ELECTRICAL CONDUCTIVITY, VISCOSITY, AND DENSITY OF THE p-NITROSODIMETHYLANILINE–PYRIDINE SYSTEM

A. S. NAUMOVA

(Presented by Professor Doctor B. V. Tronov)

The literature gives many examples of binary organic systems whose components form complex molecular compounds containing 3, 4, or a larger number of molecules of one of the components. Since such systems have been studied chiefly by the melting-point method [1, 2], it was of interest to investigate them by other methods of physicochemical analysis, in order to trace the mutual relation of the components in the liquid phase.

Kremennoy [3] carried out a thermal analysis of the nitrosodimethylaniline–pyridine system. He found the presence in the system of the compound

\[ n\text{-}(CH_3)_2NC_6H_4NO \cdot 4C_5H_5N \]

with a congruent melting point of 4.3° (p-nitrosodimethylaniline melts at 92.5–93.5°, pyridine at −42°). We studied the electrical conductivity, viscosity, and density of this system at temperatures of 30, 70, and 90°.

For the work, chemically pure nitrosodimethylaniline was taken, with m.p. 84°. The pyridine was dried for a long time with potassium hydroxide, after which it was distilled. The pyridine purified in this way had b.p. 114–115°C at 743 mm.

Electrical conductivity was measured by the Kohlrausch method in a closed vessel with platinized platinum electrodes. A tube audio-frequency generator with an amplifier was used as the current source. Viscosity was determined in an Ostwald viscometer of the closed type. Density was studied in a pycnometer of volume 1 ml.

When the components were mixed, considerable cooling was observed. The measurement results are given in Tables 1 and 2. The isotherms of the properties studied are combined in one diagram.

The dependence of the specific electrical conductivity on concentration is represented by curves 4, 5, 6 (Fig. 1). It follows from Fig. 1 that, when pyridine is added to nitrosodimethylaniline, the electrical conductivity increases, passes through a maximum, and then decreases. The maximum is more sharply expressed on the isotherm corresponding to 30°, where it occurs at 70 mole % pyridine. With increasing temperature the maximum becomes flatter and gradually shifts to about 55 mole % pyridine (at 90°).

Curve 2 (Fig. 1), constructed from the data of Table 2 and depicting the dependence of viscosity on composition (at 90°), shows that the viscosity of the system decreases rather rapidly when pyridine is added to nitrosodimethylaniline. The viscosity isotherm is a smooth curve convex toward the concentration axis (the viscosity isotherms for 30 and 70° have an analogous form). With increasing temperature the viscosity decreases (Table 2).

Table 1

Table 2

Curve 3 (Fig. 1), expressing the dependence of density on concentration (at 90°), is significantly convex away from the composition axis (the density isotherms for 30 and 70° are analogous in form, Table 2).

Discussion of Results

The viscosity isotherms of the nitrosodimethylaniline–pyridine system at all temperatures (30, 70, 90°) are convex toward the composition axis and, thus, give no indication of the formation of compounds between the components. Such a course of the curves reflects, as is known [4], the dissociation of associated molecules in the system. This is also confirmed by the considerable cooling that was observed when the components were mixed.

Fig. 1. Summary diagram of the properties of the system.
Curve 1 is the isotherm of the specific electrical conductivity corrected for viscosity at 90°; curve 2 is the viscosity isotherm at 90°; curve 3 is the density isotherm at 90°; curves 4, 5, 6 are the isotherms of the specific electrical conductivity at 30, 70, and 90°.

The density isotherms, represented by curves convex with respect to the composition axis, indicate a certain contraction occurring in the system. Such a course of the density curves, as Doležalec [5] believes, points to chemical interaction between the components of the system. However, E. V. Biron [6] showed that density curves convex with respect to the composition axis may occur in the absence of chemical interaction, and that contraction in a system may also be caused by other reasons. Consequently, density diagrams should be considered only together with diagrams of other properties.

The electrical-conductivity isotherms pass through a maximum as the concentration changes. This maximum, as M. I. Usanovich [7] showed, indicates the formation of a compound between the components of the system only if it is retained when the electrical conductivity is corrected for viscosity. Since new mixtures were prepared for the measurement of electrical conductivity and viscosity, in order to trace approximately the influence of internal friction on electrical conductivity, we calculated the electrical conductivity corrected for viscosity from the conductivity values taken (for those compositions for which viscosity was measured) from the electrical-conductivity curves we had constructed. The data thus obtained are given in Table 3. The isotherm of the corrected electrical conductivity (for 90°) is represented by curve 1 in Fig. 1.

Table 3

Electrical conductivity corrected for viscosity \(\chi \cdot \eta\)

As curve 1 shows, when the correction for viscosity is introduced, the maximum on the electrical-conductivity isotherm disappears, and the dependence of the corrected electrical conductivity on concentration is expressed by curves convex with respect to the composition axis (the isotherms for 30 and 70° are similar in appearance).

The appearance of a maximum on the electrical-conductivity curves is apparently explained by the influence on the electrical conductivity of changes in the degree of dissociation of the conducting component, its concentration, and the increase in the mobility of the ions as the viscosity of the solution decreases.

Thus, the presence in the system of the compound \(n\)-\((\mathrm{CH}_3)_2\mathrm{NC}_6\mathrm{H}_4\mathrm{NO} \cdot 4\mathrm{C}_5\mathrm{H}_5\mathrm{N}\), detected by Kremann by the melting-point method, was not reflected in the isotherms of the properties studied by us. This is possibly explained by the fact that, at the temperatures at which we carried out the measurements, which far exceed the melting temperature of the compound formed (4.3°), the latter dissociates to a considerable extent into its constituent components.

Conclusions

1. At three temperatures (30, 70, and 90°), the electrical conductivity, viscosity, and density of the nitrosodimethylaniline–pyridine system were studied.

2. Consideration of the experimental data on density, viscosity, and electrical conductivity leads to the conclusion that molecular compounds are absent in the system studied in the temperature range 30–90°C.

3. The maximum on the electrical-conductivity isotherms is apparently explained by an increase in the degree of dissociation of the conducting component and by a decrease in the viscosity of the system.

Literature

1. Technical Encyclopedia, VI, Handbook of Physical Chemistry and Chemical Quantities, Moscow, 1931.

2. Paul, Pfeiffer, Organische molekülverbindungen, Stuttgart, 1922.

3. Robert Kremann and Otto Wek, Monatsch., 40, 63, 1920.

4. N. S. Kurnakov, Introduction to Physicochemical Analysis, Moscow–Leningrad, 1940.

5. F. Dolezalek, Z. phys. Chem., 64, 737, 1908; 71, 91, 1910; 98, 395, 1921.

6. E. V. Biron, ZhRKhO, 41, 569, 1909; 42, 167, 1910; 45, 1985, 1913; 45, 2002, 1913.

7. M. I. Usanovich, Zhurn. obshch. khimii, 11, 959, 1940.