ELECTRICAL CONDUCTIVITY AND VISCOSITY OF THE ACETIC ACID—NITRIC ACID SYSTEM

A. S. Naumova

It is known that, in aqueous solution, nitric acid possesses somewhat greater strength than acetic acid.

There are indications in the literature that, when these acids are mixed, chemical interaction occurs with the formation of definite chemical compounds [7, 5, 4]. Thus, in one work [7] the formation in the system of an exothermic compound of composition \(\mathrm{HNO_3 \cdot 2C_2H_4O_2}\), distilling at a constant temperature of \(127\text{--}128^\circ\), is indicated. The existence in the system of a compound of the same composition is also noted by another author [5], who observed absorption of ultraviolet light by the acids, their esters, and salts. In one work [4], the density, viscosity, and melting point of mixtures of anhydrous acetic and nitric acids were determined. The authors of this work assume the formation of an equimolecular compound of composition \(\mathrm{HNO_3 \cdot C_2H_4O_2}\).

At one temperature, but only in the region of higher acetic acid content, the electrical conductivity of this system has been studied [6]. The results of this work show only that, with an increase in the amount of nitric acid in the system, the specific electrical conductivity increases. Thus, in the literature there are indications of the possibility of formation in the system of two compounds

\[ \mathrm{HNO_3 \cdot 2C_2H_4O_2} \quad \text{and} \quad \mathrm{HNO_3 \cdot C_2H_4O_2}. \]

It was of interest to trace whether the presence of the indicated compounds in the system would be reflected in the electrical-conductivity diagram, and how sharply the chemical interaction between the components would be expressed, the molecules of which are associated and which in aqueous solution possess acid properties differing little from one another.

In addition to electrical conductivity, we also determined the viscosity and density of the system.

Electrical Conductivity of the \(\mathrm{HNO_3—C_2H_4O_2}\) System

Absolute acetic acid was obtained by fractional freezing in sealed ampoules. The purified acid had a melting point of \(16.6^\circ\mathrm{C}\) and a specific electrical conductivity of the order of \(1 \cdot 10^{-7}\) reciprocal ohms.

Nitric acid was obtained by distilling under vacuum a mixture of concentrated sulfuric and nitric acids. The completely colorless acid obtained was immediately sealed in ampoules, in which it was stored.

At \(25^\circ\), nitric acid had a specific electrical conductivity of \(3.4 \cdot 10^{-2}\) reciprocal ohms. When the acids were mixed, heating was observed. Electrical conductivity was measured by the usual Kohlrausch method at two temperatures, 0 and \(25^\circ\mathrm{C}\). The results obtained are placed in Table 1 and represented graphically in Fig. 1.

Table 1

It follows from Figure 1 that, upon addition of the first portions of acetic acid, the electrical conductivity drops sharply. Subsequently this drop slows somewhat. Both isotherms are curves, con-

Fig. 1

vex throughout their entire length toward the concentration axis. This form of the isotherms—the absence of singular points—gives no indication of the presence of chemical interaction in the system. A diagram was also constructed showing the dependence of the temperature coefficient of electrical conductivity on composition (Fig. 2). The graph shows that in the region of higher acetic-acid content in the mixture, as dilution proceeds to approximately 67 mol. % acetic acid, a comparatively small decrease in the magni-

of the value of the temperature coefficient of electrical conductivity. With further dilution the decrease proceeds considerably more rapidly and follows a straight line. Consequently, at about 67 mol.% acetic acid there is an inflection in the curve, since at this point its course changes noticeably. Thus,

Fig. 2

the curve of the temperature coefficient of electrical conductivity gives some indication of the presence of chemical interaction in the system [2] and of the formation of the compound $\mathrm{HNO_3\cdot 2C_2H_4O_2}$, whose existence is indicated in the literature [7]. The presence of the second of the indicated compounds, $\mathrm{HNO_3\cdot C_2H_4O_2}$ [4], is not reflected in the diagram.

Table 2 gives the results of calculations of the molecular electrical conductivities, whose graphical representation is given in Figures 3 and 4.

Table 2

Molecular electrical conductivity

It follows from the figures that both curves are anomalous. The curve of molecular electrical conductivity (Fig. 3), calculated for nitric acid as an electrolyte, is a typical anomalous curve, falling sharply over a small interval of dilutions. With further dilution its fall slows somewhat.

Fig. 3

Nitric acid—a strong electrolyte in aqueous solutions—behaves in this system as a non-electrolyte.

Fig. 4

electrolyte [3,8]. The anomaly of the molecular electrical conductivity, calculated for acetic acid as the electrolyte, is expressed less sharply, although acetic acid in aqueous solutions is a weaker electrolyte than nitric acid.

Thus, on the basis of studies of the electrical conductivity of this system, one may assume the presence of chemical interaction in it and the formation of the compound $\mathrm{HNO_3 \cdot 2C_2H_4O_2}$, which apparently is an electrolyte.

Viscosity of the $\mathrm{HNO_3—C_2H_4O_2}$ System

The viscosity was studied at the same temperatures as the electrical conductivity. The measurements were carried out in a closed Ostwald viscometer. The density of the system was also measured. The results of the study of viscosity and density are summarized in Table 3.

Table 3

Both branches of the viscosity curves (Fig. 5) are concave toward the composition axis and, intersecting, form a diffuse maximum, which, with decreasing temperature, shifts to 67 mol. % acetic acid. The maximum of the 0°C isotherm is more pronounced.

