Amalgam-polarographic determination of lead and copper in gasoline fractions under conditions of the ammonium amalgam effect
Abstract Generated abstract
This paper describes an amalgam polarographic method with accumulation for determining trace lead and copper directly in gasoline fractions, aiming to reduce analysis time and avoid sample mineralization. The authors evaluate benzene, 80 percent ethanol mixtures containing ammonium chloride as supporting media, examine solution viscosity and conductivity, and optimize conditions for applying the ammonium amalgam effect. Under selected conditions, this effect increased sensitivity by about 8 to 10 times while preserving proportionality between anodic peak height and analyte concentration. A procedure using sample evaporation, dilution in 0.5 N ammonium chloride in benzene, ethanol, electrolysis, and standard additions gave reported lead and copper contents in reforming feedstock fractions with a sensitivity of approximately 5·10^-8 to 1·10^-7 percent.
Full Text
AMALGAM POLAROGRAPHIC DETERMINATION OF LEAD AND COPPER IN GASOLINE FRACTIONS UNDER THE CONDITIONS OF THE AMMONIUM AMALGAM EFFECT
Yu. A. Karbainov, G. N. Sutyagina, S. N. Tarzemyanova
(Presented by the scientific seminar of the Department of Analytical Chemistry)
The use of the method of amalgam polarography with accumulation for the direct analysis of petroleum and petroleum products (in particular, gasoline fractions) has a number of advantages: the total analysis time is considerably reduced by eliminating the stage of mineralization of the sample; the consumption of reagents required for carrying out the analysis is reduced; and the instrumentation of the analytical process is simplified.
Carrying out amalgam-polarographic measurements directly in gasoline fractions is possible only when certain diluents of the base are used in the presence of an indifferent electrolyte. According to our data, the most effective diluent is an ethyl alcohol—benzene mixture. Against the background of this diluent, a small residual current is observed, and the results obtained are readily reproducible. The viscosity and electrical conductivity of a 0.5 N NH₄Cl solution in a benzene—80% ethanol mixture were studied at various component ratios. The data obtained are presented in Table 1.
Table 1
| Solution composition | Ratio | Viscosity, $\eta$, cP | Electrical conductivity, $\chi$, $\Omega^{-1}\cdot\mathrm{cm}^{-1}$ |
|---|---|---|---|
| Benzene + 80% ethanol | 1 : 1.0 | 0.880 | $3\cdot10^{-3}\div5\cdot10^{-3}$ |
| Benzene + 80% ethanol | 1 : 1.5 | 0.960 | $3\cdot10^{-3}\div5\cdot10^{-3}$ |
| Benzene + 80% ethanol | 1 : 2.0 | 1.012 | $3\cdot10^{-3}\div5\cdot10^{-3}$ |
| Benzene + 80% ethanol | 1 : 2.5 | 1.074 | $3\cdot10^{-3}\div5\cdot10^{-3}$ |
| Benzene + 80% ethanol | 1 : 3.0 | 1.120 | $3\cdot10^{-3}\div5\cdot10^{-3}$ |
According to these data, in the amalgam-polarographic determination of elements in gasoline fractions, it is advisable to use, as the diluent, a benzene—80% C₂H₅OH mixture at component ratios of 1 : 1 or 1 : 2.
To increase the sensitivity of the amalgam-polarographic determination of lead and copper in gasoline fractions, the ammonium amalgam effect [1] was used. It has been shown that the sensitivity of the method in determining lead and copper under the conditions of the ammonium amalgam effect
increases by a factor of 8–10. The dependence of the peak height on the concentration of lead and copper ions in solution was studied. It was found that use of the ammonium amalgam effect does not disturb the proportional relationship between the height of the anodic peak and the concentration of the impurity in solution.
The influence of various factors (concentration of the ammonium salt, initial electrode radius, solution composition, etc.) on the limiting radius of an ammonium amalgam drop was studied. According to the data obtained, the optimum conditions for using the ammonium amalgam effect in the APP method are: \(r_0 = 0.035\ \mathrm{cm}\); \([\mathrm{NH}_4^+] = 0.5\ N\), \(\varphi_{\mathrm{e}} = -2.8\ \mathrm{V}\); \(\tau = 3\ \mathrm{min}\).
The procedure for determining lead and copper in gasoline fractions was as follows.
The gasoline sample taken for analysis was carefully evaporated in a quartz cup at \(57^\circ\mathrm{C}\) on a water-heated electric bath with a NOT-4-8 type temperature regulator. The moist residue was diluted with 5 ml of \(0.5\ N\) \(\mathrm{NH}_4\mathrm{Cl}\) in a benzene–80% ethanol mixture (1:2) and polarographed. Electrolysis was carried out at \(\varphi_{\mathrm{e}} = -2.8\ \mathrm{V}\) for 3 min. When the potential was decreased to \(-1.0\ \mathrm{V}\), the electrode was held without stirring for 1–2 min. The polarogram was then recorded. The percentage impurity content was determined by the standard-addition method.
Table 2
Content of lead and copper in gasoline fractions
| No. | Gasoline sample | Pb | Cu |
|---|---|---|---|
| 1 | Feedstock for reforming Fr. 62–85°C |
\((3.8 \pm 0.62)\cdot 10^{-8}\) | \((2.4 \pm 0.46)\cdot 10^{-7}\) |
| 2 | Feedstock for reforming Fr. up to 180°C |
\((1.7 \pm 0.31)\cdot 10^{-7}\) | \((3.26 \pm 0.28)\cdot 10^{-7}\) |
The sensitivity of the determination is \(1\cdot 10^{-7} \div 5\cdot 10^{-8}\%\). The results of the analysis of gasoline fractions, processed by the method of mathematical statistics, are presented in Table 2.
REFERENCES
- A. G. Stromberg, Yu. A. Karbainov, S. N. Karbainova, G. N. Sutyagina, Proceedings of the Third All-Union Conference on the Analytical Chemistry of Nonaqueous Solutions and Their Physicochemical Properties, Gorky, 1971, p. 118.