Chemistry

V. E. Plyushchev, S. B. Stepina, L. I. Lepeshkova

Isotripolyhalides of Alkali Elements Similar in Properties and Their Application for Purifying Rubidium Compounds from Cesium

(Presented by Academician I. V. Tananaev, October 22, 1962)

The successes achieved very recently in solving the problem of separating alkali elements similar in properties are, to a considerable extent, associated with the use of their polyhalides (¹–⁵). Thus, as a result of applying the method of double precipitation of rubidium chlorobromoiodate of composition \( \mathrm{Rb[J(ClBr)] \cdot H_2O} \), a method was developed (¹,²) for purifying technical rubidium salts from potassium, with the production of especially pure preparations of this element containing only 0.0002% potassium. Subsequently, (³) a technological scheme was outlined for deep purification of \( \mathrm{RbCl} \) through rubidium chlorobromoiodate. In this way the most difficult task of removing a potassium microimpurity from rubidium salts was solved; moreover, since a compound of the type \( \mathrm{Me[J(ClBr)]} \) exists only for cesium, precipitation of \( \mathrm{Rb[J(ClBr)] \cdot H_2O} \) achieves purification of rubidium salts from all alkali elements except cesium.

With the aid of cesium diiodobromide \( \mathrm{CsBrJ_2} \), the problem of removing a rubidium microimpurity from cesium salts was solved. The use of \( \mathrm{CsBrJ_2} \) makes it possible to purify cesium salts also from all other alkali elements, since in the series of their compounds \( \mathrm{CsBrJ_2} \) is the only one with the anion \( \mathrm{BrJ_2^-} \). Already a double precipitation of \( \mathrm{CsBrJ_2} \) makes it possible (⁴,⁵), from technical \( \mathrm{CsBr} \) containing 5% rubidium and 1.5% of the remaining alkali elements, to obtain a product with an impurity content of not more than 0.05%, including 0.02% rubidium. The high purification factor (10–20) and selectivity of extracting \( \mathrm{CsBr} \) from a mixture with other alkali elements give the method substantial advantages over those proposed earlier. Two further recrystallizations of \( \mathrm{CsBrJ_2} \) make it possible to obtain \( \mathrm{CsBr} \) containing not more than 0.0003–0.0005% rubidium, i.e., such cesium compounds as cannot yet be obtained by any of the other known methods.

The successful solution of the most difficult problem of removing a potassium microimpurity from rubidium salts and the development of a method for removing microimpurities of all alkali elements from cesium salts brought to the fore the question of solving a similar general problem as applied to rubidium compounds. Along this path it remained to overcome difficulties associated with the separation of rubidium and cesium (removal of a microimpurity of the latter). Experience in the application of polyhalides of alkali elements similar in properties showed that they may also be useful for work in the indicated direction.

However, the above-mentioned achievements in obtaining pure rubidium and cesium compounds are associated with the use of heteropolyhalides, in particular heterotripolyhalides (⁵), among which there are no compounds that would make it possible to free rubidium salts from a cesium microimpurity. Therefore our attention was directed to another group of polyhalides—isopolyhalides. For alkali elements similar in properties, polybromides (\(\mathrm{MeBr_3}\)) and polyiodides (\(\mathrm{MeJ_3}\)) are known; moreover, for polybromides the existence of only tribro-

cesium (6–9) and rubidium (10,11). Cesium tribromide can be isolated both from solution (6–8) and obtained by a dry method (9). Rubidium tribromide is isolated from aqueous solutions under conditions analogous to those for obtaining cesium tribromide. At the same time, solid potassium tribromide of composition \(\mathrm{KBr}_3\) has not been isolated, and only the existence (12,13) of the crystalline hydrate \(\mathrm{KBr}_6 \cdot 1.5\mathrm{H}_2\mathrm{O}\) is known. Thus, polybromides could be used at most for purifying rubidium and cesium salts from other cations, but not from one another. Moreover, rubidium and cesium polybromides partially decompose upon recrystallization from water (6,10,14).

Polyiodides are more stable, in particular cesium triiodides (6,15–18) and rubidium triiodides (10,17,19), which can be isolated from solutions and recrystallized from water. It is characteristic here that the solubility of \(\mathrm{CsJ}_3\) is 300 times lower (10) than the solubility of \(\mathrm{RbJ}_3\). This difference was used by us to develop a method for removing a microimpurity of cesium from rubidium salts.

It was indicated in the literature (10) that isomorphous relationships exist between \(\mathrm{RbJ}_3\) and \(\mathrm{CsJ}_3\). To verify this circumstance and to estimate the degree of cocrystallization of cesium with \(\mathrm{RbJ}_3\), the cocrystallization coefficients (Khlopin constants) were determined by the method of isothermal removal of supersaturation (20) at variable amounts of the precipitated \(\mathrm{RbJ}_3\). The crystallization coefficients \((k)\) were calculated by the formula:

\[ k=\frac{x(b-y)}{y(a-x)}, \]

where \(x, y\) are the amounts of the micro- and macrocomponent (in our case Cs and \(\mathrm{RbJ}_3\)) in the solid phase, in grams; \(a, b\) are the amounts of the micro- and macrocomponent in the liquid phase before crystallization (in grams).

The method for determining these quantities was as follows. In an aliquot portion of a \(\mathrm{RbJ}_3\) solution, saturated at temperature \(T_1\), with a known content of the principal component \((\mathrm{RbJ}_3)\) and impurity (Cs), an accurately weighed portion of \(\mathrm{RbJ}_3\), likewise with a known concentration of cesium impurity,* was dissolved on heating to temperature \(T_2\). From these data it was easy to calculate the content of the micro- and macrocomponents in the liquid phase before crystallization \((a, b)\). Then the resulting solution was slowly cooled to temperature \(T_1\), after which, for a definite time, the same in all experiments, a mechanical stirrer with a large number of revolutions was turned on. After removal of supersaturation in the mother solution, the cesium content was determined.

