2.6.3 Physical Characteristics of Voltaile Oils
It is a well known fact that the volatile oils usually differ from each other with regard to their chemical constitutions. However, they invariably possess a number of physical characteristics as stated below, namely:
(a) Odour: Most volatile oils do possess very pleasant and characteristic odour which vary considerably from one specimen to another.
Detection When a drop of the volatile oil is soaked on a filter paper, an expert may judge its quality and genuinity and may also differentiate between the authentic pure sample from the adulterated one by their individual odours.
(b) Nature: In general, the volatile oils are mobile liquids at ordinary temperatures. However, there are a few exceptions, for instance:
(i) Anise Oil: It solidifies at 15oC and melts at 17oC,
(ii) Rose Oil: It solidifies at 17oC and melts at 19oC, and
(iii) Oil of Mentha and Oil of Thyme: They distinctly deposit a solid substance upon
chilling i.e., menthol and thymol respectively, and leaving behind a liquid portion as a
‘mother liquor’. The former is termed as “Stearoptene” and the latter is known as
“Oleoptene”.
(c) Volatility: The essential oils are mostly volatile completely, with the exception of a few of them e.g., ‘oil of lemon’, ‘oil of orange’, that contain also an additional non-volatile substance of gummy nature. Both the volatile and their pure components do possess high vapour pressures, and hence evaporate completely and rapidly when exposed to atmosphere.
Detection Volatile oils do not leave a stain when soaked on a piece of filter paper, whereas a fixed oil does. Thus, it also checks its adulteration.
(d) Colour: Invariably, the colour of freshly obtained volatile oils are more or less colourless, but on prolonged storage they usually undergo both oxidation and resinification thereby rendering it dark in colour. The darkened volatile oil can be redistilled to obtain once again the colourless sample.
Prevention The volatile oils must be stored in a cool, dry place and preferably filled upto the brim in amber glass bottle having an airlight stopper.
(e) Refractive Index: The refractive index of volatile oils vary from 1.42 to 1.61. They are mostly characterized by high refractive indices.
Detection The pure authentic volatile oils have definite refractive index as specified in official compendia, whereas the adulterated oils will show different values.
(f ) Optical Rotation: A large number of volatile oils exhibit optical activity by virtue of the chemical constitution of the oil(s) or its constitution. It gives some vital informations with regard to the source and anthenticity of the oil sample, namely:
(i) Both optical rotation and specific rotation offer a fairly dependable and reliable clue whether the volatile oil is either genuine or adulterated,
(ii) It also establishes the source and variety of the volatile oil, for instance: American oil of Turpentine is dextro-, whereas the French oil of Turpentine is levo-, and
(iii) It ascertains whether the chemical constituent is either isolated from the volatile oil or obtained synthetically, for example: Menthol isolated from pippermint oil is exclusively levo-rotatory, whereas the synthetic menthol could be either racemic or levo. Likewise, the natural camphor is dextro, whereas the synthetic one could be either racemic or levo.
(h) Solubility: The majority of volatile oils are immiscible with water, but are soluble in absolute alcohol and several other organic solvents e.g., ether, chloroform, carbon disulphide, acetone, hexane, ethyl acetate etc.
Exceptions
(i) Oil of Rose gives a turbid solution due to the very presence of paraffin hydrocarbons that are very sparingly soluble in alcohol, and
(ii) Many a times certain volatile oils on being dissolved in organic solvents render them turbid due to the presence of traces of moisture which may be eliminated by treating the volatile oil with a small amount of powdered anhydrous sodium sulphate crystals.
In addition to the above stated physical parameters there are certain other equally valuable and important characteristic data for the identification as well as detection of adulteration in a sample of volatile oil, namely: boiling range, flash point, evaporation residue, molecular refraction and the like.
