Electrical Conductivity, Viscosity, and Density of Formic Acid–Toluidine Systems

A. S. Naumova

(Presented by Prof. B. V. Tronov, Doctor of Chemical Sciences)

The physicochemical analysis that we carried out for systems containing formic acid in mixtures with aniline and pyridine [1] made it possible to establish the presence of chemical interaction between the components of these systems. In order to trace the behavior of formic acid when it is mixed with other amines, we studied the electrical conductivity, viscosity, and density of formic acid–toluidine systems.

Experimental Part

The formic acid used in the work, after purification by repeated freezing, crystallized at 8°; para-toluidine, recrystallized from xylene, had a melting point of 45°; ortho-toluidine, purified by double distillation, boiled at 196° and a pressure of 755.3 mm.

Electrical conductivity was measured by the Kohlrausch method in a closed vessel. A vacuum-tube audio-frequency generator with an amplifier was used as the current source. Measurements of internal friction were carried out in an Ostwald viscometer of the closed type. Density was determined in a pycnometer with a narrow neck.

All properties were studied at several temperatures.

Formic Acid–Para-Toluidine System

Mixing formic acid with para-toluidine was accompanied by heating.

Measurements of electrical conductivity, viscosity, and density were carried out at 50, 60, and 70°. The results obtained in the study of electrical conductivity are presented in Fig. 1.

Only one isotherm is shown in the graph (for 50°), since electrical conductivity changes little with temperature. This latter fact is apparently explained by the fact that increasing the temperature from 50 to 70° causes a comparatively small decrease in viscosity. The electrical-conductivity isotherm (Fig. 1) is a curve passing through a narrow maximum at 90 mol.% acid. At 50 mol.% there is also a point of inflection on the curve, which at lower temperatures, owing to an even greater increase in viscosity, will apparently pass into a minimum.

The data obtained in the study of viscosity and density are shown graphically in Figs. 2 and 3. The dependence of viscosity on concentra-

…conductivity (Fig. 2) is represented by curves convex away from the composition axis. The isotherms have a clearly pronounced irrational maximum, occurring at 50 mole % and not appreciably changing its abscissa with a change in temperature. The presence of a maximum on the viscosity curves indicates, as is known, chemical interaction between the components [2].

Fig. 1. Isotherm of electrical conductivity (at 50°) of the formic acid—p-toluidine system.

The isotherms characterizing the change in density with change in the composition of the system (Fig. 3) have an S-shaped character. We obtained a diagram of a similar form for the system formic acid—pyridine [1]. Apparently, it may be considered [3] that the S-shaped course of the density curves, as well as of the viscosity curves [4], indicates the presence of chemical interaction in the system.

Fig. 2. Viscosity isotherms of the formic acid—p-toluidine system.

Formic Acid—o-Toluidine System

(the work was carried out with the participation of A. I. Kartushinskaya)

Mixing of the components of this system was accompanied by considerable heating. For most mixtures, during their preparation, not only strong heating was observed but also simultaneous crystallization¹; therefore the properties were studied in the supercooled state of the system.

Determinations of electrical conductivity, viscosity, and density were carried—

Fig. 3. Density isotherms of the formic acid—p-toluidine system.

were carried out at 0, 25, and 50°. The curves expressing the dependence of electrical conductivity on composition are given in Fig. 4.

Fig. 4 shows that the isotherms of specific electrical conductivity pass through a maximum at approximately 93 mol.% acid. With a change in temperature, no noticeable displacement of the maximum is observed. It also follows from Fig. 4 that at 50°, beginning at approximately 80 mol.%, as the acid content in the system decreases, the electrical conductivity decreases more slowly than at the other temperatures. This is apparently a result of the fact that, at this temperature, the viscosity changes only slightly with composition.

Fig. 4. Conductivity isotherms of the formic acid—o-toluidine system.

Fig. 5. Viscosity isotherms of the formic acid—o-toluidine system.

The isotherms expressing viscosity as a function of the composition of the system are shown in Fig. 5. As follows from Fig. 5, the viscosity diagram consists of smooth curves with a maximum, especially sharply pronounced at 0°. With increasing temperature, the viscosity decreases very strongly, and the maximum on the curves becomes increasingly diffuse. The maximum value of the viscosity occurs at 60 mol.% acid and does not change its abscissa with change in temperature.

The curves characterizing the dependence of density on composition are given in Fig. 6.

The density isotherms have a weakly expressed S-shaped form. The location of the inflection point on the curves is difficult to determine.

Discussion of the Results

The form of the composition–property diagrams obtained by us indicates the irrational character of both systems studied [2].

The course of the isotherms of electrical conductivity, viscosity, and density of the formic acid–para-toluidine system reflects the presence of chemical interaction between the components. The maximum in the viscosity diagram, corresponding to 50 mol. %, makes it possible to assume that an equimolecular compound of the following composition is formed in the system:

\[ \mathrm{HCOOH}\cdot\mathrm{C}_6\mathrm{H}_4 \begin{matrix} \diagup \mathrm{NH}_2\\ \diagdown \mathrm{CH}_3 \end{matrix} \]

Fig. 6. Density isotherms of the formic acid–ortho-toluidine system.

Fig. 7. Isotherms of corrected specific electrical conductivity of the formic acid–para-toluidine system.