The cocrystallization coefficients found for three different amounts of precipitated \(\mathrm{RbJ}_3\) proved to be very close to one another (Table 1);

Table 1

Results of determination of Khlopin constants \((K)\) at 25°

Thus, in the present case the magnitude of the crystallization coefficient does not depend on the amount of the precipitated macrocomponent. This serves as confirmation of the presence of isomorphism between \(\mathrm{RbJ}_3\) and \(\mathrm{CsJ}_3\).

* The cesium microimpurity was determined in all cases by the method of flame photometry.

A study of the distribution of cesium impurity between the crystalline phase and the solution during crystallization of \( \mathrm{RbJ}_3 \) thus showed that this process proceeds with concentration of cesium in the precipitate and depletion of cesium in the mother liquor (the mean value of \(K\) is 23.3). This provided grounds for concluding that it is possible to remove microimpurities of cesium from an \( \mathrm{RbJ} \) solution by precipitating cesium during the partial separation of rubidium in the form of \( \mathrm{RbJ}_3 \). The latter, consequently, should play the role of a carrier for removing microquantities of cesium from the solution.

To determine the optimal conditions for separating \( \mathrm{RbJ}_3 \) into the precipitate (concentrations of the components in solution, temperature regime), the solubility diagram in the system \( \mathrm{RbJ}—\mathrm{J}_2—\mathrm{H}_2\mathrm{O} \), studied in detail \((^{19})\) over a wide temperature range, was used. It follows from it that crystallization should be carried out in the interval from 0 to \(5^\circ\), at component concentrations in the solution determined by the segment of the \( \mathrm{RbJ}_3 \) crystallization branch adjacent to the eutonic point \( \mathrm{RbJ} + \mathrm{RbJ}_3 \). As a result of a series of experiments, the influence on the degree of purification of an \( \mathrm{RbJ} \) solution from cesium was established for the amount of \( \mathrm{RbJ}_3 \) separated, the initial content of cesium impurity, the number of precipitations, and the intensity of stirring during crystallization, which has a favorable effect on the purification of the solution from cesium impurity.

It was established that the greatest purification factor (10–20) in a single precipitation is attained at a high initial content of cesium impurity (in our experiments, up to 5%); at a lower initial cesium content (\(<0.1\%\)) the purification factor decreases. At the same time, the purification factor depends little on the amount of \( \mathrm{RbJ}_3 \) separated into the precipitate. Therefore, by successive precipitation from the same solution of small portions, rather than by a single precipitation of a considerable amount of \( \mathrm{RbJ}_3 \), better results can be achieved (Table 2).

Table 2

Dependence of the degree of purification on the number of fractionation stages

* 50% of the \( \mathrm{RbJ} \) contained in the solution was separated.
** A total of 40% of the \( \mathrm{RbJ} \) contained in the solution was separated.

The essence of the method recommended for purifying rubidium salts from microimpurities of cesium by precipitation of isotripolyhalides is as follows. Finely ground crystalline iodine is dissolved in an aqueous \( \mathrm{RbJ} \) solution heated to 60–80°, calculated so as to separate \(\sim 10\%\) of the dissolved \( \mathrm{RbJ} \) (weight ratio \( \mathrm{RbJ} : \mathrm{H}_2\mathrm{O} : \mathrm{J}_2 = 5 : 5 : 1 \)) in the form of the first fraction of \( \mathrm{RbJ}_3 \) contaminated with cesium. The mixture is stirred until the iodine is completely dissolved. Crystallization of \( \mathrm{RbJ}_3 \) from the resulting solution is carried out with vigorous stirring and cooling with ice to \(+5^\circ\). After this, the precipitated crystals are filtered off, and precipitation of a second fraction of \( \mathrm{RbJ}_3 \) is carried out under the same temperature conditions, calculated to separate from the solution the same amount of \( \mathrm{RbJ} \). In total, \(n\) precipitations of \( \mathrm{RbJ}_3 \) are carried out from the mother liquor (usually 3–4 times, depending on the initial cesium content), giving altogether \((n+1)\) fractions of contaminated \( \mathrm{RbJ}_3 \). The final mother liquor is evaporated at 120–130° to dryness, and the dry residue is calcined first at 150° to remove the bulk of the iodine, and then at 300–350° for its complete removal.

The rubidium iodide obtained after calcination contains 0.01% cesium, when its initial content in \( \mathrm{RbJ} \) was from 0.25 to 2.5%. The yield of purified rubidium in the direct cycle is 55%; the remaining 45% of \( \mathrm{RbJ} \) is contained in the cesium-enriched precipitate separated during the \((n+1)\)-fold precipitation of \( \mathrm{RbJ}_3 \) in the purification process. The precipitate of contaminated \( \mathrm{RbJ}_3 \) after preliminary-

the subsequent calcination to RbI is returned to the main part of the purification process scheme, which reduces losses of RbI to a minimum. The iodine removed during calcination can be captured by one of the known methods.

Of course, other rubidium salts besides the iodide can also be purified after converting them to RbI, for example through the tetraoxalate \(^{(21)}\) and the carbonate.

Thus, as is evident from the present communication and our preceding work \(^{(5)}\), polyhalides of alkali elements that are similar in their properties can be successfully used to obtain rubidium and cesium compounds of high purity.

Moscow Institute of Fine Chemical Technology
named after M. V. Lomonosov

Received
19 IX 1962

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