SOP: Ester Value of Essential Oils
GIFSQ — Global Institute for Food Safety & Quality
Principle
The esters in the oil are saponified by refluxing with a known excess of alcoholic KOH; the unused alkali is back-titrated with standard HCl, and the alkali consumed is converted to the ester value and to the percentage of esters using the molecular weight of the ester.
Apparatus required
- Conical flask of 250 ml capacity with ground glass joint
- Condenser which fits the flask
Reagents
a. 0.5 N alcoholic KOH solution: dissolve 33 g of KOH in 1000 ml of rectified spirit, allow to stand, and decant or filter; adjust to 0.5 N using alcohol.
b. 0.1 N alcoholic KOH solution.
c. 0.5 N HCl.
d. Phenolphthalein indicator.
Procedure
Weigh the sample into the conical flask accurately (±1 mg), depending upon the expected ester value — if the value is below 50, the weight of material required is 4.5 to 5.0 g; for every increase of 20 in the ester value up to 140, reduce the weight taken by 0.5 g, and thereafter by 0.2 to 0.3 g
↓
Add 25 ml of 0.5 N alcoholic KOH solution and 10 drops of phenolphthalein indicator, and reflux for 1 hr unless otherwise specified (see the analytical conditions for bergamot oil below)
↓
Cool and titrate the excess alkali with 0.5 N HCl using phenolphthalein as indicator
↓
Carry out a blank determination in the same way except for the material
Calculation
Ester value = ((Blank titre − Sample titre) × N of HCl × 56.1) / Weight of sample
% Esters = (Ester value × Molecular wt of ester) / 561
Where,
- Blank titre = ml of 0.5 N HCl used in the blank titration
- Sample titre = ml of 0.5 N HCl used in the sample titration
- N of HCl = normality of the HCl solution
- Weight of sample = weight of the oil taken
Analytical conditions:
| Oil | Wt of sample (g) | Reflux time (min) | Factor for calculation: g of ester per ml of 0.5 N KOH |
|---|---|---|---|
| Bergamot oil | 2.0 | 30 | 0.09815 as linalyl acetate |
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
- Methods of Sampling and Test for Natural and Synthetic Perfumery Material (First Revision), IS: 326—1968, ISI, New Delhi.
- EOA Specifications and Standards, EOA No. 1, Essential Oil Association of USA, Inc.