Separation of Certain Alkali Metal and Alkaline Earth Cations by Electrochromatography G. HARLOWE EVANS' and HAROLD H. STRAIN 111.
Argonne National Laboratory, Lemont,
The electrochromatographic separation of the alkali metal ions from the alkaline earth ions varied greatly with the composition and pH of the background electrolytic solution. I n most acidic solutions, both the alkali metal and the alkaline earth ions migrated rapidly as cations with much cross contamination of the zones. In weakly acid and alkaline solutions of complex-forming acids, such as the polyvalent citric and ethylenediaminetetraacetic acids, the alkaline earth ions formed complexes t h a t migrated as anions and separated completely from the uncomplexed alkali metal cations. The separation of barium, strontium, calcium, and magnesium from one another was most effective w-hen these ions were but partially in the complex form. In acid solutions, barium migrated fastest, followed closely by strontium, calcium, and magnesium. In weakly acid and alkaline solutions of the polybasic acids, magnesium migrated fastest, followed h y calcium, strontium, and barium. The separation of the alkali metal ions from one another was not affected by variation of the background electrolytic solution. Under all the conditions t h a t were tested, cesium, rubidium, and potassium formed a single, rapidly migrating zone, followed in turn by sodium and lithium. There was no simple relationship between this electrochromatographic sequence of t h e alkali metal ions and the chromatographic sequences observed in columns and in paper.
another and from the alkali metal ions. For example, the alkaline earth cations should form complexes with polyvalent acids contained in the background electrolytic solution, the complexes migrating to the anode. This use of polyvalent acids should facilitate the complete separation of the alkaline earth ions from the alkali metal ions, which do not form complexes, and should also increase the separation of the alkaline earth ions from one another. As indicated by the stability of the complexes, these electrochromatographc effects should vary with the polyvalent acid and with the composition and pH of the background electrolytic solution. Sorption of the ions by the stabilizing paper should also improve the separation of the alkaline earth ions, but, unfortunately, sorption causes trailing, overlapping, and cross contamination of the zones (14).
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1 Present address, Physical Science Department, Illinois State Normal University, Normal, Ill.
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IFFERENTIAL electrical migration in moist paper has provided effective separations of mixtures of various inorganic cations (6,8, IO, 13-15). As a rule, mixtures of ions of different groups of the periodic table are readily separable ( I S 16), whereas ions of the same group are difficultly separable (6, 9, 12-14). Thus far, however, there have been few systematic, electrochromatographic investigations of mixtures of similar ions. For the elaboration of an electrochromatographic scheme of analysis, knowledge of the separability of similar kinds of ions, such as the alkali metal ions and the alkaline earth ions, is required, A few experiments have already indicated that mixtures of the alkaline earth ions, radium, barium, strontium, and calcium, are not readily separable by electrical migration in paper moistened with lactic acid (14). Similarly, electrical migration of the alkali metal ions in paper moistened with aqueous ammonia indicated that lithium, sodium, and potassium were completely separable from one another, but that potassium and rubidium were not separable from each other (6). Migrations in paper moistened with lactic acid instead of ammonia also showed that cesium and rubidium were not separable ( I S ) . Unless there were unexpected solvation or sorption effects, potassium, rubidium, and cesium would not, therefore, be readily separable either in ammonia or in lactic acid solutions. Several conditions might be expected to improve the electrochromatographic separation of the alkaline earth ions from one
