Assay of Volatile Oils

Assay of volatile oils Volatile oils are actually plant oils(Essential oils). These are generally obtained by steam distillation & solvent extraction. These different from fixed & fatty oils (in properties & chemical composition). These are used in perfumes, cosmetics, soaps, foods ,drinks, incense items & in household cleaning products. The compositions of the oils are different among different plants. These are generally obtained mostly in complex mixture. In generall , hydrocarbon: terpene is the main constituent of the volatile oils. Volatile oils: peppermint oil, lemon oil, aniseed oil etc. It is apparent that terpenes are only slightly odouriferous. Hence, the characteristic odour of the oil is attributed to various compounds i.e. alcohols, esters, ketones & phenols. Volatile oils may be the terpene derivatives. In the comparatively small proportion of the odouriferous substances terpene hydrocarbon merely acts as a diluent.

Basis of analytical standards: i)Physical measurement, -Refractive index, optical rotation etc, R. index: Peppermint oil : 1.460—1.467 Lemon oil : 1.474— 1.476 Aniseed oil : 1.553—1.560 ii)Chemical analysis: It is based on quantitative determination of one particular compound. or group of closely related compounds, i.e. ester value, phenol content, acid value, aldehyde content etc. Nowadays, Gas chromatography is the more fovourable tool for the determination of volatile oils. e.g. 1)Determination of esters by the general method of assay. 2)Determination of free menthol .(in peppermint oil) 3)Determination/limit of aldehyde in volatile oils(in eucalyptus oil) 4)Determination of cineole.(in eucalyptus oil) 5)Determination of eugenol , phenol (in clove oil)

Determination of free menthol: in peppermint oil C10H20 O (R.OH)

Oil is measured & acetylated by boiling with double its volume of acetic anhydride. Little sodium acetate is also added. Boiling was conducted in a small Kjeldahl flask under an air-cooled reflux condenser. R.OH + (CH3 CO)2O → R.O.OC.CH3 +CH3COOH The acetylated oil is heated with water on a water bath to hydrolise excess anhydride.

The aceylated oil is separated and washed successively with brine(sodium carbonate –water). It is dried over anhydrous sodium sulphate. A weighed portion of the acetylated oil is then boiled under reflux with N/2 alcoholic KOH. Hydrolysis of menthyl acetate along with other esters occur(their value should be addressed). R.O.OC.CH3+ KOH = R.OH+ CH3.COOK The excess of KOH is determined by titration with N/2 HCl. Determination of eugenol: in clove oil

Take a graduated narrow flask( ~0.1ml). Transfer a measured volume of clove oil and add aqueous potassium hydroxide solution. Shake the mixture. The eugenol is converted to it soluble potassium salt.

The volume of the unabsorbed oil is subtracted from the initial volume gives the volume of eugenol. Determination/limit of aldehyde: in ecucalyptus oil. R..CHO + NH2.OH, HCl → R..CH:N-OH + HCl + H2O Hydroxylamine HCl Aldoxime A measured volume of oil is shaken in the stoppered tube with the hydroxyl ammonium chloride reagent and benzene. The liberated HCl is titrated against N/2 alcoholic KOH using methyl orange indicator until persistent yellow colour appears.