POLAROGRAPHY AND AMALGAM STRIPPING POLAROGRAPHY DIRECTLY IN EXTRACTANTS (EAPN)

Yu. A. Karbainov

(Presented by the scientific-methodological seminar of the KhTF)

The combination of extraction with polarography has the following advantages: high selectivity, the possibility of working with both macro- and microconcentrations of a substance, etc. An undesirable operation in this combined method of analysis is re-extraction, since this process complicates and lengthens the analysis and, moreover, leads to additional losses of substance and analytical errors. Recently, a fundamentally new and promising direction has begun to develop in the use of extraction methods in polarographic analysis, the essence of which consists in determining trace impurities directly in extractants. The promise of this direction lies in eliminating re-extraction operations, which leads to a significant reduction in the total analysis time, a reduction in reagent consumption, etc. In addition, eliminating such operations as re-extraction, destruction of the complex, and removal of the organic phase, which are directly or indirectly associated with losses, should lead to improved reproducibility and faster analysis.

Let us consider the use of the indicated method in classical polarography. In work [1], instead of the re-extraction or destruction of the complex by evaporation that was usually used, it was proposed to use, as the background, a homogeneous ternary system, one component of which is an organic substance—the extract. The authors use extraction of diethyldithiocarbonates (DDC) of Tl, Cu, and Pb with chloroform from a citrate buffer solution at pH 11, after which the extract containing DDC complexes of Tl, Pb, and Cu in CHCl₃ is mixed with methyl cellosolve and water in the ratio 25:56:19, after which it is analyzed.

Values of $\varphi_{1/2}$ and $i_d$ are given for the indicated elements. In work [2], the polarographic behavior of DDC complexes of Bi, Cu(+), Tl(+) and Pb was studied on a background containing: 5 ml CHCl₃ + 1.7 ml conc. HCl (H₂SO₄, HClO₄), 11.3 ml methyl cellosolve, and 2 ml H₂O. It was shown that the wave height is proportional to the concentration of metal ions in solution in the range $1 \cdot 10^{-3} \div 1 \cdot 10^{-5}$ g-ion/l. Values of $\varphi_{1/2}$ and of the diffusion-current constants for Bi, Cu(+), Tl, and Pb under the indicated conditions are given. The practical application of the method for analytical purposes is shown by the example of determining $5 \cdot 10^{-4} \div 5 \cdot 10^{-5}\%$ Pb in NaCl reagent.

More promising with respect to sensitivity is the combination of extraction with amalgam stripping polarography with accumulation (ASP).

In works [3, 4, 5] a method of extraction amalgam stripping polarography with accumulation (the EASP method) is proposed, the essence of which consists in determining traces of elements directly in extractants by the method of amalgam stripping polarography with accumulation. The possibility has been shown of determining \(10^{-6} \div 10^{-7}\%\) impurities of Pb, Cu, Tl(+), Bi, In, and Cd directly in chloroform solutions of 8-hydroxyquinoline and dithizone, as well as in an ethyl acetate solution of diethyldithiocarbamate (DDC). In contrast to the previously described ternary systems using water as solvent, in the present case binary nonaqueous systems were studied: extract–methyl alcohol. In the experiments, the salt \(NH_4NO_3\) was used as indifferent electrolyte. The electrical conductivity of chloroform–methanol and ethyl acetate–methanol mixtures was studied at different contents of chloroform and ethyl acetate in the mixture, but at a constant electrolyte concentration in it equal to 0.1 M. It was shown that the electrical conductivity of a chloroform–methanol and ethyl acetate–methanol mixture in a ratio of 1:1 and at the indicated electrolyte concentration corresponds, in order of magnitude, to the electrical conductivity of a 0.1 N KCl solution (\(0.011\ \Omega^{-1}\ \text{cm}^{-1}\)), which fully satisfies the requirements imposed on polarographic analysis. In order to increase the sensitivity of the extraction ASP method, two new techniques are proposed in the cited works. The first consists in carrying out preliminary concentration of the metal in a drop of mercury at the boiling point of the solution. This technique makes it possible to increase the sensitivity of the extraction ASP method, on average, by one order of magnitude. The second technique consists in carrying out preliminary concentration of the metal in a mercury drop under conditions of the ammonium amalgam effect [4] at a temperature close to the boiling point of the solution. This technique makes it possible to increase the sensitivity of the extraction ASP method by more than 1.5 orders of magnitude. With the aid of these techniques, the maximum sensitivity attained for Pb, Cu, Cd, Tl(+), In, and Bi was on average \(2.5 \div 5 \cdot 10^{-9}\) g-ion/l.