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Figure 1. Electrical migration of alkali metal and alkaline earth ions in paper moistened with 0.05M citric acid solution 5 v o l t s per c m . for 3 hours, pH 2.3
Because of its promise in the evolution of a system of analysis. based upon electrochromatography, complex formation has now been tested as an aid for the separation of mixtures of alkali metal ions and alkaline earth ions. To this end, various polyvalent acids have been added to the background electrolytic solution. The complex-forming capacity of these solutions has also, been varied by changing the hydrogen ion concentration, usually by the addition of ammonia. APPARATUS AND PROCEDURE
The background electrolytic solutions, usually 0.05M, were prepared by dissolving the free acids, the free acids plus the calculated quantity of ammonia, or the ammonium salts of the acids in distilled water. The pH was then determined with a glass electrode. The mixture of ions usually submitted to electriral migration was prepared from the chlorides, Owing to the variation of the sensitivity of the flame spectrophotometer, with which they were determined, the ions were employed a t the following molar con-
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centrations: barium, 0.0225; calcium, 0.0075; cesium, 0.005; potassium, 0.0025; lithium, 0.0025; sodium, 0.0025; rubidium, 0.0025; and strontium, 0.0075. The total molar concentration of these cations, 0.0525.M, barely exceeded the molar concentration of the background electrolytic solution, a condition which leads to the formation of symmetrical zones, provided there is no sorption of the ions by the paper ( 1 4 ) . As magnesium was located most readily by fluorescence techniques, separate spots of magnesium and of barium solutions, each 0.05111, were also submitted to electrical migration. All the paper employed for the migrations was from a single roll, Eaton-Dikeman, Grade 301, 0.03 inch thick. Before use, sheets about 61 by 152 em. (2 by 5 feet) were wrapped in polyethylene sheeting and washed by downward percolation with 6 N nitric acid for a day, then with 10% acetic acid for another day, and finally with distilled water for 2 or 3 days. The polyethylene mapping was removed, and the paper sheets were spread out on clean polyethylene to dry. To prevent contamination with sodium, this mashed paper v a s always handled with forceps or with clean rubber gloves. When dry, i t was cut into strips 5.1 em. by about 140 cm. (2 by 55 inches), with clean, chromiumplated scissors. These strips were marked with a transverse lead pencil line 60 cm. from the end to be attached to the anode, and this pencil line, 5.1 cm. long, was marked a t 2-mm. intervals so that 25 portions of the mixture, 10 pl. each, could be added, forming a narrow zone. Parallel lines CI ere then ruled a t 1-cm. intervals from this marked line for 20 cm. toward the anode and for 40 em. toward the cathode. The strips formed by the ruling were numbered, and locations for the terminals of a Simpson voltmeter were marked a t 20 em. tonard the anode and at 40 em. ton ard the cathode ( 4 ) . Lines to mark the positions of the direct current elcctrodes were drawn a t 52 cm. toward the anode and a t 58 cm. toward the cathode. The primary direct current electrodes were, therefore, about 110 em. apart, and the terminals of the direct current voltmeter were 60 em. apart. The strip of marked paper was placed on a sheet of polgethylene, supported on a long, horizontal board, and moistened uniformly with the background electrolytic solution. The excess electrolytic solution was removed by light blotting with facial tissue paper (Kleenex) or with a fresh strip of the washed and dried filter paper, and the blotted paper was covered with a sheet of polyethylene and allowed to stand overnight.
