UDC 541.13

EFFECT OF PROPANOL ADDITIVES ON THE ELECTRICAL CONDUCTIVITY OF SOLUTIONS

I. A. Tikhomirov, G. S. Tikhonov, E. R. Gofman, M. I. Bokor

(Submitted by the scientific seminar of the Faculty of Physics and Technology)

The electrical conductivity of chloride salts of lithium, sodium, and potassium was measured in aqueous–propanol solutions with different propanol contents (10, 30, 50, 70, 85 wt. %). It is shown that, with a decrease in dielectric permittivity, the electrical conductivity decreases owing to the formation of ion pairs. Illustrations 2, tables 1, references 6.

At present, much attention is being devoted to the development of continuous methods for separating ions by means of ion-exchange technology.

One of the principal methods that make it possible to carry out a continuous separation process is ion exchange with the imposition of an electric field [1–3]. To select the optimum conditions for such processes, it is necessary to know the electrical conductivity of the ion-exchanger–solution system, which is mainly limited by the electrical conductivity of the solution. Since the use of aqueous–organic solutions increases the efficiency of exchange [4], it becomes necessary to study the electrical conductivity of aqueous–organic solutions.

In the present work, the electrical conductivity of chloride salts of lithium, sodium, and potassium was measured in aqueous–propanol solutions with different alcohol contents.

Experimental Part

The electrical conductivity of the solutions was measured by us in a cell with platinized electrodes at \(t = 25 \pm 0.1^\circ\mathrm{C}\) using the apparatus shown in Fig. 1. Two cells were used to measure electrical conductivity: one for measuring the electrical conductivity of highly conducting electrolytes (cell constant—8.725), the other for measuring poorly conducting electrolytes (cell constant—0.968). The cell constants were determined using KCl standard solutions of various concentrations. Chemically pure propyl alcohol was used as the solvent. Having determined the cell constant, the specific electrical conductivity of the solution \(\chi\) was calculated by the formula

\[ \chi = K/R, \tag{1} \]

where \(K\) is the cell constant, and \(R\) is the resistance of the electrolyte.

Knowing the specific electrical conductivity, one can calculate the equivalent electrical conductivity:

\[ \lambda=\frac{\chi\cdot 1000}{C}. \tag{2} \]

Fig. 1. Diagram of the experimental setup.

Here \(C\) is the concentration of the solution, mg-equiv/ml. The results of measurements of the electrical conductivity of lithium, sodium, and potassium chlorides in aqueous-propanol solutions with alcohol concentrations of 10, 30, 50, 70, and 85% are presented in Fig. 2.

From the dependence \(1/\lambda=f(c\lambda)\), the values of the limiting equivalent electrical conductivity \(\lambda_0\) were determined; their values as a function of the dielectric constant \((\varepsilon)\) of the solution are given in Table 1.

From the data presented in Fig. 2 it is evident that, with an increase in the dilution \(V=1/C\), the equivalent electrical conductivity increases; this agrees with the theoretical propositions concerning the dependence of the electrical conductivity of electrolyte solutions on concentration.

Table 1

The decrease in electrical conductivity with increasing propanol content in the solution can be explained from the standpoint of ion association

Fig. 2. Dependence of equivalent electrical conductivity on dilution: △ — lithium, □ — sodium, ○ — potassium. 1 — aqueous solution, 2 — 10, 3 — 30, 4 — 50, 5 — 70, 6 — 85% propanol

into ion pairs. For ions with a single charge, their association may be represented as follows:

\[ C^{+} + A^{+} \rightleftharpoons [CA]^{0} \rightleftharpoons CA, \tag{3} \]

where \([CA]^0\) denotes the ion pair, and \(CA\) the undissociated molecule. The former is formed only due to the action of Coulomb forces, while the latter results from the formation of an electronic bond. As the dielectric constant decreases, the force of interaction between ions according to formula (4) increases; the probability of formation of associates increases, which leads to a decrease in electrical conductivity

\[ F = \frac{z^{2}e^{2}}{\varepsilon r}, \tag{4} \]

where \(F\) is the force of interaction between ions; \(e\) is the charge of the ion, \(z\) is the valence of the ion, and \(r\) is the distance of closest approach of the ions.

It was also shown in [6] that, with an increase in the fraction of propanol in the solution, the radii of the solvated ions decrease, which affects the increased formation of associates. The formation of ion pairs leads to a decrease in the number of particles carrying charge and, consequently, to a decrease in electrical conductivity.

The electrical conductivity of solutions also depends on the viscosity of the solutions \((\eta)\).

As the viscosity of the solution increases, the electrical conductivity decreases (see Table 2).

The decrease in electrical conductivity in the series \(\lambda_{\mathrm{KCl}} > \lambda_{\mathrm{NaCl}} > \lambda_{\mathrm{LiCl}}\) is explained by the decrease in the mobility of the ions in the same order.

Thus, additions of an organic solvent (propanol) to aqueous solutions of the salts LiCl, NaCl, and KCl decrease the electrical conductivity of these solutions, and the effect is more pronounced the higher the alcohol concentration in the solution. It follows from this that alcohol additions will affect the efficiency of electrochromatographic separation processes.

REFERENCES

1. B. N. Laskorin, N. M. Smirnova, M. N. Gantman. Ion-exchange membranes and their application. Moscow, Gosatomizdat, 1961.
2. I. A. Tikhomirov, G. S. Tikhonov. ZhFKh, 44, 1504 (1970).
3. I. A. Tikhomirov, G. S. Tikhonov. Elektrokhimiya, 6, 1817 (1970).
4. V. I. Gorshkov, Yu. Z. Korolev. Vestnik MGU, Khimiya (Series II), No. 1, 16 (1966).
5. G. Akerlof. J. Amer. Chem. Soc., 54, 4125 (1932).
6. M. Goffredi, T. Shedlovsky. J. Phys. Chem., 71, 2176 (1967).