Refractive Index Measured at 589 nm from 283.15 to 358.15 K to Evaluate Temperature Invariance of Berthelot, Gladstone–Dale, Lorentz–Lorenz, and Eykman Equations

Molar refractivity is temperature independent, and its relationship to molar volume is described by a refractive index function. Four refractive index functions describing the relationships between molar refractivity, refractive index, and molar volume were evaluated: Berthelot, Gladstone–Dale, Lorentz–Lorenz, and Eykman. Experimental measurements of refractive index (n, 0.000001 readability) and density (ρ, 0.001 kg·m–3 readability) were obtained for pure compounds at five temperatures, from 283.15 to 358.15 K at atmospheric pressure, with nine replicates per temperature condition. In total, 72 compounds from the n-alkanes, 1-alkenes, 1-alkynes, cyclic compounds, alkyl aromatics, 1-alcohols, carboxylic acids, and those containing sulfur were included in the study. Using the experimental data as input, the variation in molar refractivity with temperature was calculated using each of the four refractive index functions. It was found that the Eykman refractive index function, (n2 −1)/(n + 0.4), resulted in the lowest temperature-dependent variation in molar refractivity. The rank order of performance of the refractive index functions was consistent for all groups of compounds, with Eykman being the best (0.03% relative variability in molar refractivity with temperature), followed by Gladstone–Dale, Lorentz–Lorenz, and Berthelot.