REFRACTIVE INDEX OF VISCOUS MATERIALS: ASTM D1747
EXPLANATION
Refractive index is a fundamental physical property that can be used in conjunction with other properties to characterize pure hydrocarbons and their mixtures. The use of refractive index in correlative methods for the determination of the gross composition of viscous oils and waxes often requires its measurement at elevated temperature.

Refractive index is the ratio of the velocity of light (of specified wavelength) in air, to its velocity in the substance under examination. The relative index of refraction is defined as the sine of the angle of incidence divided by the size of the angle of refraction, as light passes from air into the substance. If absolute refractive index (that is, referred to vacuum) is desired, this value should be multiplied by the factor 1.00027, the absolute refractive index of air. The numerical value of refractive index of liquids varies inversely with both wavelength and temperature.

This test method covers the measurement of refractive indexes, accurate to two units in the fourth decimal place, of transparent and light-colored viscous hydrocarbon liquids and melted solids which have refractive indexes in the range between 1.33 and 1.60, and at temperatures from 80 to 100° C. Temperatures lower than 80° C may be used provided that the melting point of the sample is at least 10° C below the test temperature.

This test method is not applicable, within the accuracy stated, to liquids having colors darker than ASTM Color No. 4. ASTM color as determined by Test Method ASTM D1500, to liquids which smoke or vaporize readily at the test temperature, or to solids melting within 10° C of the test temperature.

The instrument can be successfully used for refractive indices above 1.60, but since certified liquid standards for ranges above 1.60 are not yet available, the accuracy of measurement under these conditions has not been evaluated.

TEST SUMMARY
The refractive index normally is measured by the critical angle method using monochromatic light from a sodium lamp. The instrument is previously adjusted by means of calibration obtained with certified liquid standards.

TEST PRECISION
Repeatability: 0.00007
Reproducibility: 0.0006