressed oil suggests the possibility of the addition of terpenes, or other volatile constituents; a high value may indicate the addition of foreign material, such as, rosin, fixed oils, or high boiling sesquiterpenes. In the case of rectified oils such as turpentine, a high value may indicate improper or lack of rectification, or polymerization due to age or improper storage. In the case of certain solids, such as camphor, thymol, or menthol, a high evaporation residue will indicate insufficient purification.
It is important to study the odor of an oil as it volatilizes during the heating. Often "by-notes" of foreign low boiling adulterants or contaminants may be discovered. The odor of the final residue while still hot should also be carefully studied for the addition of high boiling adulterants, such as cedarwood.
The consistency of the residue, both when hot and cool, and the color sometimes indicates the presence of particular adulterants. For example, an orange oil, which has a brittle residue instead of the usual soft waxy residue, should be carefully investigated for rosin.
Acid numbers and saponification numbers may be determined on suspicious residues: rosin usually raises the acid number considerably; fixed oils raise the saponification number.
48 For oils containing mostly high boiling constituents (such as cassia and bay), use an air-cooled condenser.

The fact that essential oils are complex mixtures makes an exact determination of the nonvolatile residue very difficult. "Constant weight" cannot conveniently be attained because of the fact that waxes and other high boiling nonvolatile material tend to retain or "fix" some of the lower boiling constituents. "The United States Pharmacopoeia" defines constant weight as the value obtained when "two consecutive weighings do not differ by more than 0.1 per cent, the second weighing following an additional hour, of drying."49 Even after "constant weight," according to this definition, has been attained, further prolonged heating will give much lower results. Hence, a certain standardization of technique becomes necessary.
Procedure: Weigh accurately (to the closest milligram) a well cleaned Pyrex evaporating dish that has been permitted to stand in a desiccator for 30 min. To this tared dish add the requisite amount of oil or solid (weighed to the closest centigram) and heat on a steam bath for the prescribed length of time. Then permit the evaporating dish to cool to room temperature in the desiccator and weigh (to the closest milligram). Calculate the nonvolatile residue obtained, the so-called "evaporation residue," and express as a percentage of the original oil. 
Calculate the nonvolatile residue obtained, evaporation residue

It is well to bear in mind that the size, shape, aiid composition of the evaporating dish employed in such a determination, as well as the size of sample and time of heating, will influence the analytical result obtained.
Flat bottom evaporating dishes of Pyrex glass are very satisfactory; they offer the further advantage of more easily permitting an observation of the color and opacity of the residue. Conventional Pyrex evaporation dishes, 80 mm. in diameter and 45 mm. deep, are to be recommended. The use of such dishes tends to minimize the formation of polymerization products in most cases.
Certain exceptional products will require special treatments, however; evaporation residues on such materials as diacetyl are meaningless because of the rapid formation of polymerization products unless the determinations are carried out in vacuum with the application of little or no heat.
In evaluating oleoresins, evaporation residues should also be determined. Here it is best to express the results as "loss of weight on heating” The analytical results obtained will include the loss of volatile solvent as well as the loss of part of the naturally occurring essential oil. An abnormally high value often indicates the incomplete removal of the volatile solvent used in the manufacture of the oleoresin.

49 Twelfth Revision, 3. The Thirteenth Revision, 7, limits the difference to not more than 0.05%.

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