METHODS FOR DETERMINING TRACE ELEMENTS IN CRUDE OILS AND PETROLEUM PRODUCTS (REVIEW)

G. N. Sutyagina

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

At the present time, with the discovery of oil fields in Western Siberia and the construction on their basis of the largest petrochemical complexes in the Tobolsk and Tomsk region, questions associated with the development of reliable and highly sensitive methods for determining metal-containing impurities in crude oils, as well as in petroleum products, are acquiring especially important practical significance.

As is known, major difficulties in the production and use of fuel are caused by the presence in crude oils of metals that change the activity of catalysts during catalytic cracking and affect the quality of petroleum products [1]. In addition, the study of trace elements in crude oils makes it possible to obtain information that may subsequently clarify certain questions of the genesis of oils [2]. Spectral methods have found the broadest application in the analysis of crude oils and petroleum products [3].

Among the methods used recently for the determination of trace elements in crude oils and petroleum products, photometric methods of analysis predominate. In a number of cases they have high sensitivity, are selective, use inexpensive equipment, and are comparatively simple in application.

In determining vanadium in the ash of crude oils and petroleum products, the phosphotungstate method is most often used; its sensitivity is 0.1 µg/ml [4]. It has been established that the results of the quantitative determination of vanadium in crude oils by this method are strongly affected by other trace elements and, above all, nickel and iron, which must be removed beforehand [5].

For the photometric determination of vanadium in crude oil and petroleum products present in amounts of \(10^{-3} \div 10^{-5}\%\), n-methoxybenzoic acid is used [6].

A photometric method has been developed for determining vanadium in petroleum products in the form of a complex with N-benzoylphenylhydroxylamine. Iron, titanium, molybdenum, and tungsten interfere with the determination [7].

In determining vanadium in liquid fuel and crude oil, xylenol orange is used [8], which forms an orange complex with pentavalent vanadium in a weakly acidic medium. The sensitivity of the determination is 0.16 µg/ml. The method makes it possible to determine \(> 1 \cdot 10^{-4}\%\) vanadium. Nickel (0.01 µg), aluminum (0.1 µg), copper, cobalt and chromium (0.5 µg), iron (1 µg), zinc, lead, and titanium do not interfere with the determination.

A method has been developed for the photometric determination of vanadium in ash

mineral oils, based on measurement of the intensity of the coloration arising in HCl solutions in the presence of \((\mathrm{CH_3CO})_2\mathrm{O}\), \(\mathrm{CH_3COOH}\), and methyl salicylate [9]. Colored compounds are also formed in this case by cobalt, molybdenum, and tungsten. Iron, aluminum, copper, chromium, uranium, and cadmium do not interfere with the determination of vanadium. The method is also applicable to the determination of vanadium in oil.

For the detection of very low concentrations of vanadium in petroleum products, a photometric method for determining vanadium with diphenylcarbazone is used [10]. The sensitivity of the method is \(0.08\ \mu\mathrm{g}/\mathrm{ml}\). Iron and chromium must be removed beforehand.

As photometric reagents for determining vanadium in the ash of oil, hematoxylin [11], solochrome fast gray [12], 3,3-dimethylnaphthidine [13], and diphenylbenzidine [14] have been proposed. A spectrophotometric method has been developed for determining vanadium with 4-(2-pyridylazo)-resorcinol [15]. Tungsten, niobium, titanium, and others interfere with the determination. The method is used in determining vanadium contained in petroleum fractions in amounts from \(1\) to \(5 \cdot 10^{-4}\%\).

For the photometric determination of copper contained in petroleum products at not less than \(10^{-6}\%\), sodium diethyldithiocarbamate is used [16]. Lead and iron do not affect the determination of copper. In the presence of heavy metals, and especially nickel, cobalt, vanadium, and iron, 1-(2-pyridylazo)-2-naphthol is used as the photometric reagent [17].

Colorimetric methods are widely used for determining lead in amounts of \(10^{-4}\%\) in isooctane and \(n\)-heptane, which are used as antiknock additives to fuels. This method is based on decomposition of the sample with bromine and extraction of lead with nitric acid. The pH of the extract is brought to 9.5–11 and treated with a solution of dithizone in chloroform. The sensitivity of the method is \(0.3 \cdot 10^{-4}\%\) [18].

Most photometric methods for determining nickel in oils and petroleum products are based on formation of a colored complex with dimethylglyoxime in an alkaline medium [19]; \(\alpha\)-furildioxime is also used as a photometric reagent for determining nickel in oils [13].

Cobalt can be determined with a high degree of sensitivity by photometric methods. Ammonium ferricyanide is used for determining cobalt in naphthenates [16]. However, the effectiveness of this method decreases in the presence of a number of other metals present in the samples. A more sensitive method for determining cobalt in oils is based on formation of a cobalt complex with nitroso-R salt [20]. Among other photometric reagents, nitroso-naphthols and nitroso-cresols [16] and 1-(2-pyridylazo)-2-naphthol [21] have also been proposed. Manganese is determined colorimetrically in the form of the permanganate ion after oxidation with sodium or potassium periodate [22]. It is recommended that the determination of manganese in this case be carried out in alkaline solution in order to prevent the influence of chromium [23]. In the absence of chromium, manganese is determined in the form of the peroxide complex \(\mathrm{Mn(3+)}\), formed in the presence of triethanolamine [24].

Polarographic methods of analysis, which possess high sensitivity, can be very effective in analyzing traces of metals in oils and petroleum products. A polarographic method for determining lead and copper contained in petroleum products in amounts of \(10^{-7}\%\) was described by Samuel and Brunnock [25]. Both elements are found in an acidic solution obtained by treating the sample with bromine, extracting with hydrochloric acid, or decomposing the organic substance with a mixture of nitric and hydrochloric acids.

A method has been proposed for the polarographic determination of vanadium, nickel, iron, and cobalt in petroleum ash from a single weighed portion of a petroleum product [26]. Manganese is determined polarographically in the form of the triethanolamine complex [27].

For the determination of alkali and alkaline-earth metals in oils, flame photometry is used [28]. Among other methods for determining elements, in particular vanadium and nickel, X-ray fluorescence [29], electron paramagnetic resonance [30], and neutron activation analysis [16] are used; the latter is applied to the determination of concentrations much lower than \(10^{-7}\%\).

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