PHYSICOCHEMICAL ANALYSIS OF THE SYSTEMS: FORMIC ACID—PYRIDINE AND FORMIC ACID—ANILINE

A. S. NAUMOVA

In our previous studies [1] we investigated the electrical conductivity of binary systems containing acetic acid in mixtures with aniline and pyridine. It was of interest to trace the behavior of formic acid in mixtures with the same organic bases. Formic acid, like acetic acid, is to a considerable degree and firmly associated, but, in addition, has a large dielectric constant and is a stronger acid.

Experimental Part

The formic acid taken for the work crystallized at 8°. Aniline boiled at 174° and 747 mm pressure. Pyridine had a boiling point of 115–116° at 756 mm pressure.

As methods of investigation we used electrical conductivity, viscosity, and, for one of the systems, thermal analysis.

Electrical-conductivity measurements were made by the usual Kohlrausch method. Viscosity was determined in a closed-type viscometer.

Both electrical conductivity and viscosity were studied at several temperatures.

Thermal analysis of the formic acid—aniline system was carried out by K. A. Prokof’eva visually-polythermally. The liquidus line on the diagram was determined from the appearance of the first crystals. Each mixture for determining the crystallization temperature was prepared separately.

The Formic Acid—Pyridine System

Mixing of the components was accompanied by considerable heating. The results of measurements of specific electrical conductivity are given in Table 1 and presented graphically in Fig. 1.

Fig. 1 shows that the electrical-conductivity curves pass through a maximum at 90 mol.% acid. The bend present on the isotherms of high temperatures shifts with change in the latter and, in addition, at low temperatures passes into a minimum corresponding to 75 mol.% acid.

The data obtained in the study of viscosity and density are given, respectively, in Tables 2 and 3.

The viscosity isotherms are shown in Fig. 2, and the densities in Fig. 3. The viscosity diagram consists of curves with a sharply expressed maximum, which falls at 77 mol.% acid and does not change its abscissa with change in temperature.

Fig. 1. Isotherms of the specific electrical conductivity of the formic acid—pyridine system.

Fig. 2. Isotherms of the viscosity of the formic acid—pyridine system.

Fig. 3. Isotherms of the density of the formic acid—pyridine system.

Table 1

Specific electrical conductivity

Table 2

Viscosity of the formic acid—pyridine system

Table 3

Density of the formic acid—pyridine system

The density isotherms have an S-shaped character. At first, that is, at low acid concentrations, the density of the solutions changes little; then, beginning at approximately 30 mol.%, it rises sharply and increases up to approximately 80 mol.% acid, after which its change again slows.

The electrical-conductivity and viscosity isotherms obtained by us have essentially the same form as the isotherms of these properties studied by Udovenko V. V. and Airapetova R. P. [2].

The formic acid—aniline system

(work carried out with the participation of Prokop'eva K. A.)

When formic acid was mixed with aniline, considerable heating was also observed.

The results of measurements of the specific electrical conductivity are summarized in Table 4. The electrical-conductivity isotherms (Fig. 4) pass through a narrow maximum at 90 mol.% and an inflection point at 50 mol.% acid.

The results of density and viscosity measurements are given in Table 5. The dependence of viscosity on composition is represented graphically in Fig. 5.

It follows from the figure that the clearly expressed maximum on the viscosity isotherms, with increasing temperature, shifts somewhat toward the more viscous component ($\mathrm{C_6H_5NH_2}$). With decreasing temperature, the viscosity increases considerably.

Table 4

Specific electrical conductivity

Fig. 4. Isotherms of the specific electrical conductivity of the formic acid—aniline system.

Fig. 5. Viscosity isotherms of the formic acid—aniline system.

Table 5

Viscosity and density of the formic acid—aniline system

The liquidus curve (Table 6, Fig. 6), studied by K. A. Prokop’eva, has three maxima. The diffuseness, especially of two of them (Fig. 6), does not make it possible to determine precisely what stoichiometric ratios of the components they correspond to.

Table 6

Temperature of separation of the first crystals

Fig. 6. Liquidus curve of the formic acid—aniline system.

