STUDY OF THE AMALGAM-POLAROGRAPHIC BEHAVIOR OF CERTAIN ELEMENTS IN MIXED SOLVENTS USED AS DILUENTS OF PETROLEUM

Yu. A. Karbainov, V. S. Smorodinov, S. N. Karbainova,
G. N. Sutyagina

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

To determine the optimum conditions for the amalgam-polarographic determination of metallic impurities directly in petroleum and petroleum products (for example, gasoline fractions), studies were carried out—using lead, cadmium, and antimony as examples—of the amalgam-polarographic behavior of these elements in various mixed solvents: water–ethanol mixtures, which may serve as one of the components of mixed solvents used as diluents for “crude” petroleum; carbon tetrachloride–ethanol and chloroform–ethanol mixtures, which may serve as diluents for dehydrated petroleum.

Fig. 1. Conductivity of water–ethanol mixtures at a constant electrolyte concentration equal to 0.1 N \(NH_4NO_3\).

As the indifferent electrolyte in this work, \(NH_4NO_3\) was used, and the amalgam-polarographic measurements were therefore carried out against a background of 0.2 N \(NH_4NO_3\) in 96% \(C_2H_5OH\) or another water–ethanol mixture.

It was shown (Fig. 1) that the electrical conductivity of such solutions is, in order of magnitude, not lower than the electrical conductivity of aqueous 0.1 N KCl and is sufficient for carrying out polarographic measurements. In these experiments, the reference electrode used was—

A layer of mercury was placed at the bottom of the electrolyzer, and all potential values in the tables and figures are given relative to this reference electrode. The working electrode was a stationary mercury dropping electrode. The electrode radius was \(r = 0.04\) cm. The studies were carried out using 20, 40, 60, 80, and 96% aqueous–ethanol mixtures as examples.

Table 1 presents the experimentally found values of the potentials of the anodic peaks of antimony, lead, and cadmium, as well as the values of the accumulation potentials for these elements, found from the dependence of \(I_a\) on \(\varphi_e\).

Table 1

It follows from the table that the numerical values of the anodic-peak potentials for Sb, Pb, and Cd under the conditions studied differ considerably from one another, which makes it possible to record the indicated anodic peaks on a single polarogram. As an example, Fig. 2 presents a polarogram of the anodic peaks of Sb, Pb, and Cd recorded in 80% ethyl alcohol.

Fig. 2. Polarogram of the anodic peaks of antimony, lead, and cadmium against a background of 0.1 N \(NH_4NO_3\) in an 80% aqueous–ethanol mixture \([Cd^{\cdot\cdot}] = 7.5 \cdot 10^{-6}\ \frac{\text{g-ion}}{\text{l}}\); \([Sb^{\cdot\cdot\cdot}] = 4.0 \cdot 10^{-6}\ \frac{\text{g-ion}}{\text{l}}\); \([Pb^{\cdot\cdot}] = 5 \cdot 10^{-6}\ \frac{\text{g-ion}}{\text{l}}\). Electrolysis time: 3 minutes.

It follows from Table 1 that, with an increase in the proportion of alcohol in the aqueous–ethanol mixture, the values of the anodic-peak potentials shift toward more positive values. Conversely, the magnitude of the accumulation potentials, with an increase in the alcohol fraction, shifts for all the elements studied toward more negative potential values. The nature of the dependence of the anodic-peak height on the electrolysis potential is presented, for the example of a 40% aqueous–ethanol mixture, in Fig. 3. The decrease in the height of the anodic peak of antimony, beginning from \(\varphi_e = -1.1\) and further in

Fig. 3. Dependence of the height of the anodic peaks of antimony, lead, and cadmium on the electrolysis potential against the background of 0.1 N \(NH_4NO_3\) in a 40% aqueous-ethanol mixture.

the negative region, can be explained by the possibility of formation of the so-called “ammonium amalgam” [1]. In the process of reduction of ammonium ions on the electrode, as shown in [2], only an extremely small part of the product of this reaction, ammonia, goes into the formation of “ammonium amalgam,” while the greater part passes into the solution with formation of \(NH_4OH\). As a result, the medium in the near-electrode layer must be alkaline. The alkaline medium, in turn, leads to hydrolysis of the antimony salt. This, apparently, can explain the decrease in the height of the antimony peak in the region of strongly negative potentials. The decrease in the height of the anodic peak in the strongly negative region may also be associated with the evolution of gaseous hydrogen on the electrode, as a result of which part of the working surface of the electrode is screened.

