Refractivity intercept

Kurtz and Ward 22) have described a composite function of the refractive index and density which they call the refractivity intercept. The equation is R. Int = n — d/2. The value of the refractivity intercept lies in the fact that for hydrocarbon isomers it is more constant than most other functions. Its chief uses are the rapid checking of physical property data found in the literature and distinguishing between naphthenes, paraffins, and aromatics. 

Smittenberg, J., Refractivity Intercept and Refractivity Quotient of Series of Homologous... 

The refractive index is also needed to calculate the refractivity intercept in the determination of carbon type composition (Speight, 2000). 

The viscosity-gravity constant (ASTM D-2501) and the refractivity intercept (r) have been used for characterizing oils of widely different viscosity (ASTM D-2140). 

Refractivity intercept- Refractive index, density, viscosity... 

The tij -KVQC and the a -VGC methods do not require determination of the molecular weight. The refractivity intercept is calculated by the formula... 

Refractivity intercept This is calculated (ASTM D2159) from the density (d) and refractive index (n) (both at 20°C) using the sodium D line (ASTM D1218), where... 

Early compositional analyses on petroleum (and lubricating oil base stocks) were focused on quantifying the three major hydrocarbon types present, namely paraffins, naphthenes or cycloparaffins, and aromatics. In that period (the 1920s to the 1950s), the availability of instrumental techniques was essentially nil in terms of our viewpoint today, since spectroscopic methods were in their infancy, as was electronics technology. Accordingly, research workers used the limited tools available at that time—density, refractive index, molecular weight, and elemental analyses. Based on work with model compounds, these led to compositional relationships between structure and these measurements and development of the concepts of VGC, refractivity intercept, and the n-d-M method. 

It can be seen that measurement of the VGC for a sample establishes which line a sample may lie on, but cannot determine the point on that line which is necessary for that composition. That required development of a further triangular diagram that could be superimposed and where the intersections of lines gave the composition. This additional diagram was based on the following concept of refractivity intercept, of interest in its own right. 

Source S. S. Kurtz, Jr., and W. A. Ward, The Refractivity Intercept and the Specific Refraction Equation of Newton. I. Development of the Refractivity Intercept and Comparison with Specific Refraction Equations, Journal of the Franklin Institute 222 563-592 (1936). With permission. 

S. S. Kurtz, Jr., R. W. King, W. J. Stout, D. G. Parkikian, and E. A. Skrabek, Relationship Between Carbon-Type Distribution, Viscosity-Gravity Constant, and Refractivity Intercept of Viscous Fractions of Petroleum, Analytical Chemistry 28 1928-1936 (1956). 

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