The absolute refractive index of a medium is the ratio of the speed of light in a vacuum to the speed of light in that medium. The refractive indices of the low-sulfur diesel which feature in the final row of the table at the end of the previous section are 1.4595 and 1.4745. Again a mild trend of increase with density is evident, a typical value of 1.448 having been noted in Chapter 4 for kerosenes and values of around 1.42 having been noted in Chapter 3 for gasoline. The refractive index of n-cetane, the benchmark hydrocarbon against which diesel is assessed, is 1.432. This is also equivalent to the ratio of the total refractive index of the medium to the absolute refractive index of the medium with respect to which the assessment is being made. A lot of studies regarding diesel fuel blends properties are presented in the literature. Some of the significant properties of diesel fuel blends can be evaluated from other blends properties. For example, density and viscosity of biodiesel blends can be predicted based on the blend refractive index.
The dielectric constant for diesel fuels varies across quite a wide range, from about 2 to about 6, making the correlation used for the lighter fractions for calculating the refractive index from the dielectric constant unsuitable for diesel. The meaning of the correlation, which even for a pure hydrocarbon compound is only approximate, is not altogether lost when diesel is considered. Taking the mean of the two values of the refractive index earlier in this paragraph and squaring it gives a value for the dielectric constant of 2.15, which is within the range given though certainly at the low end of it. The dielectric constant of n-cetane at 20°C is 2.08, giving a value of the refractive index of – √2.08 = 1.442
A reader utilizing the correlation in the box above might well make the judgment that the term v0.003 is so close to unity across the range of values of the kinematic viscosity of diesel that it can be omitted without any loss of precision.
More than that, the refractive index can be used as a consistent physical property to predict transesterification reaction evolution. As a consequence, the refractive index of diesel fuel blends is significant in order to distinguish these blends or to monitor the growth of the transesterification procedure of vegetable oils or animal fats. The refractive index of diesel fuel blends can be experimentally resolute or evaluated based on refractive indices of diesel fuel and biodiesel.