The most important determinations for these are total alkali and silica.
Total alkali is estimated by dissolving 2 grammes in distilled water, and titrating when cold, with N/1 acid, using methyl orange as indicator.
Silica may be determined by dissolving 1 gramme in distilled water, rendering the solution acid with HCl, and evaporating to complete dryness on the water-bath, after which the residue is moistened with HCl and again evaporated, this operation being repeated a third time. The residue is then heated to about 150° C., extracted with hot dilute HCl, filtered, thoroughly washed, dried, ignited in a tared platinum crucible, and weighed as SiO2.
Essential Oils.
As already stated, these are very liable to adulteration, and an examination of all kinds of oil is desirable, while in the case of the more expensive varieties it should never be omitted.
Specific Gravity.—As with fats and oils, this is usually taken at 15° C., and compared with water at the same temperature. In the case of otto of rose and guaiac wood oil, however, which are solid at this temperature, it is generally observed at 30° C. compared with water at 15° C.
The specific gravity is preferably taken in a bottle or U-tube, but if sufficient of the oil is available and a high degree of accuracy is not necessary, it may be taken either with a Westphal balance, or by means of a hydrometer.
Optical Rotation.—For this purpose a special instrument, known as a polarimeter, is required, details of the construction and use of which would be out of place here. Suffice it to mention that temperature plays an important part in the determination of the optical activity of certain essential oils, notably in the case of lemon and orange oils. For these Gildemeister and Hoffmann give the following corrections:—
Lemon oil, below 20° C. subtract 9' for each degree below, above 20° C. add 8' for each degree above.
Orange oil, below 20° C. subtract 14' for each degree below, above 20° C. add 13' for each degree above.