Dried aerial parts of mint contain about 0.4% to 0.8% essential oil of which 40% to 60% is menthol. It is typically isolated from the essential oil of mint by cold freezing, during which menthol crystallizes. The liquid portion is poured off and crystallized menthol is pressed between filter papers and subsequently purified by recrystallization.

Method

1. The volatile oil of peppermint herb is separated by water and steam distillation. The oil is then freed of moisture by passing it through a bed of anhydrous sodium sulphate.
2. The dried oil is then taken in well-sealed plastic containers and frozen to about minus 60°C for about 7 days. Menthol in the oil separates as flaky crystals which are filtered out.
3. The mother liquor still contains some menthol along with menthone and other terpenes. Add 8 g boric acid for 100 ml of the dementholized essential oil and further boil for 3 h to distill off menthone.
4. Convert the borate of menthol by saponification with 50 ml of 15% sodium hydroxide by heating under reflux for 1 h.
5. Cool the resultant solution to separate the remaining menthol.
6. Collect the flakes of menthol together, dry them in a desiccator and note the yield.

Identification

1. Heat a few crystals of menthol taken in a watch glass over a water bath. The entire material volatilizes without leaving any residue.
2. To 0.5 g of menthol taken in a test tube, add equal quantity of camphor or thymol. Liquefaction of the material indicates the presence of menthol.
3. To 0.25 g of menthol, add 5 ml of sulphuric acid and mix well. There is formation of yellowish-red turbidity. Allow to stand for 24 h. Formation of a transparent oily layer with no odour of menthol indicates its presence.