1984 Volume 27 Issue 6 Pages 500-505
Determination of calibration factors (F), area/mg•ml-1, for saturates, monoaromatics and diaromatics from the specific gravity (15/4°C) of oil has been attempted.
For this purpose, a relation between the specific gravity of oil and Δn (difference in refractive index between a substance and n-hexane) of each hydrocarbon type, which has been reported in Part 17), was applied. In the present study, a method of converting Δn, obtained from the relation, to F with pure compounds was investigated.
Fig. 1 shows an example of a relation between Δn and F obtained with n-hexane solutions of pure hydrocarbons. As shown in Fig. 1, F was not proportional to Δn. This appeared to be mainly caused by the property of refraction of the solutions, because the trend was observed even when column was not attached. In addition, it can be speculated from the fact that Δn/mg•ml-1 for the n-hexane solution is not proportional to Δn of solutes (Fig. 2).
Figs. 3 and 4 show effects of column length and packing on the relation between Δn and F, respectively. As shown in the Figs., no outstanding change by these effects was observed. However, the relation between Δn and F seemed to be extremely sensitive to the minor impurities in n-hexane as can be seen in Figs. 3 and 4 (compare data on Nucleosil 50-5). Accordingly, six compounds, whose value of F is situated near the solid line in Fig. 1, were selected as standard materials for obtaining the relation between Δn and F. They are listed in Table 1. The obtainable relation can be expressed by a general formula: F=A{1-B exp (-CΔn)}. Constants A, B and C are determined by the least squares with Δn and F of those compounds.
The Δn of each hydrocarbon type, determined from the specific gravity of oil, was actually converted to F with the relation mentioned above. Tables 2 and 3 show calculated F by the present method comparing with measured F by the conventional method3). As can be seen from the Tables, calculated values are in good agreement with measured values. Exceptionally, calculated F of saturates determined with amino groupbonded silica is smaller than measured F. Correction must be needed. Data in parenthesis indicate that irrversible adsorption of components occurs on silica. Therefore, attention must be payed to recovery in using silica.
Although the base of oil must be known for applying this method to analysis, for this, the UOP characterization factor9) from kinematic viscosity and API gravity is useful.