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primary graphite electrodes (from about 550 to 700 or 800 volts), and i t was maintained for 3 or 4 hours. The current carried by the moist paper strips varied with the background electrolytic solution. Strips moistened with 0.05121 lactic, tartaric, and citric acids and with the ammonium salts carried from 5 to 20 ma.; strips rrith the tetraammonium salt of ethylencdiaminetetraacetic acid (EDTA) carried some 50 ma.; and strips with 0.0512f hydrochloric acid carried 80 ma. or more. Heating of the covered strips was prevented by directing an electric fan a t them during the electrolysis. After the migration, the electrodes were removed, and the ruled, numbered sections of the paper were cut into strips (1 cm. wide). These strips, each of which contained about 0.5 ml. of solution, were placed in sequence in 10-ml. portions of mater contained in centrifuge tubes and allowed to stand overnight. Then the extracted paper strips were removed, the solutions vere centrifuged to remove filter fibers, and the metal ions were determined with a Beckman flame spectrophotometer, Alodel DU, fitted with the oxygen-acetylene flame attachment and with a photomultiplier tube for wave lengths less than 625 mp. The slit widths ranged from 0.01 mm. for strontium to 0.26 mm. for rubidium. Measurements of the transmittance mere made a t the following wave lengths (mp): barium, 553; calcium, 622; cesium, 853.1 ; potassium, 768; lithium, 670.8; sodium, 589; rubidium, 780; and strontium, 460.7. The strontium interfered with the lithium determination; the calcium with the barium Corrections for this interference were determined with standard solutions and applied to the readings. Relative transmittance values, which are proportional to the quantity of the ion present, have been utilized for comparison of the rrlative proportions of the ions in various sections of the paper as illustrated in Figures 1 to 4. I n the migrations n ith magnesium and barium, 100-pl. portions of each solution n-ere added as separate zones to the paper strips moistened m-ith the background electrolytic solution. After the migration, the barium zone m-as located by spraying with a freshly prepared solution of sodium rhodizonate. The magnesium zone was sprayed with an alcoholic solution of 8-quinolinol and then with ammonium hydroxide. In ultraviolet light, it appeared as a fluorescent spot. The migration procedure with electrodes on the moist paper ( 1 6 ) was selected because electro-osmotic effects are usually much smaller than those observed with electrodes placed in reservoirs of the background electrolytic solution (4). Electroosmosis in the paper strips in contact n-ith the graphite electrodes was determined in blank experiments with zones of hydrogen peroxide as the flow indicator ( 1 6 ) . In most of these tests, the electro-osmotic displacement was less than 0.5 to 1 em. This small displacement has. therefore, been omitted from the electrical migration values. Separate experiments with electrodes in reservoirs of the background electrolytic solution showed that the electro-osmotic properties of the paper were similar to those of another lot examined earlier. With 0.1M lactic acid, electro-osmosis was toward the anode (4). RESULTS
NUMBER
Figure 2. Electrical migration of alkali metal and alkaline earth ions in paper moistened with 0.05M monoammonium citrate solution 5 volts per cm. for 3 hours, pH 4.0
The polyethylene cover was removed from the moist paper, and the primary electrodes consisting of blocks of graphite (0.5 X 2 X 5 inches) (nuclear reactor graphite, Grade CS) were placed on the paper. The solution to be examined was pipetted in 10-p1. portions onto the 25 marked spots along the first line drawn on the paper. The paper was covered again with the polyethylene, and direct current potential was applied to the graphite electrodes until 300 volts were recorded on the Simpson voltmeter with platinum terminals attached to the paper 60 em. apart (5 volts per em.). This secondary potential on the Simpson voltmeter was adjusted every 5 minutes by variation of the potential on the
The electiical migration of alkaline earth and alkali metal ions iii a citric acid solution (0.05M) is illustrated by Figure 1. Similar results were obtained when the migration time was 4 hours instead of 3, except that the distance of migration was proportionately greater. Relative to barium in the acid solutions, magnesium migrated at about the same rate as lithium. Analogous results Tvere likewise obtained rrith solutions of lactic, tartaric, and hydrochloric acids with migration periods of 3 and 4 hours. With hydrochloric acid, the alkaline earth zones were close together in the sodium zone as indicated by Table I, R-hereas in the organic acids the separation of barium, calcium, and strontium was much greater. As the pH of the solutions was increased, the migration of the alkaline earth ions decreased relative to the migration of the alkali metal ions. I n the lactic acid (pH 2.6), the alkaline earth ions formed zones ahead of the lithiiim zone. In ammonium