The principal factor in increasing the sensitivity of the extraction ASP method under these conditions is an increase by 1.5–2 orders of magnitude in the electrolysis constant. The latter characterizes the intensity of delivery of the substance to the electrode in the process of preliminary electrolysis and depends mainly on the intensity of stirring of the solution. The change in the electrolysis constant as a function of temperature and of the ammonium amalgam effect, separately and under their combined influence, was evaluated. The values of the activation energies of the convective diffusion process were calculated. The practical application of the extraction ASP method was shown using as an example the analysis of LiCl reagent (chemically pure) for the content of Pb, Cu, Cd, and In. In determining \(10^{-6} \div 10^{-7}\%\) Pb and Cd in LiCl reagent, a \(1 \cdot 10^{-4}\%\) solution of dithizone in \(CHCl_3\) [4] was used for extraction; for Cu, a 1% solution of oxine in \(CH_3Cl\) [3]; for In, a 2% solution of diethyldithiocarbamate in ethyl acetate [5]. After completion of extraction, the extract was mixed with a 0.2 N solution of \(NH_4NO_3\) in \(CH_3OH\) and polarographed.

The total duration of one analysis is 15–20 minutes. In works [6, 7, 8] the reversibility of the electroreduction process of Pb, Cu, Cd, Tl, In, and Bi in extracts at a mercury dropping electrode was studied. It was shown that the electroreduction process of the ions Tl(+), Cu(2+), In(3+), and Bi(3+) in all the above-mentioned binary systems is irreversible, whereas that of Pb(2+) and Cd(2+) ions is reversible.

Using the De Ford and Hume method, the composition and instability constants of Pb and Cd complexes with oxine, dithizone, and DDC in extracts were studied. The concentration of oxine in the chloroform–methanol mixture was varied within the range 0.1–3.1% [6], the concentration of dithizone from \(5 \cdot 10^{-3}\) to 0.1% [7], and the concentration of diethyldithiocarbamate in the ethyl acetate–methanol mixture up to 0.1–3.1% [8].

By Leden’s graphical method, the composition and instability constants of the complexes predominating in solution were calculated.

The method of extraction amalgam polarography with accumulation is still at the very beginning of its development, and in the near future one may expect its wide application in the analysis of high-purity materials.

In those cases where there is no need to carry out extraction separation of elements, the most promising approach (acceleration of the analysis and improvement of its reproducibility, reduction of reagent consumption) should be considered to be carrying out the analysis without separating the matrix. In this case, obviously, the most favorable situation is when the matrix itself serves as the electrolyte in the analysis.

In [9], the electrical conductivity of binary mixtures of \(\mathrm{SiCl_4}\) with aliphatic oxygen-containing compounds was studied. It was shown that \(\mathrm{SiCl_4}\) in a mixture with \(\mathrm{H—C_3H_7OH}\) (93.4%) has an electrical conductivity of the same order of magnitude as the conductivity of a \(0.1\,N\) KCl solution. In [10, 11], the possibility was shown of determining \(10^{-6}\)–\(10^{-7}\)% impurities of a number of elements directly in a medium of high-purity silicon tetrachloride.

REFERENCES

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3. A. G. Stromberg, Yu. A. Karbainov. In: Collected Works of Postgraduate Students of the Tomsk Polytechnic Institute (in press).

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10. Yu. A. Karbainov, A. G. Stromberg. ZhAKh, 11, (1964).

11. Yu. A. Karbainov, A. G. Stromberg. Izv. TPI (in press).