Figure 4 presents the dependence of the height of the anodic peak in 40, 60, and 80% aqueous-ethanol mixtures on the concentration of antimony, lead,

Fig. 4. Dependence of the height of the anodic peaks on the concentration of antimony, lead, and cadmium ions in solution against the background of 0.2 N \(NH_4NO_3\) in 40% \(C_2H_5OH\) (1), 60% \(C_2H_5OH\) (2), and 80% \(C_2H_5OH\) (3).

and cadmium in solution at \(t = 22^\circ C\). According to these data, the sensitivity of the determination of Sb, Pb, and Cd increases as the proportion of alcohol in the mixture increases in the range 40–80%. It is interesting to note that the sensitivity of the determination of antimony, lead, and cadmium in 20% ethyl alcohol and the sensitivity of the determination in 96% ethyl alcohol are approximately the same. The sensitivity of the determination of these ele-

…elements decreases as the proportion of alcohol in the mixture increases in the range 20–45%. This regularity can be explained using data [3] on the viscosity of water–ethanol mixtures. For clarity, these data are presented in Fig. 5. An increase in the viscosity of water–ethanol

Fig. 5. Viscosity of water–ethanol mixtures according to data [3].

mixtures in the range 10–43% (at 20°C) and 10–50% (at 25°C) leads to a decrease in the diffusion coefficients of antimony, lead, and cadmium ions in this range, which, in turn, other conditions being equal, leads to a decrease in the sensitivity of the determination. Conversely, a decrease in the viscosity of water–ethanol mixtures in the range 50–90% leads to an increase in the diffusion coefficients of the ions of the elements studied and, consequently, to an increase in the sensitivity of the determination. To confirm this conclusion, the values of the diffusion coefficients of antimony, lead, and cadmium ions in water–ethanol mixtures, found by us from the Ilkovič equation at \(t = 22^\circ\mathrm{C}\), are given below (Table 2).

Table 2

Table 3 presents the data obtained by us on the viscosity and electrical conductivity of nonaqueous mixed solvents: carbon tetrachloride–ethanol, chloroform–ethanol, and benzene–ethanol at \(t = 22^\circ\mathrm{C}\) and at different ratios of the components in the mixture, but at a constant concentration of electrolyte in it, equal to 0.1 N \( \mathrm{NH_4NO_3} \). According to these data, the viscosity of the nonaqueous mixed solvents studied increases as the proportion of ethyl alcohol increases. Consequently, when these solvents are used as diluents of petroleum, the sensitivity of amalgam-polarographic determinations should decrease as the proportion of alcohol in the mixture increases.

As a result of the studies carried out on mixed solvents used as diluents of petroleum, the following conclusions may be drawn:

Table 3

1. As one of the components of mixed solvents used as diluents for “crude” petroleum, water–ethanol mixtures may be employed. In this case, in order to achieve the maximum sensitivity of amalgam-polarographic determinations in them, it is best to use mixtures containing 20% or less, or 80% or more, ethyl alcohol. Forty-percent or fifty-percent mixtures are unsuitable, since the viscosity of these mixtures is maximal, and the sensitivity of amalgam-polarographic determinations is correspondingly minimal.

2. As diluents for dehydrated petroleums, the following mixtures may be recommended: carbon tetrachloride–ethanol, chloroform–ethanol, and benzene–ethanol, with the ratio of the components in the mixture either 1 : 2 or 1 : 1. In this case, for carrying out amalgam-polarographic measurements directly in petroleums, it is best to use a benzene–ethanol mixture (1 : 1) or (1 : 2) as the diluent (against the background of this diluent a small residual current is observed, and the results obtained are readily reproducible).

LITERATURE

1. Yu. A. Karbanov, A. G. Stromberg. Journal of Analytical Chemistry, 8, 769 (1965).

2. A. V. Konkova. Abstract of a Candidate’s dissertation. Tomsk, 1969.

3. Chemist’s Handbook, vol. 3, 1965.