Yu. A. KARBAINOV, A. G. STROMBERG

STUDY OF THE ELECTRICAL CONDUCTIVITY OF BINARY MIXTURES WITH ALIPHATIC OXYGEN-CONTAINING COMPOUNDS FOR THE DETERMINATION OF MICROIMPURITIES IN HIGH-PURITY SiCl₄

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

Stranathan and Strong [1] in 1927 studied the viscosity and electrical conductivity of the system SnCl₄—CH₃COOH. Later, the electrical conductivity, viscosity, and density of this system were studied in greater detail by Usanovich and Kalabegovskaya [2]. In this system, stannic chloride is regarded by the authors as an aprotic acid, and glacial acetic acid as a base [3]. The sharp increase in electrical conductivity observed at 22–23 mol/% SnCl₄ is explained by the authors by the presence of acid–base interaction leading to the formation of a compound of complex character.

As far as we know, there are no data in the literature on the electrical conductivity of binary mixtures of silicon tetrachloride with organic oxygen-containing compounds. The task of the present work is to study the electrical conductivity of binary mixtures of silicon tetrachloride with aliphatic oxygen-containing compounds in order to find a suitable system for determining microimpurities \((10^{-5} \div 10^{-7})\) in high-purity silicon tetrachloride by the method of amalgam polarography with accumulation. For measuring electrical conductivity, which was carried out by the Kohlrausch method, a vessel specially designed for solutions with high resistance was used. The installation circuit included: a GZ-2 audio-frequency generator, an MO-47 direct-current bridge, and, as a null instrument, a Duoskop electronic oscilloscope. Silicon tetrachloride was stored in sealed quartz ampoules, which were broken immediately before the measurements. The studies were carried out under conditions that completely excluded access of moisture. But even in this case the formation of SiCl₄ decomposition products was sometimes observed, creating a certain difficulty in the work. All reagents were thoroughly dehydrated. Glacial acetic acid, for example, was additionally dehydrated by repeated freezing out. Sodium acetate was dehydrated by ignition in a crucible. Absolute n-propyl and isopropyl alcohols were used in the work. We studied the electrical conductivity of binary mixtures of silicon tetrachloride with acetic acid, chloroacetic acid, ethyl ester of chloroacetic acid, anisole, n-propyl and isopropyl alcohols at 18° in the concentration range from 0 to 30 mol.% SiCl₄.

In order to increase the electrical conductivity of the system SiCl$_4$—CH$_3$COOH, additions of dehydrated sodium acetate were made. The measurement results are collected in Table 1 and shown graphically in Fig. 1,

Table 1

where the molecular percentages are plotted on the abscissa, and the specific electrical conductivity on the ordinate. On all curves a sharp maximum of electrical conductivity is observed in the region of 6–8 mol. % SiCl$_4$. Curve 1 of the specific electrical conductivity passes through a sharp maximum at a concentration of 6.65 mol. % SiCl$_4$, $\chi_{\max}=12.6\cdot 10^{-6}\ \Omega^{-1}\text{cm}^{-1}$. Further, the curve of specific electrical conductivity also drops sharply, and already at 50 mol. % SiCl$_4$, $\chi=0.40\cdot 10^{-6}$. As a result of adding sodium acetate, calculated as 1 g per 200 ml CH$_3$COOH, $\chi_{\max}=19.8\cdot 10^{-6}$; and when the concentration of sodium acetate is increased fourfold, the maximum value of the electrical conductivity reaches $\chi_{\max}=63.0\cdot 10^{-6}$, i.e., approximately five times greater than the conductivity of the binary system. With increasing concentration of sodium acetate, the maximum of electrical conductivity shifts toward higher mol. % SiCl$_4$. In the case with monochloroacetic acid (curve 2), $\chi_{\max}=15.4\cdot 10^{-6}$ at 3.3 mol. % SiCl$_4$; with the ethyl ester of m-chloroacetic acid, $\chi_{\max}\simeq 4.1\cdot 10^{-6}$; in the case with anisole, $\chi_{\max}\simeq 4.6\cdot 10^{-6}$. The system SiCl$_4$—CH$_3$CH$_2$CH$_2$OH has the greatest value of electrical conductivity, where $\chi_{\max}=12.86\cdot 10^{-3}$ at 6.3 mol. % SiCl$_4$. For isopropyl alcohol, the maximum value of the electrical conductivity is $\chi_{\max}=9.91\cdot 10^{-3}$. This system can be used for the amalgam-polarographic determination of microimpurities in high-purity silicon tetrachloride. The systems SiCl$_4$—HCOOH and SiCl$_4$—C$_6$H$_5$N are of definite interest.

Theoretically, it could have been predicted that the system SiCl$_4$—HCOOH should be analogous to the system SiCl$_4$—CH$_3$COOH. However, when the components were mixed, their vigorous interaction was observed, accompanied by strong heating of the reaction-

molecular weight, and the formation of a gel-like product. It goes without saying that, for the reasons indicated above, it was not possible to study the electrical conductivity of this system. Attempts to measure the electrical conductivity of the system \(\mathrm{SiCl_4—C_5H_5N}\) were likewise unsuccessful. With even the slightest additions of \(\mathrm{SiCl_4}\) to pyridine, the components reacted vigorously, with formation of by-products.

Fig. 1. Specific electrical conductivity of systems with aliphatic oxygen-containing compounds in the concentration range from 0 to 30 mol.% \(\mathrm{SiCl_4}\) at \(18^\circ\mathrm{C}\): curve 1—acetic acid; 2—chloroacetic acid (\(65^\circ\)); 3—acetic acid + sodium acetate (1 g per 200 ml); 4—acetic acid + sodium acetate (2 g per 100 ml); 5—isopropyl alcohol; 6—\(n\)-propyl alcohol.

