Amalgam Polarographic Determination of Copper and Lead in Crude Oils of Western Siberia at Elevated Temperature of the Analyzed Solution
Abstract Generated abstract
This paper examines amalgam polarography with accumulation for determining trace copper and lead in crude oils from Western Siberia, addressing limitations of slower and less accurate spectral methods. The authors study temperature effects on anodic peaks after matrix separation and in direct oil analysis using alcohol and benzene based diluents with indifferent electrolyte, establishing practical electrolysis conditions for lead and copper determinations. Heating during preliminary electrolysis substantially increases sensitivity, while direct analysis shortens sample preparation and gives relative errors reported at about 10 to 15 percent. The developed procedures are applied to oils from several fields, with tabulated copper and lead contents and statistical estimates of uncertainty.
Full Text
AMALGAM-POLAROGRAPHIC DETERMINATION OF COPPER AND LEAD IN WEST SIBERIAN OILS AT ELEVATED TEMPERATURE OF THE ANALYZED SOLUTION
Yu. A. Karbainov, G. N. Sutyagina, S. N. Tarzemyanova,
T. I. Burmistrova, A. N. Zykova
(Presented by the scientific seminar of the Department of Analytical Chemistry)
By the present time, a considerable amount of factual material has already been accumulated on methods—primarily spectral methods—for determining metallic impurities in oils. However, all these methods are not free from substantial shortcomings, the chief of which are labor intensity, the long duration of the analysis, and the low accuracy of determination.
One of the promising methods for oil analysis may be the method of amalgam polarography with accumulation (the APA method), which advantageously combines high sensitivity (down to \(10^{-8}\%\) and lower) with simplicity of instrumental design. The use of the APA method in oil analysis can be carried out in two directions: with preliminary separation of the base (for example, by ozonation), or by direct amalgam-polarographic analysis of this base. Presented below are the results of investigations obtained using both approaches.
To determine the optimum conditions for the amalgam-polarographic determination of copper and lead in oils after preliminary separation of the base at an elevated temperature of the analyzed solution, studies were carried out on the influence of temperature on the anodic peaks of lead against a background of \(0.1N\) HCl and of copper against a background of \(0.1N\) NH\(_4\)F.
The temperature of the analyzed solution was measured directly in the electrolyzer with an accuracy of \(\pm 0.2^\circ\)C. All series of experiments were duplicated 3–4 times. In studying the influence of temperature on the anodic peaks at the stage of preliminary electrolysis, electrolysis was carried out at the limiting-current potential under conditions of variable temperature. The experimental procedure is described in detail in [1].
As an example, Fig. 1 (curve 1) presents the dependence of the height of the anodic peaks of lead against a background of \(0.1N\) HCl on temperature. Analysis of these data shows that increasing the temperature of the analyzed solution from 22 to 75°C makes it possible to increase the sensitivity of the determination by an average factor of 7–8.
The results of studies on the influence of temperature on the anodic peaks of lead at the stage of anodic dissolution of the amalgam are also presented in Fig. 1 (curve 2). The experimental conditions were as follows: concentration of metal ions in solution \(4.2 \cdot 10^{-7}\ \frac{\text{g-ion}}{\text{l}}\); electrolysis time 3 min; mercury-drop radius \(r = 0.04\) cm; rate of potential change \(\omega = 1.4 \cdot 10^{-2}\) V/sec. According to these data, the sensitivity of deter-
Fig. 1. Dependence of the height of the anodic peaks of lead on temperature.
... separation of lead, by changing the temperature at the stage of anodic dissolution from 16 to 75°C, can be increased only by a factor of 1.5. Thus, in practical work it is advisable to raise the temperature of the background solution only at the stage of preliminary electrolysis.
According to our data, the dependence of the height of the anodic peaks of lead and copper on the concentration of ions in solution in the range \(1 \cdot 10^{-7} \div 5 \cdot 10^{-8}\)
\[ \frac{\text{g-ion}}{\ell} \]
is proportional. The numerical values of the potentials of the limiting current for copper and lead under the conditions studied are, respectively: \(-0.5\) and \(-0.7\) V relative to the saturated calomel electrode.
The amalgam-polarographic determination of lead and copper in oils after preliminary separation of the matrix is reduced to the following. The sulfate ash obtained after decomposition of the corresponding oil sample is treated with hydrochloric acid, tridistillate, and each time the contents are evaporated to a moist residue, after which it is dissolved either in 0.1 N HCl (for the determination of lead) or in 0.1 N NH\(_4\)F (for the determination of copper). Electrolysis of the solution is carried out at \(t = 70^\circ\)C in a sealed cell equipped with a reflux condenser with a water seal [1].
