Micro determination of eugenol, thymol and vanillin in volatile oils and plants

Abstract

Two simple, rapid and sensitive spectrophotometric methods are described for the determination of some phenolic compounds in volatile oils and plants. The first method is based on their reaction with aqueous sodium nitrite (5%) and hydrochloric acid (10%) solutions, followed by the addition of sodium hydroxide (10%) solution to give red–yellow coloured products measured at 450, 396 and 385 nm for eugenol, thymol and vanillin, respectively. The method is highly selective for eugenol: Beer's law is obeyed typically in the concentration range 1–100 μg/mL. The second method depends on the addition of a known excess of standard potassium permanganate in neutral solution and measuring (at 520 nm) the excess unreacted after 5 min at 25 ± 5°C. The decrease in absorbance was found to be proportional to the amount of eugenol, thymol and vanillin reacted: Beer's law is obeyed typically in the concentration range 0.5–10 μg/mL. The methods could be applied to the determination of eugenol in clove oil and dried flower buds, thymol in thyme dried herb, and vanillin in vanilla pods. The results obtained were comparable with those from the official methods. Menthol, anisole and methyl salicylate were found not to interfere with the proposed method. © 1998 John Wiley & Sons, Ltd.