The viscosity diagram also indicates the existence in solution of the compound $\mathrm{HNO_3 \cdot 2C_2H_4O_2}$.

In order to eliminate the influence of viscosity on the electrical conductivity, we calculated the so-called “reduced” electrical conductivities (Table 4, Fig. 6).

The form of these latter curves differs from that of the uncorrected curves. Such a course of the “reduced” electrical conductivities can apparently be explained by the fact that the electrolyte formed in the system has an electrical conductivity greater than the electrical conductivity of only one of the components ($\mathrm{C_2H_4O_2}$).

Thus, the conductivity of the system must be composed of the conductivities of the acids and of the newly formed compound; moreover, in the region of higher nitric-acid content, its conductivity will undoubtedly predominate, while the influence of the conductivity of the complex should find its greatest reflection on the isotherm at those ratios of the com-

components at which the greatest amount of it is formed. As the graph shows, this latter occurs at a concentration of 22 mol.%.

Fig. 5

of acetic acid. It may be thought that the presence of the complex will be revealed most clearly if the considerable influence on the conductivity

Fig. 6

of the nitric-acid conductivity system is eliminated. To test this, we made use of the experimental data of Tartakovskaya¹. She stu—

Table 4

Corrected electrical conductivity

the electrical conductivity of the system HNO₃—CH₂ClCOOH. On the basis of her own data and the data of other authors, she comes to the conclusion that chemical interaction is absent in the system, while its conductivity is determined by the intrinsic conductivity of nitric acid, which, upon dilution with the second component, decreases as a result of the decomposition of the complex electrolyte \((\mathrm{HNO}_{3})_{x}\) into simple, nonionogenic HNO₃ molecules. By subtracting Tartakovskaya’s numerical data for the reduced electrical conductivity of the system HNO₃—CH₂ClCOOH from the corresponding data of ours for the system HNO₃—C₂H₄O₂, we obtained the difference \((\Delta \chi_{0} \cdot \eta_{0})\), the values of which are given in Table 5 and are represented graphically in Fig. 7.

Table 5

It is seen from the figure that the curve passes through a clearly expressed maximum (approximately at 30 mole % acetic acid), thereby confirming

Fig. 7

the presence of chemical interaction in the system and the formation of an electrolyte.

Conclusions

1. The electrical conductivity and viscosity of the system \(\mathrm{HNO_3—C_2H_4O_2}\) were studied at temperatures of \(0^\circ\) and \(25^\circ\mathrm{C}\).

2. The maximum of the viscosity isotherms and the bend in the curve of the temperature coefficient of electrical conductivity indicate the existence in the system of an exothermic compound of composition \(\mathrm{HNO_3 \cdot 2C_2H_4O_2}\).

3. The presence of chemical interaction in the system is revealed on the conductivity isotherms only when the influence of viscosity on this property is eliminated. The special points of the diagrams of “reduced” electrical conductivity and of the difference of two “reduced” electrical conductivities are located in the region of higher nitric acid content.

4. It is possible that the discrepancy between the positions of the special points of the indicated diagrams (Figs. 6, 7) and the ordinate of the compounds occurred because the calculation of the molecular percentages of acetic acid was carried out only for simple molecules, without taking into account the dimers \((\mathrm{C_2H_4O_2})_2\). The presence of dimers in this system is quite possible, especially at higher concentrations of acetic acid. Confirmation of the presence of dimers in the system may be provided, for example, by the viscosity isotherms (a concave form, instead of the curves usually [1] convex toward the composition axis). Noticeable dissociation of the molecules \((\mathrm{C_2H_4O_2})_2\) apparently occurs only in dilute solutions, in which the dissociated molecules also give a sufficient amount of the product of interaction with nitric acid. This, in our opinion, is also reflected in the curves of corrected electrical conductivity (bend) and of the difference of two corrected electrical conductivities (maximum).

It is quite possible that, if in calculating the molecular content of acetic acid one takes into account not only simple molecules but also dimeric molecules, then the singular points of the diagram (Figs. 6 and 7) will correspond more or less exactly to the composition of the compound $\mathrm{HNO_3 \cdot C_2H_4O_2}$, whose existence is also indicated by other investigators [4].

REFERENCES

1. N. S. Kurnakov.—Introduction to Physico-Chemical Analysis. 1940.
2. M. Usanovich—Proceedings of the meetings of the Leningrad Chemical Society for 1932, p. 25; Proceedings of the VI Mendeleev Congress, II, issue 2, 174; ZhOKh, 11, p. 773. 1941.
   M. Usanovich and F. Terpugov.—ZhOKh, 2, p. 447. 1932.
3. M. Usanovich—Reports of the Academy of Sciences of the USSR, 1, p. 518. 1935.
4. Briner—Susz and Favarger, Helv. chim. Acta, 18, p. 375. 1935.
5. Hantzsch—Ber. d. D. chem. Ges. 58, p. 941. 1925.
6. Kolthoff and Willeman.—J. Amer. chem. Soc. 56, p. 1007. 1934.
7. Pictet and Genequand.—Ber. d. D. chem. Ges. 35, p. 2526. 1903.
8. Rabinowitsch—Zeitschr. phys. Chem. 99, p. 338. 1921; 99, p. 433. 1921.

1. Dissertation work. ↩