The sensitivity of the method under the developed conditions is \(1 \cdot 10^{-6} \div 5 \cdot 10^{-7}\)%. The duration of analysis of four parallel ash samples and one blank sample is about 1.5 hours.
The use of the APN method for direct analysis of oils has a number of advantages in comparison with those described above: the total analysis time is considerably shortened by eliminating the stage of mineralization of the oil or petroleum-product sample; the reproducibility of the analytical results is improved, since at the stage of mineralization of the matrix both losses of part of the impurity being determined and contamination introduced from outside are possible; the consumption of reagents required for carrying out the analysis is reduced, etc.
Carrying out amalgam-polarographic measurements directly in oils is possible only when certain matrix diluents are used in the presence of an indifferent electrolyte. The most effective diluent of oils among those studied by us [2] is an ethyl alcohol–benzene mixture at a component ratio of 1:1 or 1:2; ammonium nitrate may be used as the electrolyte.
The oil sample taken for analysis was preliminarily subjected to alcohol treatment for semiquantitative separation of organics (mainly saturated hydrocarbons) from the matrix. These studies were carried out as follows. The appropriate oil sample was mixed with a 25-fold amount of ethyl alcohol.
Table 1
Lead content
| Field | Well no. | Number of measurements, \(n\) | Arithmetic mean, \(\bar{x}\cdot 10^{5}\) | Standard error, \(S_{\bar{x}}\cdot 10^{5}\) | Student parameter, \(t_{0.95}\) | Absolute deviation, \(E\) | Experimental result \(\cdot 10^{5}\) |
|---|---|---|---|---|---|---|---|
| 1. Ubinskoye | 305 | 5 | 0.81 | 0.02 | 2.78 | 0.05 | \(0.81 \pm 0.05\) |
| 2. Ubinskoye | 315 | 5 | 0.35 | 0.018 | 2.78 | 0.049 | \(0.35 \pm 0.05\) |
| 3. Ubinskoye | 324 | 5 | 4.10 | 0.28 | 2.78 | 0.778 | \(4.10 \pm 0.78\) |
| 4. Ubinskoye | 328 | 5 | 1.57 | 0.06 | 2.57 | 0.154 | \(1.57 \pm 0.154\) |
| 5. Ubinskoye | 346 | 5 | 2.23 | 0.12 | 2.78 | 0.33 | \(2.23 \pm 0.33\) |
| 6. Kartopinskoye | 8 | 5 | 0.39 | 0.009 | 2.78 | 0.025 | \(0.39 \pm 0.025\) |
| 7. Tolumskoye | 13 | 5 | 4.86 | 0.14 | 2.78 | 0.389 | \(4.86 \pm 0.39\) |
| 8. Luginetskoye | 162 | 8 | 1.27 | 0.045 | 2.36 | 0.106 | \(1.27 \pm 0.106\) |
| 9. Severnoye | 205 | 5 | 2.84 | 0.14 | 2.78 | 0.39 | \(2.84 \pm 0.39\) |
| 10. Severnoye | 206 | 5 | 2.36 | 0.13 | 2.78 | 0.36 | \(2.36 \pm 0.36\) |
| 11. Severnoye | 207 | 5 | 3.52 | 0.19 | 2.57 | 0.50 | \(3.52 \pm 0.50\) |
| 12. Bagryanovskoye | 330 | 5 | 0.77 | 0.015 | 2.78 | 0.041 | \(0.77 \pm 0.04\) |
| 13. Samotlorskoye | 324 | 5 | 1.34 | 0.08 | 2.78 | 0.22 | \(1.34 \pm 0.22\) |
| 14. Yuzhno-Teremshanskoye | 335 | 5 | 1.85 | 0.06 | 2.78 | 0.13 | \(1.85 \pm 0.13\) |
| 15. Megionskoye | 514 | 5 | 0.33 | 0.006 | 2.78 | 0.016 | \(0.33 \pm 0.016\) |
Table 2
Copper content
| No. | Field | Well No. | Interval | Number of measurements, \(n\) | Arithmetic mean, \(\bar{x}\cdot 10^5\) | Standard error of the arithmetic mean, \(S_{\bar{x}}\cdot 10^5\) | Student parameter, \(t_{0.95}\) | Absolute deviation, \(E\) | Results of determination, \(\cdot 10^5\) |
|---|---|---|---|---|---|---|---|---|---|
| 1. | Ubinskoe | 305 | 1851—1858 | 5 | 4,47 | 0,038 | 2,78 | 0,104 | 4,47±0,10 |