In order to determine whether the maximum on the electrical-conductivity isotherms is retained when the influence of viscosity on it is excluded, we calculated the “reduced” electrical conductivity. The curves of the dependence of the “reduced” electrical conductivity on composition are shown in Fig. 7.

It follows from Fig. 7 that correction of the electrical conductivity for viscosity does not eliminate the maximum. This indicates the formation of a complex compound in the system [5]; moreover, the addition of para-toluidine to formic acid causes a slower decrease in the corrected electrical conductivity than in the uncorrected conductivity.

The curves showing the dependence of the temperature coefficient of viscosity on composition (Fig. 8) pass through a maximum somewhat shifted from the abscissa corresponding to 50 mol. %. Thus, when the temperature changes, the viscosity changes most strongly at the equimolecular ratio of the components.

Fig. 8. Curves of the absolute temperature coefficient of viscosity of the formic acid–para-toluidine system.

The results of studying the electrical conductivity, viscosity, and density of this system indicate that, when formic acid is mixed with para-toluidine, chemical inter-

interaction with manifestation of an acid–base function, and a compound of equimolecular composition is formed.

The considerable heating that accompanies the mixing of formic acid with orthotoluidine indicates that the components of the system enter into chemical interaction. The presence of chemical behavior in the system is also indicated by the course of the isotherm of electrical conductivity, by the viscosity [2] (the curves pass through a maximum), and, apparently, by the S-shaped character of the density diagrams [3]. The maximum value of the viscosity occurs at 60 mol.% acid, that is, corresponds to the composition:

\[ 3\mathrm{HCOOH}\cdot 2\mathrm{C}_6\mathrm{H}_4 \begin{matrix} \mathrm{NH}_2\\ \mathrm{CH}_3 \end{matrix} \]

It is possible that, owing to the thermal instability of the compound, the maximum value of the viscosity has proved to be shifted from the abscissa of the compound and that the latter has a different composition.

Assuming that viscosity in this system has a substantial effect on electrical conductivity, and wishing to follow the electrical conductivity with change in composition independently of viscosity, we eliminated this factor. The curves of the “reduced” electrical conductivity are shown in Fig. 9.

Fig. 9. Isotherms of the corrected specific electrical conductivity of the formic acid—orthotoluidine system.

In the isotherms of the corrected electrical conductivity the maximum is preserved, confirming the presence of chemical interaction in the system [5]. The graph also shows that the considerable change in viscosity with increasing temperature has a substantial effect on the electrical conductivity.

When the influence of viscosity on electrical conductivity is eliminated, the isotherms of the latter are considerably transformed and assume a complex form. In addition, the temperature coefficient of electrical conductivity becomes negative. With an increase in temperature from 0 to 50°, the corrected specific electrical conductivity decreases, which is explained by the excessively large drop in viscosity with increasing temperature. The latter indicates that the complex compound being formed is thermally unstable and decomposes with increasing temperature.

The complex course of the curves of “reduced” electrical conductivity may be explained by the fact that not one but several compounds are formed in the system, the composition of which changes with concentration.

We have also calculated the values of the absolute and relative temperature coefficients of electrical conductivity.

The absolute temperature coefficient of electrical conductivity almost completely repeats the course of the electrical-conductivity isotherms; therefore we do not present its graphical representation. The diagram of the relative temperature coefficient of electrical conductivity as a function of ...

...of dependence on the composition of the system (Fig. 10) differs in form from the diagram of the property itself. The maximum on the curves of the temperature coefficient occurs at approximately 55 mol.% acid. The diagrams of the temperature coefficient, both relative and absolute (Fig. 11), have a form analogous to the viscosity diagrams, that is, they pass through a maximum at 60 mol.% acid.

Conclusions

1. At three temperatures, the electrical conductivity, viscosity, and density of the systems formic acid—ortho- and para-toluidine were studied.

Fig. 10. Curves of the relative temperature coefficient of electrical conductivity of the formic acid—orthotoluidine system.

Fig. 11. Curves of the absolute temperature coefficient of viscosity of the formic acid—orthotoluidine system.

1. In both systems, chemical interaction occurs between the components.

Literature

1. Naumova A. S. Collection of articles on general chemistry. 2, 778, 1953; Bulletin of the Tomsk Polytechnic Institute. 77, 778, 1953.

2. Kurnakov N. S. Introduction to Physicochemical Analysis. Moscow–Leningrad: Publishing House of the Academy of Sciences of the USSR, 1940.

3. Trifonov N. A. Viscosity of Liquids and Colloidal Solutions. 2, 76. Publishing House of the Academy of Sciences of the USSR, 1944.

4. Usanovich M. I. Viscosity of Liquids and Colloidal Solutions. 2, 85. Publishing House of the Academy of Sciences of the USSR, 1944.

5. Usanovich M. I. Journal of General Chemistry. 11, 959, 1940.

1. The readily crystallizing compounds formed in the system are complex, not valence, compounds, since they are comparatively unstable. Crystals of the compound, insoluble in benzene, when placed in a mixture of benzene and an alkali solution, disappear. As a result of the decomposition of the complex compound, the liberated toluidine dissolves in the benzene layer, and the acid in the alkali solution. ↩