This section is from the book "A Research On The Eucalypts Especially In Regard To Their Essential Oils", by Richard T. Baker, Henry G. Smith. Also available from Amazon: A Research On The Eucalypts And Their Essential Oils.
Although several methods for the quantitative determination of cineol have been employed, chiefly based on the reactions and combinations it forms with various reagents, yet, it must be admitted that none of them gives results absolutely correct, so that a successful method for determining accurately the amount of cineol in Eucalyptus oil, under all conditions, has yet to be discovered.
With present methods advantage is taken of the loose combinations cineol forms with phosphoric acid or arsenic acid of certain strengths, or by absorption with resorcinol. Difficulties, however, present themselves, particularly the instability of the compounds with the acids under atmospheric conditions. With resorcinol the trouble is that other substances present in Eucalyptus oils besides the cineol are also absorbed.
With the phosphoric acid or arsenic acid methods the result depends, to a certain extent, on the manipulation, as well as the method of procedure adopted, so that, for commercial purposes at any rate, some uniform method should be recognised, such process being carried out always in a similar manner.
To largely remedy this defect of the "personal equation" we have devised a Rapid Phosphoric Acid Method, using petroleum ether, as in this way both quantities and times are fixed and the results satisfactory. (In connection with this problem see "Perfumery and Essential Oil Record," August, 1919, p. 211. Bennett and Salomon.)
The quantitative determinations for cineol recorded in the first edition of this work (published 1902) were all carried out with the phosphoric acid method as employed at that time. It was then considered necessary for the cake of cineol phosphate to be repeatedly pressed between fresh paper until grease spots ceased to appear. This rigid method of procedure is now known to have given too low results, more particularly in a hot climate like that of Sydney, and it might therefore have been permissible to add a certain percentage to the figures there given. We have, however, in this edition retained the results as previously recorded, but have added the letters O.M. (meaning old method) to denote the particular process employed.
In other cases where the more recent application of the phosphoric acid method has been used those letters are omitted, or the process employed is noted. Recently the resorcinol method has been utilised to a considerable extent, but in all cases where this has been done, it is so stated; with several of the oils the necessary corrections have been made for the non-cineol substances absorbed, or for the alcohols present, and with a very large number the results obtained with the phosphoric acid method are also given for comparison.
This method for the quantitative determination of cineol is a complicated process, and not likely to become of general application; besides it has no advantage over more simple methods. Although the compound is formed in oils somewhat poor in cineol, and is thus sometimes useful for qualitative work, yet the process does not give a very stable product, and results are low.
This method, which depends on the comparative ease with which cineol crystallises at a low temperature, has also been suggested for the quantitative determination of cineol (Helbing's Pharmacological Record, VIII, 1892.) It is not possible, however, to remove all the cineol in this way, so that the process is defective, and of little use for oils poor in cineol; thus it is not of general application.
This method, proposed by F. D. Dodge (Journ. Industrial and Engineering Chemistry, Vol. IV, August, 1912), is based on the fact that, in the cold, cineol is practically unacted upon by a dilute solution of potassium permanganate, while many other constituents are oxidised into soluble compounds. The process has not come into general use and the results are not always reliable. (See also "Perfumery and Essential Oil Record," Nov., 1912, and Oct., 1913.)