We are inclined to explain the origin of the electrical conductivity by the formation of current-conducting complexes with \(\mathrm{SiCl_4}\). As Usanovich [4] states, in the case of ethers, compounds of the oxonium type should be formed. In our case it is preferable to assume the following scheme for their formation and dissociation:

\[ \begin{aligned} &\begin{array}{c} \mathrm{C_2H_5}\\[-2pt] \mathrm{ClH_2C{-}C(=O)} \end{array} \!\!>\mathrm{O}\cdots \mathrm{SiCl_4} \quad \begin{array}{c} \\[-2pt] \cdots \mathrm{SiCl_4} \end{array} \;\rightleftarrows\; \left[ \begin{array}{c} \mathrm{C_2H_5}\\[-2pt] \mathrm{ClH_2C{-}C(=O)} \end{array} \!\!>\mathrm{O}\cdots \mathrm{SiCl_3} \quad \begin{array}{c} \\[-2pt] \cdots \mathrm{SiCl_3} \end{array} \right]^{++} + 2\mathrm{Cl^-} \end{aligned} \]

\[ \begin{aligned} &\begin{array}{c} \mathrm{CH_3}\\[-2pt] \mathrm{C_6H_5} \end{array} \!\!>\mathrm{O}\cdots \mathrm{SiCl_4} \quad \begin{array}{c} \\[-2pt] \cdots \mathrm{SiCl_4} \end{array} \;\rightleftarrows\; \left[ \begin{array}{c} \mathrm{CH_3}\\[-2pt] \mathrm{C_6H_5} \end{array} \!\!>\mathrm{O}\cdots \mathrm{SiCl_3} \quad \begin{array}{c} \\[-2pt] \cdots \mathrm{SiCl_3} \end{array} \right]^{++} + 2\mathrm{Cl^-} \end{aligned} \]

In the system \(\mathrm{SiCl_4—CH_3COOH}\), silicon tetrachloride plays the role of an aprotonic acid. The dissociation of acetic acid under these conditions-

conditions is ruled out because of the absence of a proton acceptor. Here one may propose the following scheme for the formation of a complex compound:

\[ \mathrm{SiCl_4\cdot 2CH_3COOH + CH_3COOH \rightleftarrows SiCl_4\cdot 3CH_3COOH.} \]

Table 2

Initially, \(\mathrm{SiCl_4\cdot 2CH_3COOH}\) is formed—a compound possessing stronger acidic properties than \(\mathrm{CH_3COOH}\); it enters into an acid–base interaction with acetic acid, with the formation of \(\mathrm{SiCl_4\cdot 3CH_3COOH}\). The latter, most probably, dissociates according to the scheme:

\[ \mathrm{SiCl_4\cdot 3CH_3COOH \rightleftarrows SiCl_3\cdot 3CH_3COOH^{+} + Cl^{-}.} \]

Analogously in the case of the alcohols, since the alcohols here play, just as acetic acid does, the role of a base, \(\mathrm{SiCl_4}\) plays the role of an aprotic acid.

Conclusions

1. The electrical conductivity of binary mixtures of silicon tetrachloride with aliphatic oxygen-containing compounds (acetic acid, chloroacetic acid, ethyl ester of chloroacetic acid, anisole, propyl alcohol, and isopropyl alcohol) was studied at \(18^\circ\mathrm{C}\) over the concentration range from 0 to 30 mole % silicon tetrachloride. In addition, the electrical conductivity of one ternary system, silicon tetrachloride—acetic acid—sodium acetate, was studied at different concentrations of sodium acetate.

2. It has been shown that in all binary systems, at a silicon tetrachloride content of 6–8 mole %, a maximum in electrical conductivity is obtained. The greatest value of the electrical-conductivity maximum among the systems studied is observed for binary systems with propyl and isopropyl alcohols \((0.013\) and \(0.010\ \mathrm{ohm^{-1}\,cm^{-1}})\). The addition of sodium acetate to the binary system silicon tetrachloride (8 mole %)—acetic acid (92 mole %) makes it possible to increase the electrical conductivity by a factor of 5 in comparison with the conductivity of the binary system.

3. The question of the role of complex formation in increasing the electrical conductivity of the systems studied is discussed. In the case with acetic acid, for example, it is assumed that first a complex compound of the type \(\mathrm{SiCl_4\cdot 2CH_3COOH}\) is formed, which, being a strong complex acid, enters into acid–base interaction with \(\mathrm{CH_3COOH}\). As a result, the compound \(\mathrm{SiCl_4\cdot 3CH_3COOH}\) is formed, which ensures the corresponding electrical conductivity of the system by dissociating into ions:

\[ \mathrm{[SiCl_4\cdot 3CH_3COOH] \rightleftarrows [SiCl_3\cdot 3CH_3COOH]^{+} + Cl^{-}.} \]

1. The system silicon tetrachloride (92%)—propyl alcohol (8%)—in terms of the magnitude of its electrical conductivity should be suitable for

for the determination of \(10^{-6} \div 10^{-7}\%\) impurities in high-purity silicon tetrachloride by the method of amalgam polarography with accumulation.

References

1. I. D. Stranathan, I. Strong. J. Phys. Chem. 31, 1420, 1927.
2. M. Usanovich, E. Kalabanovskaya. The system \(\mathrm{SnCl_4}\)—\(\mathrm{CH_3COOH}\). ZhOKh 7, 1235 (1947).
3. M. Rabinovich. Physicochemical analysis of the system \(\mathrm{SnCl_4}\)—\(\mathrm{CH_3COOH}\). Ukrainian Chemical Journal 3, 410 (1928).
4. M. Usanovich. Electrochemistry of ethereal solutions. ZhOKh 6, 452 (1932).