ON THE POSSIBILITY OF DETERMINING TRACES OF HEAVY METALS IN OIL AND PETROLEUM PRODUCTS BY AMALGAM POLAROGRAPHY WITH ACCUMULATION

A. A. KAPLIN, N. I. BELOUSOVA, A. G. STROMBERG

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

Studies aimed at developing rapid, highly sensitive methods for analyzing impurities of heavy metals in the oils and petroleum products of Western Siberia are at present acquiring great theoretical and practical importance. In the course of these studies the following questions of petroleum geochemistry may be clarified:

1. The dependence of the quantitative distribution of microelements in oils on the depth of occurrence, group composition, and the content of asphalt-resinous substances in them.

2. The dependence of the quantitative distribution of microelements in oils on the composition and nature of the waters accompanying the oil.

In addition, at present the study of microelements has acquired important practical significance, since the corrosive effect of certain microelements on steel equipment has been established, as well as their poisoning effect on catalysts during cracking [1, 2]. Complete removal of microelements from oil and products of its refining can be carried out only if the exact quantities of the elements are known. This circumstance stimulates researchers to develop new, more sensitive methods for determining traces of microelements in oils and petroleum products.

For studying the quantitative distribution of microelements in oils and products of their refining, the method of amalgam polarography with accumulation can be successfully used; it advantageously combines high sensitivity (down to \(10^{-8}\%\) and lower) with simplicity in the instrumental arrangement of the analysis.

In the present work, the possibility of amalgam-polarographic determination of traces of zinc and lead in oils and petroleum products has been studied. According to literature data [5], the zinc–dithizone complex is formed directly in an oil–carbon tetrachloride solution. This is taken as the basis for separating zinc from oil by extracting it in the form of a dithizone complex. For the subsequent transfer of zinc into the aqueous phase, concentrated hydrochloric acid proved to be the most suitable re-extractant under our conditions. Other re-extractants proved unsuitable because, during re-extraction, a noticeable amount of emulsion was formed, and it did not appear possible to eliminate it. Because of the emulsion, phase separation was practically not achieved. A 0.5 M solution of ammonium fluoride was used as the indifferent electrolyte. The dependence of the height of the anodic

the zinc peak on the concentration of its ions in a 0.5 M NH\(_4\)F solution is linear. The dependence of current on potential reaches a limiting value at \(\varphi = -1.5\) V (relative to the SCE).

The studies were carried out on an OH-101 polarograph (Hungary). In the work an electrolyzer with insertable beakers was used; as the indicator electrode, a mercury film electrode with a film thickness of 5–10 \(\mu\) was used; the reference electrode was the SCE. Oxygen was removed from the solution being analyzed by purging with purified nitrogen before polarography and during accumulation.

On the basis of the investigations carried out, the following procedure is proposed for determining microconcentrations of zinc in oil.

A mixture of 10 ml of carbon tetrachloride, 1 ml of oil, and 3 ml of a 0.01% dithizone solution is shaken for three minutes in a separatory funnel. Zinc is then re-extracted with concentrated hydrochloric acid. After a sharp boundary between the aqueous and organic phases has been achieved, the re-extract is transferred to the beaker for polarography and carefully evaporated to moist salts. After evaporation, 10 ml of a 0.5 M ammonium fluoride solution (supporting electrolyte) is added to the beaker, and zinc is accumulated for five minutes at a potential of 1.5 V. The zinc content in the sample was determined by the method of additions of a standard solution.

The sensitivity of the instrument is \(2 \cdot 10^{-8}\) A/mm. The rate of potential change is 400 mV/min. The sensitivity of the procedure under the developed conditions is \(n \cdot 10^{-5}\)%. The duration of the analysis is about three hours. The relative error is not more than 15%.

In addition, we have developed a procedure for determining lead in gasoline by amalgam polarography with accumulation. The procedure is based on preliminary separation of the matrix by evaporation, followed by decomposition of the residue with concentrated nitric acid and final determination of lead against a background of 0.1 M hydrochloric acid solution. The amalgam-polarographic behavior of lead on the indicated background was studied in [6]. The lead content in gasoline was determined by the method of additions of a standard solution. Using the developed procedure, an analysis of a gasoline sample was carried out. The lead content in the sample is \(1.2 \div 1.5 \cdot 10^{-5}\)%.

The sensitivity of the procedure under the developed conditions is \(n \cdot 10^{-5}\)%. The relative error is not more than 15%.

Conclusions

1. A procedure has been developed for determining traces of zinc in oil. The method is based on separating zinc from the matrix by extraction as a dithizonate complex with carbon tetrachloride, followed by re-extraction and determination of zinc by the APN method.

2. A procedure has been developed for determining lead in gasoline. The sensitivity of the procedure is \(n \cdot 10^{-5}\)%.

References

1. A. V. Agafonov, I. E. Gel’ms, E. I. Rabinovich. “Chemistry and Technology of Fuels and Oils,” No. 6, p. 6, 1960.

2. Cottin André, “Gaz aujourd’ neu,” 92, No. 1, 9, 1968.

3. Serbănescu A. “Petrol si gaze,” 16, Nos. 3–4, 219, 1965. RZhKhim. 2P 89, 1966.

4. V. O. Kraushkin, S. B. Kazakov, V. N. Povkhovich. “Donobigi,” Academy of Sciences of the Ukrainian SSR, No. 6, 810, 1966.

5. J. M. Howard. Anal. Chem. 37, 596, 1965.

6. A. A. Kaplin, R. F. Zarubina, N. A. Kolpakova. “Zav. lab.,” No. 1, p. 11, 1971.