Separation of a Binary Lithium-Sodium Ion Mixture by Two-Dimensional Electrochromatography in a Slit-Type Column
Abstract Generated abstract
This study examines the continuous separation of lithium and sodium cations by two-dimensional electrochromatography in a slit-type column, combining solution flow through the column with a perpendicular constant electric field. Experiments used either KU-2 cation-exchange resin in the H-form or quartz sand as packing, with concentration ratios measured by flame spectrophotometry at different times and positions in the cathode region. The results show an initial sodium breakthrough followed by lithium, indicating that sampling must begin at the sodium breakthrough to minimize zone mixing. Separation was more effective with the cation exchanger than with inert quartz sand, suggesting that ion exchange contributes substantially in addition to differences in electrophoretic mobility.
Full Text
SEPARATION OF A BINARY LITHIUM—SODIUM ION MIXTURE BY TWO-DIMENSIONAL ELECTROCHROMATOGRAPHY IN A SLIT-TYPE COLUMN
G. S. Tikhonov, I. A. Tikhomirov
(Presented by the scientific seminar of the Research Institute of Nuclear Physics)
The principle of separating ionic mixtures by the method of two-dimensional chromatography has now found wide application in analytical chemistry [1], [2], since it provides rapid and effective separation of the initial mixture.
Such a separation method can also be carried out by using simultaneously the process of chromatographic separation of ions and the method of separating them by virtue of the difference in their mobilities under the action of a constant electric field, when these two processes are directed mutually perpendicular [3], as, for example, in the case of planar chromatography on paper in the separation of small quantities of ions.
In the present work we investigated the possibility of two-dimensional electrochromatographic separation of cations in a slit-type column, which would permit continuous separation of cations for both analytical and preparative purposes.
We used a column with dimensions \(18 \times 18\) cm and a slit width of 0.3 cm. A schematic representation of this column is shown in Fig. 1. As the packing we used KU-2 cation-exchange resin with a grain size of 0.3 mm, or an inert filler—quartz sand with the same grain size 2. The experiment was carried out as follows.
Fig. 1. Schematic diagram of the separation process
First, the column was filled with KU-2 cationite, previously swollen in distilled water, in the H-form, or with quartz sand. Then, near the anode space [1] in the upper part of the column, the initial mixture of salts was fed through a rheometer in the form of an aqueous solution of lithium chloride and sodium chloride salts. In the lower part of the column, also near the anode, continuous sampling was carried out, and, in order to determine the separation process, until establish—
…of the column’s stationary state, samples were taken at definite time intervals along the entire length of its lower part.
Thus, over the entire height of the column, continuous flow of the solution of the initial mixture was ensured. Simultaneously with switching on the feed, i.e., simultaneously with the introduction of the solution of the initial mixture into the column, a constant voltage from a rectifier was applied to the electrodes. The direction of action of the constant electric field was chosen perpendicular to the direction of flow of the initial-mixture solution passing through the column; therefore the direction of motion of the sodium ions is determined by line 5, and the direction of motion of the lithium ions by line 4 (Fig. 1).
To ensure the most effective separation process, the rate of dripping of the solution was selected so that the linear velocity of motion of the ions was equal to their linear velocity when moving under the action of a constant electric field. On the basis of the basic separation scheme, the greatest spreading-apart of the cation zones occurs in the cathode part of the column—3. Therefore, to study the separation process, we took samples precisely in the cathode part of the column. Quantitative analysis of the samples was carried out on an SP-6 flame spectrophotometer.
To study the separation of the initial mixture as a function of time in the nonstationary state, an experiment was carried out with sampling in the cathode part of the column at definite time intervals. The results of the experiment are presented in Table 1. In this case, KU-2 cation exchanger was used as the packing.
Table 1
Change in the concentration of sodium and lithium ions in the cathode part of the column as a function of time
| No. | Operating time, hours | Ratio of quantitative content (sodium:lithium), initial | Ratio of quantitative content (sodium:lithium), in the sample | Strength of the constant electric field \(E\), V/cm |
|---|---|---|---|---|
| 1 | 7 | 2.78 | 31.60 | 4 |
| 2 | 8 | 2.78 | 15.10 | 4 |
| 3 | 9 | 2.78 | 4.33 | 4 |
| 4 | 13 | 2.78 | 5.50 | 4 |
| 5 | 17 | 2.78 | 4.61 | 4 |
| 6 | 21 | 2.78 | 5.00 | 4 |
It follows from the table that in the cathode part, at the initial moment of time, a “breakthrough” of sodium ions is observed, and then, following the sodium ions, the front of lithium ions arrives; as a result, the quantitative content of both ions in the mixture changes sharply.