The eutectic points lie at 0,8 mol.% HCOOH and −7°, at 27,8 mol.% HCOOH and 25°, at 65,2 mol.% HCOOH and 10°, and at 85,1 mol.% HCOOH and −42°.

Discussion of Results

The composition–property diagrams obtained by us for the formic acid–pyridine system show that the system belongs to the irrational type.

The course of the isotherms of electrical conductivity, viscosity, and density (S-shaped character) indicates the presence of chemical interaction between the components. The minimum on the electrical-conductivity diagram corresponds to, and the maximum of the viscosity isotherms closely approaches, the abscissa corresponding to 75 mol.% acid. This gives grounds to assume that a compound of composition \(3\mathrm{HCOOH}\cdot\mathrm{C}_5\mathrm{H}_5\mathrm{N}\) is formed in the system.

The conclusion that the S-shaped form of the density isotherms also indicates chemical interaction is made by us on the basis that, in Trifonov’s opinion [3], conclusions drawn with respect to various types of viscosity isotherms “... may also be extended to other physical properties, for which rectilinear isotherms may be accepted for normal systems and concave ones for systems containing associated components.” Viscosity diagrams having an S-shaped character are usually regarded as a sign of chemical interaction between the components of the system [4].

In order to make sure that the increase in electrical conductivity is caused by the formation of a new electrolyte, we corrected the electrical conductivity for viscosity. The data obtained are given in Table 7 and in Fig. 7.

It follows from Fig. 7 that on the isotherms of corrected electrical conductivity the maximum is retained, thus confirming the existence of a complex compound in the system [5].

We also calculated the values of the absolute temperature coefficients of electrical conductivity and viscosity (Table 8), whose graphical representation is given, respectively, in Figs. 8 and 9.

Table 7

Corrected specific electrical conductivity

Fig. 7. Isotherms of the corrected electrical conductivity of the formic acid–pyridine system.

Fig. 8. Absolute temperature coefficient of electrical conductivity of the formic acid–pyridine system.

Table 8

Absolute temperature coefficient of electrical conductivity \((C)\) and viscosity \((\gamma)\)

The inflection on the curve of the temperature coefficient of electrical conductivity (Fig. 8) [6] and the sharp maximum on the diagram of the temperature coefficient of viscosity (Fig. 9) correspond to 75 mol.% acid.

Thus, the singular points of these diagrams also indicate the existence in the system of a compound of the above composition.

It is possible that the presence of a maximum on the curve of the temperature coefficient of electrical conductivity (about 80 mol.% acid) and on the isotherms of the corrected electrical conductivity (at 80 mol.% acid) is due to the formation in the system of still another compound of composition \(4\mathrm{HCOOH}\cdot \mathrm{C}_5\mathrm{H}_5\mathrm{N}\).

The formic acid—aniline system is also irrational.

Fig. 9. Absolute temperature coefficient of viscosity of the formic acid—pyridine system.

Fig. 10. Isotherms of corrected electrical conductivity of the formic acid—aniline system.

The isotherms of specific electrical conductivity (Fig. 4) have an inflection at 50 mol.% acid and, at 90 mol.% acid, pass through a maximum that does not shift with changes in temperature. This maximum does not disappear when the specific electrical conductivity is corrected for viscosity (Table 9, Fig. 10), thereby indicating that the conductivity of the system is determined by the presence of a complex compound formed through interaction of the components of the system [5].

Fig. 10 also shows that the inflection on the isotherm of corrected electrical conductivity (at 60°), lying approximately at 60 mol.% acid, with decreasing temperature turns into a maximum corresponding to 65 mol.%.

The appearance on the isotherms of corrected electrical conductivity of a second maximum also causes the formation of a minimum.