| 2. | Ubinskoe | 315 | 1880—1888 | 5 | 0,533 | 0,005 | 2,78 | 0,01 | 0,53±0,01 |
| 3. | Ubinskoe | 324 | 1903—1916 | 5 | 0,58 | 0,026 | 2,78 | 0,072 | 0,58±0,07 |
| 4. | Ubinskoe | 328 | 1879—1893 | 5 | 0,567 | 0,044 | 2,78 | 0,12 | 0,57±0,12 |
| 5. | Ubinskoe | 346 | 1898—1913 | 5 | 0,251 | 0,009 | 2,78 | 0,025 | 0,25±0,02 |
| 6. | Kartopinskoe | 8 | 1884—1894 | 5 | 1,29 | 0,044 | 2,78 | 0,123 | 1,29±0,12 |
| 7. | Tolumskoe | 13 | 2089—1998 | 5 | 0,80 | 0,048 | 2,78 | 0,133 | 0,80±0,13 |
| 8. | Luginetskoe | 162 | 2332—2332 | 6 | 0,30 | 0,012 | 2,57 | 0,031 | 0,30±0,03 |
| 9. | Severnoe | 205 | 1558—1569 | 4 | 0,26 | 0,015 | 3,18 | 0,048 | 0,26±0,05 |
| 10. | Severnoe | 206 | 1766—1779 | 5 | 0,694 | 0,008 | 2,78 | 0,022 | 0,69±0,02 |
| 11. | Severnoe | 207 | 1898—1900 | 4 | 1,20 | 0,085 | 2,78 | 0,23 | 1,20±0,23 |
| 12. | Vartovskoe | 330 | 2570—2556 | 5 | 1,21 | 0,044 | 2,78 | 0,123 | 1,21±0,12 |
| 13. | Samotlorskoe | 324 | 58 | 5 | 1,12 | 0,013 | 2,78 | 0,037 | 1,12±0,037 |
| 14. | Yu-Teremshanskoe | 335 | 7767—1785 | 5 | 2,90 | 0,032 | 2,78 | 0,08 | 2,90±0,08 |
| 15. | Megionskoe | 514 | 2124—2139 | 5 | 1,48 | 0,016 | 2,78 | 0,044 | 1,48±0,04 |
(96%) was added, and the resulting mixture was shaken vigorously for 10–15 min. The alcohol phase was separated, transferred to a quartz cup that had been checked for purity, and carefully evaporated. The residue was treated with a mixture of concentrated hydrochloric and sulfuric acids, evaporated to a moist residue, and dissolved in 0.1 N HCl or 0.1 N NH₄F. In parallel, a “blank” experiment was carried through all stages of the analysis. For analysis, an aliquot portion of the solution was taken and polarographed. In this way the amount of copper and lead that had passed into the alcohol phase was estimated. As a result of these investigations it was shown that extraction of copper and lead with 96% ethyl alcohol under the specified conditions practically did not occur.
The oil sample obtained after alcohol treatment was diluted with 0.1 N NH₄FO₃ solution (ethyl alcohol–benzene), and an aliquot portion was polarographed. Electrolysis of the solution was carried out at \(t = 60^\circ\mathrm{C}\). Figure 2 presents polarograms of the anodic peaks of lead and copper,
Fig. 2. Polarograms of the anodic peaks of copper and lead: a) copper, b) lead. Concentration of ions in the solution: 2 — \(5 \cdot 10^{-7}\); 3 — \(1 \cdot 10^{-6}\); 4 — \(2.5 \cdot 10^{-6}\) g-ion/l.
obtained from the TN-23 oil sample (polarograms No. 1) and by the method of additions of a standard solution (polarograms No. 2–4). The relative error, according to statistical processing of 6 anodic peaks obtained in parallel experiments, does not exceed 10–15%. The sensitivity of the determination is \(5 \cdot 10^{-6}\%\).
The methods described above were used for the analysis of West Siberian oils for copper and lead content. The results of the analysis are presented in Tables 1 and 2.
REFERENCES
-
A. G. Stromberg, Yu. A. Karbainov, S. N. Karbainova. “Zavodskaya laboratoriya,” 3, 257 (1970).
-
Yu. A. Karbainov, V. S. Smorodinov, S. N. Karbainova, G. N. Sutyagina. Study of the amalgam-polarographic behavior of certain elements in mixed solvents used as oil diluents.—In the present collection.