Menthol content in peppermint oils, other volatile oils, plant material, formulations and other preparations may conveniently be estimated by GC methods.

A. Method I

1. Weigh accurately 2 g of menthol in a 250 ml conical flask with a ground glass jointed neck. Add 5 ml of a mixture of 4 ml acetic anhydride and 1 ml of anhydrous pyridine and boil under reflux for 30 min.
2. Through the condenser add 30 ml warm distilled water and continue heating for 15 min.
3. Remove from the heat source and cool completely. Disconnect the condenser and rinse it with 15 ml of distilled water.
4. Titrate the contents of the flask with 0.5 M alcoholic potassium hydroxide (KOH) solution using phenolphthalein as indicator. Let the titred value be ‘a’ ml.
5. Carry out a blank determination to determine the number of millilitres of the titrant required to neutralize the acetic acid produced by 5 ml of the acetylizing mixture. Let the titred value be ‘b’ ml.
6. (b-a) gives the number of millilitres of titrant equivalent to the acetic acid used up for the acetylation of menthol. Determine the percentage of free menthol from the equivalent factor of 0.5 M alcoholic KOH, which is 0.0781 g.