To determine the distribution of the concentrations of lithium and sodium ions along the length of the column, an experiment was also carried out using KU-2 cation exchanger in the H-form as the packing. The results of the experiment are presented in Table 2.
In order to determine the influence of ion exchange on the separation process, an experiment was carried out in which quartz sand was used as the filler. The results of this experiment are presented in Table 3.
As can be seen from the table, the maximum separation of lithium ions from sodium ions occurs in the cathodic part of the column at the moment of “breakthrough.” Then—
Table 2
Distribution of the concentration of lithium and sodium ions along the length of the column as a function of time
| No. | Operating time, h | Distance from the feed point, mm | Ratio of quantitative content (sodium : lithium), initial | Ratio of quantitative content (sodium : lithium), in the sample | Strength of the constant electric field \(E\), V/cm |
|---|---|---|---|---|---|
| 1 | 7,0 | 45 | 0,94 | 3,00 | 4 |
| 2 | 9,5 | 45 | 0,94 | 0,76 | 4 |
| 3 | 12,5 | 45 | 0,94 | 0,50 | 4 |
| 4 | 15,5 | 45 | 0,94 | 0,63 | 4 |
| 5 | 7,0 | 90 | 0,94 | 23,30 | 4 |
| 6 | 9,5 | 90 | 0,94 | 0,97 | 4 |
| 7 | 12,5 | 90 | 0,94 | 0,83 | 4 |
| 8 | 15,5 | 90 | 0,94 | 0,85 | 4 |
| 9 | 7,0 | 140 | 0,94 | 40,00 | 4 |
| 10 | 9,5 | 140 | 0,94 | 4,70 | 4 |
| 11 | 12,5 | 140 | 0,94 | 4,00 | 4 |
| 12 | 15,5 | 140 | 0,94 | 4,20 | 4 |
Table 3
Distribution of the concentration of lithium and sodium ions along the length of the column as a function of time when the column is filled with quartz sand
| No. | Operating time, h | Distance from the feed point, mm | Ratio of quantitative content (sodium : lithium), initial | Ratio of quantitative content (sodium : lithium), in the sample | Strength of the constant electric field \(E\), V/cm |
|---|---|---|---|---|---|
| 1 | 16,0 | 45 | 1,0 | 0,95 | 4 |
| 2 | 18,5 | 45 | 1,0 | 0,85 | 4 |
| 3 | 24,5 | 45 | 1,0 | 0,79 | 4 |
| 4 | 29,5 | 45 | 1,0 | 0,75 | 4 |
| 5 | 16,0 | 90 | 1,0 | 1,66 | 4 |
| 6 | 18,5 | 90 | 1,0 | 0,89 | 4 |
| 7 | 24,5 | 90 | 1,0 | 0,90 | 4 |
| 8 | 29,5 | 90 | 1,0 | 0,84 | 4 |
| 9 | 16,0 | 140 | 1,0 | 10,00 | 4 |
| 10 | 18,5 | 140 | 1,0 | 1,62 | 4 |
| 11 | 24,5 | 140 | 1,0 | 1,18 | 4 |
| 12 | 29,5 | 140 | 1,0 | 1,15 | 4 |
the predominant content of sodium ions in the samples decreases noticeably. This is explained by the fact that, following the sodium ions, lithium ions enter the cathodic space. Consequently, continuous sampling
through the column must be begun at the moment of “breakthrough” of sodium ions into the cathode space in order to avoid mixing the zones of both cations.
Comparing Tables 2 and 3, it may be noted that, when a cation exchanger is used as the packing, the separation process is more effective than when quartz sand is used. This is explained by the fact that in the former case, along with separation of the ions due to the difference in mobilities, the ion-exchange process has a substantial influence on the separation, whereas when the column is packed with quartz sand this process is absent.
Conclusions
-
The fundamental possibility has been established of separating a binary lithium–sodium ionic mixture by the method of two-dimensional electrochromatography in a slit-type column, in which a constant electric field is applied perpendicular to the direction of flow of the initial solution through the column.
-
It has been established that ion separation proceeds more effectively when a cation exchanger is used as the packing than when an inert packing—quartz sand—is used.
REFERENCES
- I. Hais, K. Macek. Paper Chromatography, IL, Moscow, 1962.
- K. M. Olshanova, M. A. Potapova, V. D. Kopylova, N. M. Morozova. Handbook of Ion-Exchange, Partition, and Precipitation Chromatography, Khimiya, Moscow, 1965.
- R. Lederer. Introduction to Electrophoresis and Related Methods, IL, Moscow, 1956.