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are supplemented by some new results on optical absorptivity in
dilute crude oil solutions.
2. Results and Discussion
2.1. Samples and Equipment
The virgin crude oils were collected from well-heads at Ro-
mashkinskoye reservoir (Tatarstan). Crude #1, employed in RI ex-
periments, had a density of 875.6 g/l, contained 3.6 wt. %
asphaltenes. For comparative absorptivity measurements we used
crude #2 (895 g/l, 3.5 wt. % asphaltenes) and solid n-heptane as-
phaltenes precipitated from a blend of Tatarstan crude oils. The
samples were stored in the dark, in air. As a solvent, a “chemically
pure” grade toluene was used.
The RI measurements were performed in an Abbe-type refrac-
tometer IRF-454-B2M (KOMZ, Kazan, Russia). For toluene at 20°C
the measured RI was 1.4967, close to the value of 1.4969 quoted by
the producer. The effects of crude oil concentration in toluene on
optical absorption have been studied in a spectrometer (Evdokimov
et al., 2003a,b), equipped with a set of narrow-band light filters
(KFK-2 Photocolorimeter). The UV/Vis absorption spectra have
been measured in a FLUORAT®-02-PANORAMA analyzer
equipped with a high-pressure xenon arc lamp. All measurements
were performed at 20°C and at ambient pressure.
2.2. Refractive Index in Concentrated Solutions
Figure 1 shows the results of RI measurements in oil/toluene
solutions for a wide range of oil concentrations, from 0.011 to
50.25 vol. % of oil (note a log scale). For concentrated solutions (≥ 1
vol. % oil) each data point was obtained with a new sample, pre-
pared by mixing the required volume of the crude oil with ≈10 ml
of toluene. No effects of the manner of mixing (e.g., introducing oil
into toluene or vice versa) were observed. Owing to the dark and
opaque nature of our crude oil, it was not possible to measure the
RI of solutions with >50 % of oil. In studies of such opaque crudes
(or bitumen) there is a well-established technique for estimation of
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