PHOTOMETRIC DETERMINATION OF TRACES OF VANADIUM IN OILS OF WESTERN SIBERIA

G. A. Chervenchuk, Yu. A. Karbainov

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

For the determination of traces of vanadium in oils, photometric methods of analysis are widely used. One of the most common methods for determining vanadium in oil ash is the phosphotungstate method [1–3]. For the determination of \(10^{-3}\)—\(10^{-5}\%\) vanadium in oil, 4-methoxybenzoic acid is used [4]. Xylenol orange is used in determining vanadium in oil [5] and in liquid fuel when the vanadium content is greater than \(10^{-4}\%\) [6]. A method has been developed for the photometric determination of vanadium in ash of mineral oils, based on measuring the intensity of the color that arises in hydrochloric-acid solutions in the presence of acetic anhydride, acetic acid, and methyl salicylate [7]. As photometric reagents in the determination of vanadium in oils, \(N\)-benzoylphenylhydroxylamine [8], 3-3′-dimethylnaphthidine [9], hemotoxylin [10], durable gray solochrome [11], diphenylcarbazone [12], and others are used.

Among the reagents described in the literature for the photometric determination of vanadium, we selected diphenylcarbazone as the most sensitive and comparatively readily available.

The sensitivity of the method is \(0.05\ \mu\text{g}/\text{ml}\). Diphenylcarbazone forms a rose-orange complex with pentavalent vanadium at \(\mathrm{pH}=5\) in an acetone–acetic medium [13, 14]. In this work, the conditions for formation of the vanadium complex with diphenylcarbazone were studied on pure salts and on the objects under investigation. The data obtained are presented in Tables 1 and 2.

Table 1

Dependence of the optical density of the complex on the pH of the solution

The tabulated data make it possible to draw the following conclusions:

1. The optimal medium for formation of the complex is \(\mathrm{pH}=5.30\)—\(5.50\). The solution medium was created with acetic acid and alkali, and was monitored on an LPU-01 pH meter.

2. The maximum optical density of the complex develops 2–3 min after addition of the reagent and thereafter continuously decreases.

Diphenylcarbazone also forms complexes with ions of silver, aluminum, cadmium, cobalt, divalent copper, trivalent iron, divalent manganese, nickel, divalent lead, trivalent antimony, and divalent tin.

Table 2

Dependence of optical density on time

The study investigated the effect of aluminum, iron, antimony, titanium(IV), cadmium, copper, manganese, nickel, and lead on the formation of the vanadium complex.

It was established that iron, antimony, titanium, cadmium, cobalt, and lead, in amounts equal to the vanadium content, only slightly change the optical density of the vanadium complex, whereas aluminum, copper, and nickel increase the density by 0.015–0.02, 0.035–0.04, and 0.013, respectively. Therefore, for the determination of vanadium with diphenylcarbazone, we chose the standard-addition method, which takes into account the influence of foreign ions.

All investigations were carried out with a vanadium concentration equal to 2.5 µg in 25 ml. The proposed method for determining vanadium in oils is based on decomposition of the oil sample with concentrated sulfuric acid in a ratio of (1:1) to the sample, with the addition of 1 ml of 30% hydrogen peroxide (for oxidation of vanadium), heating on a sand bath until a semi-coke forms, followed by ignition of it in a muffle furnace at 520–540°C; dissolution, with heating, of the ash residue in a minimal amount of hydrochloric acid (1:1), followed by dilution with bidistillate to a definite volume depending on the sample taken.

For analysis, an aliquot containing 0.15–0.3 g of oil is taken. To the aliquot are added one drop of 0.1% alcoholic phenolphthalein solution and, successively, 2 N NaOH solution and acetic acid (1:7) until the color of the indicator changes, 6 ml of acetate buffer with pH = 5, then 5 ml of 0.1% acetone solution of diphenylcarbazone. The volume of the solution in the flask is brought to the mark with water and mixed. After 3 min, the optical density of the complex is measured on an FEKN-57 in cuvettes with a layer thickness of 50 mm, using green light filter No. 5 with a transmission maximum of 536 nm. The reference solution is bidistilled water. In parallel, an oil sample with an addition and a blank sample are carried through all stages of the analysis.

The vanadium content is calculated by the formula

\[ \% V = \frac{C_v (D_{\mathrm{p}} - D_0) V \cdot 100} {(D_{\mathrm{p}/\mathrm{d}} - D_{\mathrm{p}}) P \cdot V_a 10^6}. \]

$C_v$ — addition of vanadium, in µg;
$D_{\mathrm{p}}$ — optical density of the sample;
$D_0$ — optical density of the blank sample;
$V$ — volume of the volumetric flask with the oil or petroleum-product sample;

$D_{\mathrm{p/d}}$ — optical density of the sample with added vanadium;
$P$ — oil sample weight;
$V$ — aliquot volume.

Using the described procedure, 15 oil fields were analyzed. The results of the analyses were processed by methods of mathematical statistics and are presented in Table 3.

Table 3

Vanadium content

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