SOP: Refractive Index of Essential Oils

GIFSQ — Global Institute for Food Safety & Quality

Principle

The refractive index is the ratio of the sine of the angle of incidence to the sine of the angle of refraction when monochromatic sodium light of wavelength 5893 Å passes from air into the material; it is measured at 30 °C with a temperature correction applied where the determination is made at other temperatures. The refractive index and rotation of the 10% distillate are determined for the physical constants of the distillate.

Apparatus required

• Refractometer (Bausch and Lomb precision refractometer or Zeiss butyrorefractometer, with standard tables for converting the instrument reading to refractive index)
• Quartz plate with monobromonaphthalene (for checking the instrument)
• 3-bulb Ladenburg flask: main bulb 6 cm in diameter and 120 ml capacity, condensing bulbs 3.5 cm, 3.0 cm and 2.5 cm; distance from the bottom of the flask to the opening of the side arm 20 cm

Reagents

a. Alcohol (for cleaning the prisms).

b. Distilled water (for checking the instrument; the refractive index of distilled water at various temperatures is given in Table 28-5 of the book).

Procedure

Clean the prisms with alcohol and circulate water at 30 °C

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Place 2 or 3 drops of oil on the surface of the prism, close the prism and adjust the mirror until the reading is sharp

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Allow the instrument to stand for a few minutes before the reading is made, so that the sample and the instrument come to equilibrium

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Take a second reading after a lapse of a few minutes — if the reading is indistinct after running water at constant temperature for some time, the sample may not have been evenly distributed on the prism surfaces

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As refractive index is greatly affected by temperature, control the temperature accurately

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After the material has been measured, clean the prism surface carefully and dry, so that no contaminating material is carried over to subsequent measurements

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Occasionally check the instrument by means of the quartz plate that accompanies the instrument using monobromonaphthalene; if such a plate is not available, check the instrument using distilled water

Physical constants of 10% distillate:

Place 50 ml of the sample in the 3-bulb Ladenburg flask

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Distil the oil at the rate of 2.0 ml per minute until 5.0 ml has been distilled

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Determine the refractive index and the rotation of the distillate as given above and in the optical rotation procedure

Calculation

The notation for refractive index is nᴅᵗ, where t is the temperature in °C at which the determination was made.

Temperature correction: as far as possible, carry out the determination at or near 30 °C. If the refractive index is determined at other temperatures, apply the correction factor specified in the individual standard, or a factor of 0.00038 per degree Celsius if a factor is not specified. The corrections for Florida oils in USA are:

• Orange oil: 0.00045
• Lemon oil: 0.00046

If the temperature is above 30 °C, add the correction, and if below, subtract the correction.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Official Methods of Analysis, Association of Official Analytical Chemists, Washington, DC, USA, 12th edn., p. 340 (1975).