The complex course of the curves of corrected electrical conductivity apparently indicates the presence of several compounds in the system. The liquidus line (Fig. 6), obtained by K. A. Prokop’eva, also points to this latter circumstance. From Fig. 6 it follows that the crystallization curve reflects the existence in the system of three compounds, the exact composition of which is difficult to establish, because, as already noted,

Table 9

Corrected specific electrical conductivity

The maxima (especially two of them) are very diffuse. However, on comparing this diagram with the curves of other properties studied by us, it may be assumed that the maxima of the crystallization curve correspond to 20, 50, and 67 mol. % acid, thus indicating the formation of complex compounds of the following composition: \(\mathrm{HCOOH}\cdot4\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2\), \(\mathrm{HCOOH}\cdot\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2\), and \(2\mathrm{HCOOH}\cdot\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2\). These compounds have approximately the following crystallization temperatures: 35, 43, and 19°. Fig. 6 also shows that the first two compounds are thermally less stable than the last.

The presence of this more stable compound is also reflected in the viscosity isotherms (Fig. 5) in the form of a clearly expressed maximum, which, apparently because of the irrational nature of the system, does not correspond to an exactly stoichiometric ratio of the components.

We calculated the temperature coefficients of electrical conductivity (Table 10, Fig. 11) and viscosity (Table 10, Fig. 12).

As Fig. 11 shows, the curve of the temperature coefficient of electrical conductivity has a complex form. It passes through three maxima (at 20, 50, and approximately 85 mol. % acid) and two minima (approximately at 27 and 70 mol. %).

Thus, the compositions of the presumed compounds

\[ \mathrm{HCOOH}\cdot4\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2 \quad \text{and} \quad \mathrm{HCOOH}\cdot\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2 \]

correspond to maxima on the diagram of the temperature coefficient of electrical conductivity, while the compound \(2\mathrm{HCOOH}\cdot\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2\) corresponds to a minimum.

The latter complex also corresponds to the maximum value of the temperature coefficient of viscosity (Fig. 12).

It follows from Fig. 11 that the electrical conductivity of the equimolecular compound undergoes the greatest change.

Thus, the results obtained by us in studying the formic acid—aniline system make it possible to assume the presence in the system of three compounds of the indicated composition.

It is possible that in this system more than the three compounds noted on the crystallization curve are formed, whose existence is not reflected on the liquidus line but causes complications in the course of the curves of other properties (temperature coefficient of electrical conductivity, corrected electrical conductivity).

Fig. 11. Relative temperature coefficient of electrical conductivity of the system formic acid—aniline.

Fig. 12. Absolute temperature coefficient of viscosity of the system formic acid—aniline.

Table 10

Temperature coefficients of electrical conductivity and viscosity

The totality of the data obtained shows that formic acid, like acetic acid [1], chemically interacts with aniline and pyridine with the manifestation of an acid–base function.

The interaction of formic acid with these organic bases is more vigorous than that of acetic acid with the same bases, since the components of the system possess more markedly differing chemical functions. In the composition–property diagrams this interaction is expressed more distinctly.

That formic acid interacts with aniline and pyridine more vigorously than acetic acid can be judged at least from the fact that the electrical conductivity in systems with formic acid has higher values, and the viscosity isotherms pass through a more sharply pronounced maximum.

Conclusions

1. The electrical conductivity, density, and viscosity of the formic acid–pyridine system were studied at temperatures of 0, 20, 30, 40, and 60°.

2. At 30, 40, and 60°, the electrical conductivity, density, and viscosity of the formic acid–aniline system were determined. K. A. Prokop’eva studied the liquidus curve of this system.

3. From the diagrams obtained, the presence of chemical interaction between the components of both systems was established.

4. It was noted that the chemical interaction of formic acid with aniline and pyridine is more vigorous than that of acetic acid with the same bases.

Literature

1. Naumova A. S. Zhurn. obshch. khimii, 7, 1216, 1949; Zhurn. obshch. khimii, 7, 1222, 1949.

2. Udovenko V. V. and Airapetova R. P. Zhurn. obshch. khimii, 3, 425, 1947.

3. Trifonov N. A. Viscosity of Liquids and Colloidal Solutions, II, pp. 76–84, Publishing House of the Academy of Sciences of the USSR, 1944.

4. Usanovich M. I. Viscosity of Liquids and Colloidal Solutions, II, pp. 85–88, Publishing House of the Academy of Sciences of the USSR, 1944.

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

6. Usanovich M. I. Zhurn. obshch. khimii, 10, 773, 1941.