E. T. LABYKINA

APPLICATION OF THE INTERFEROMETRIC METHOD OF INVESTIGATION IN THE STUDY OF THE KINETICS OF THE REACTIONS OF BARIUM AND RUBIDIUM BROMIDES WITH HYDROGEN CHLORIDE

(Presented by the seminar of the Department of Inorganic Chemistry)

There are recommendations for the use of the interferometric method of solution analysis for two-component systems, as well as for multicomponent systems with one component of variable concentration [1, 2, 3].

In studying the kinetics of certain processes, one sometimes has to deal with two-component systems in which the concentrations of the components change synchronously, i.e., as the concentration of the reaction product increases, the concentration of the initial substance decreases correspondingly. In this case, as our studies have shown, the method of interferometric analysis can be applied successfully.

Interferometric analysis made it possible to confirm the reliability of kinetic data obtained by the gravimetric method for exchange reactions between crystalline metal bromides and gaseous hydrogen chloride [4,5]. Kinetic studies were carried out for the reactions

\[ \mathrm{RbBr} + \mathrm{HCl} = \mathrm{RbCl} + \mathrm{HBr} \quad \text{and} \quad \mathrm{BaBr}_2 + 2\mathrm{HCl} = \mathrm{BaCl}_2 + 2\mathrm{HBr}. \]

Experimental part and discussion of results

The determinations were carried out on an ITR-1 interferometer [2], which is intended for measuring the concentrations of solutions by comparing the refractive indices of reference liquids with those under investigation.

The operating principle of the interferometer is based on the phenomenon of diffraction from a double slit: a parallel beam of rays passes through a diaphragm with two openings and is collected in the focal plane of the objective of the viewing tube. As a result of diffraction of light at the diaphragm openings, a system of interference fringes is produced in the focal plane, which is observed with the aid of a strong eyepiece. In the path of the rays opposite the diaphragm openings a two-chamber cuvette is placed. The chambers of the cuvette are filled with the solvent and the test solution.

The change in refractive index in the ITR-1 is measured by the shift of the interference spectrum of the solution under investigation relative to the spectrum of the pure solvent. Readings of quantities proportional to the refractive index are made on the drum of the micrometer ...

of the instrument screw, by means of which the interference spectra of the pure solvent and of the solution are brought into coincidence.

Standard solutions of sodium, potassium, rubidium, and barium chlorides and bromides were prepared from chemically pure salts used in the preceding work [4,5]. Solutions were taken at concentrations of 0.08 M; 0.16 M; 0.2 M; 0.24 M; 0.32 M and 0.4 M. The standard solutions were compared in the interferometer with the pure solvent—bidistillate.

On the basis of the readings obtained with the instrument (Table 1), calibration graphs were constructed for the standard solutions (Fig. 1).

Table 1

Concentration of solutions and readings on the ITR-1

In studying reaction kinetics, the solutions were prepared from samples taken during the course of the reaction at various time intervals. By determining the displacement of the interference spectrum for the sample solution, a point corresponding to the given reading $m$ was found on the calibration diagram. From the magnitude of the reading $m$, using the composition–property diagram, the percentage composition of the samples $a$RbCl—$b$RbBr and $a$BaCl$_2$—$b$BaBr$_2$ was determined.

The measurement results showed that the refractive index for the chlorides and bromides of sodium, potassium, rubidium, and barium lies in a li-

Fig. 1. Calibration plots of the dependence of the indicator \(m\) on concentration. Lines 1, 2, 3, 4, 5, 6 correspond respectively to solutions: NaCl, KCl, RbCl, NaBr, KBr, RbBr.

Fig. 2. Diagram: indicator \(m\)—percentage composition of the mixture RbBr—RbCl.

linear dependence on the concentration of the solution (Fig. 1). Consequently, the additivity rule could be applied [6].

On the basis of the rectilinear dependence, composition–property diagrams were constructed for the systems RbBr—RbCl and BaBr$_2$—BaCl$_2$ (Figs. 2 and 3). The concentrations of the solutions were, respectively, 0.16 M and 0.2 M; the temperature was 26°. The resultant straight lines of these diagrams reflect the dependence of the change in the value $m$ on the composition of the system. Points $A$ and $B$ in Figs. 2 and 3, respectively, correspond to the indicator $m$ of solutions with an average molecular weight for RbBr(Cl) equal to 138.8, and for BaBr$_2$(Cl$_2$) equal to 245.0. The molecular weights of the samples were calculated on the basis of gravimetric data. Points $A$ and $B$ correspond to $A^1$ and $B^1$, which determine the percentage composition of the samples. The results obtained by the interferometric method show good agreement with the results of the gravimetric method (Tables 2 and 3).

Fig. 3. Diagram: indicator $m$—percentage composition of the BaBr$_2$—BaCl$_2$ mixture.

Table 2

Data from interferometric and gravimetric analyses of the kinetics of the reaction RbBr + HCl = RbCl + HBr (for $t = 350^\circ$)

Table 3

Data from interferometric and gravimetric analyses of the kinetics of the reaction
\(\mathrm{BaBr_2 + 2HCl = BaCl_2 + 2HBr}\) (for \(t = 300^\circ\))

Conclusions

1. The applicability of interferometric analysis has been demonstrated for two-component systems of variable composition, using diagrams: percentage composition—refractive index (or a quantity proportional to it).

2. An interferometric study has been carried out of the kinetics of reactions previously studied by the gravimetric method. The agreement of the results is good.

LITERATURE

1. N. A. Figurovskii and K. A. Pospelova. A liquid interferometer and its application for measuring concentrations. Zh. “Zav. laboratoriya”, 8, 983–985 (1936).

2. Laboratory interferometer, model ITR-1. (Description supplied with the instrument).

3. A. N. Zakharyevskii. Interferometers. Oborongiz, 1952.

4. E. T. Labýkina. Kinetics of topochemical reactions of certain metal bromides with hydrogen chloride. Izvestiya vyssh. uch. zavedenii, “Khimiya i khim. tekhnologiya,” No. 6, 944 (1961).

5. E. T. Labýkina. Kinetics of topochemical reactions of certain metal bromides with hydrogen chloride. Izvestiya Tomskogo politekhnicheskogo in-ta, 112 (1963).

6. A. I. Brodskii. Interferometric method of analysis. Zhurn. zav. laboratoriya, 8, 1282 (1936).