Photocolorimetric Determination of Trace Amounts of Aluminum in West Siberian Crude Oils

E. A. Guber, Yu. A. Karbainov, T. A. Sagachenko, A. V. Kovsh

(Presented by the scientific seminar of the Department of Analytical Chemistry)

A large number of reagents are known from the literature that form colored compounds with aluminum at the appropriate pH of the solution [1–6]. A drawback of many reagents is their low specificity, as a result of which methods for determining aluminum are complicated by the need for preliminary separation of interfering ions. The stability of many colored aluminum compounds is limited in time and depends on tenths of pH units.

Among methods free of these shortcomings are the method for determining aluminum with 8-hydroxyquinoline [2, 3] and with eriochrome cyanine R [2]. The determination of aluminum with eriochrome cyanine R is more specific, and the analysis itself is less time-consuming.

The red-violet aluminum complex with eriochrome cyanine is photometrically measured at $\lambda = 530$ m$\mu$ ($\varepsilon = 40000$). The presence of tungsten, nickel, cobalt, molybdenum, bismuth, lead, titanium, and manganese does not interfere with the determination of aluminum, i.e., those elements that usually accompany aluminum in crude oils. Vanadium, zirconium, and beryllium interfere. The influence of interfering ions is eliminated by adding masking reagents.

It is known that copper and iron form colored compounds with eriochrome cyanine whose absorption maxima are close to those of aluminum compounds [3]. The influence of iron is eliminated by introducing ascorbic acid, and that of copper by sodium thiosulfate.

Table 1

Optimal amount of ascorbic acid (4%)

We investigated: 1) the optimal amounts of ascorbic acid and sodium thiosulfate in the determination of aluminum in the presence of a tenfold excess of iron and copper; 2) the stability over time of the aluminum complex with eriochrome cyanine at pH 5.9.

The optical densities of the artificial mixtures were compared with the optical density of a solution containing 2 μg of aluminum, whose optical density was equal to 0.129. The results of the study are given in Tables 1, 2, and 3.

Table 2

Optimal amount of sodium thiosulfate
(15% solution)

As the results of the study showed, the optimal amounts are 0.2 ml of 4% ascorbic acid and 0.125 ml of 15% sodium thiosulfate. The aluminum complex with eriochrome cyanine continuously changes

Table 3

Stability of the aluminum complex with eriochrome cyanine
(pH 5.9) over time

its optical density; therefore it is recommended to photometer the solutions after 2 min, i.e., at the highest value of the optical density.

Course of analysis

The ash residue obtained from a 10 g sample of oil is dissolved in hydrochloric acid (1:1). The solution is transferred to a 50 ml volumetric flask and brought to the mark with bidistillate. Further analysis is carried out with an aliquot of the resulting solution. 10 ml of the solution is transferred to a 50 ml volumetric flask, 1 drop of methyl red indicator is added, and the solution is neutralized with 1 N ammonia solution. Then 0.2 ml of a 4% ascorbic acid solution is added, and after 2–3 min 0.25 ml of a 15% sodium thiosulfate solution. The mixture is stirred thoroughly, allowed to stand for 2–3 min, and the volume is brought to the mark with bidistillate. From the resulting solution, 25 ml is taken into a 50 ml volumetric flask. To the remaining 25 ml, 2 ml of sodium fluoride (2.4%) is added; after 2 min, 10 ml of acetate buffer is added to each flask.

Table 4

Dependence of the optical density of the solution
on the aluminum content

ra, 3 ml of eriochrome cyanine solution, and the volumes are brought to the mark with bidistillate and mixed thoroughly. The measurement is carried out after 2 min on an FEK-N-57 in cuvettes with a layer thickness of 30 mm, using green light filter No. 5 with absorption maximum \(\lambda = 530\) mµ. The aluminum content in the analyzed sample is determined from the calibration curve (Table 4).

Table 5

Aluminum content

Using the described procedure, an analysis was carried out for oils from 15 fields. The results of the analyses were processed by the method of mathematical statistics [7] and are given in Table 5.

LITERATURE

1. E. V. Alekseevsky et al. Qualitative Analysis. Moscow, Goskhimizdat, 1954.

2. F. P. Treadwell and V. G. Hall. Qualitative Analysis. Moscow, Goskhimizdat, 155, 1964.

3. V. F. Maltsev. “Factory Laboratory,” 7, 807 (1961).

4. G. Charlot. Methods of Analytical Chemistry. Moscow—Leningrad, Khimiya Publishing House, 1969.

5. E. Sandell. Colorimetric Determination of Traces of Metals. Moscow, Mir Publishing House, 1964.

6. M. I. Bulatov, I. P. Kalinkin. A Practical Guide to Photocolorimetric and Spectrophotometric Methods of Analysis. Moscow—Leningrad, Khimiya Publishing House, 1968.

7. K. Doerffel. Statistics in Analytical Chemistry. Moscow, Mir Publishing House, 1969.