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Table I. Effect of Background Solutions on Electrochromatographic Sequences of Alkaline Earth and Alkali Metal Ions (Migration period 3 or 4 hours a t 5 volts per om. Solutions 0.05M. ode a t right, : indicates start) Solutions
PH
HC1 Lactic Tartaric Citric "&-lactate ( S H d H1-citrate NHrH-tartrate ( NHdx-tartrate (NHdzH-citrate (KHP)~H~-(EDTA) (NH4)r-citrate (NHp)ll/aHir,l-(EDTA) ("4) sH-(EDTA) (SHi)r-(EDT.I)
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Sequences
+ Ca + Sr + Ba, K + Rb + C B + S a , K + Rb + Cs : C a Sr + Ba + Li Na K + Rb + Cs : Ca' + Sr + Ba + 'Li, Na, K + Rb + Cs :Ca, Sr + Ba, Li, Na, K + Rb + Cs Ca:Sr, Ba, Li, Na, K + R b + Cs Ca Sr + Ba:Li Na K + Rb + Cs C a ' + Sr. Ba:Li: Na: K + Rb + Cs Ca + Sr + Ba:Li, S a , K + Rb + Cs :Li, Pia
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Table 11. Effect of Background Solutions on Distance of Migration of Calcium, Barium, Lithium, and Cesium (-
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lactate (pH 6.7) by contrast, the alkaline earth ions formed zones behind and within the lithium zone as shown in Table I . In the monoammonium citrate, the alkaline earth ions also formed zones behind the lithium zone. The separation of these zones was somewhat better than in the lactic acid solution (Table I and Figure 2). With the monoammonium tal trate and the diammonium tartrate, the relative migration of the alkaline earths was slower than with the tartaric acid alone. But not even with the diammonium tartrate were the alkaline earth ions induced to migrate toward the anode. With diammonium citrate, magnesium and calcium migrated toward the anode, and strontium and barium migrated slowly t o the cathode (Table I and Figure 3) I n the triammonium citrate, magnesium, calcium, strontium, and barium migrated toward the anode. As shown by Figure 4, the calcium was me11 separated from strontium but the zones of strontium and of barium overlapped. The electrical migration of the alkali metal ions in ammoniacal solution (0.2M) was similar t o that shown in Figure 1. But the positions of potassium and cesium were in the reverse order. Study of the effect of EDTA upon the migration of the several cations was complicated by the slight solubility both of the free acid and of the monoammonium salt. Several commercial preparations of EDTA also contained much sodium. This impurity interfered with the detection of the sodium zone, and it was not removed by recrystallization of the EDTA from pyridine by the addition of hydrochloric acid. With the diammonium EDTA, as with the diammonium citrate, the calcium migrated to the anode and separated well from the strontium and barium which migrated together toward the cathode (Table I). With the triammonium and tetraammonium EDTA, the calcium, strontium, and barium migrated together toward the anode. By careful variation of the composition and pH of the EDTA solution, as by
use of equal volumes of the solutions of the diammonium salt and of the triammonium salt, so that the average composition was (XH,), 112H1I ~ ~ - E D T Athe , calcium, strontium, and barium were found to migrate toward the anode. Under these circumstances, the zones of calcium and strontium overlapped, but both were clearly separated from the barium (Table I). In the figures and in Table I, major emphasis was placed upon the relative migration rates of the ions and upon the separation of the zones from one another. Neglecting the minor effects of electro-osmosis, variations in the wetness of the paper, and changes in the potential gradient, one may also compare the migration rates of an ion in different solutions. A comparison of this kind is presented in Table 11, with migration values for calcium, barium, lithium, and cesium. Kith monobasic, dibasic, and tribasic acids, the migration of the cations decreased as the background electrolytic solutions Rere made more alkaline with ammonia. This decrease was less for lithium and cesium than for calcium and barium. Even when converted into an anion in the triammonium citrate solution, calcium still migrated away from the barium. But this latter result is not typical of the migration in the presence of certain other polyvalent acids, because, as reported already, calcium, strontium, and barium migrated at the same rate in the solutions of the triammonium and tetraammonium EDTA (Table I). DISCUSSION
Because the divalent alkaline earth ions migrate to the anode in neutral or alkaline solutions of the polybasic acids, the alkali metal ions should be removed completely from the alkaline earth ions by electrolysis in these complex-forming solutions. Presumably all univalent cations may be separated from all polyvalent cations by this procedure. Practical techniques for the separation of monovalent from polyvalent cations may be based on this effect. As one example, the radioactive cesium and rubidium produced as nuclear fission products should be readily separable from all the other products by electrical migration in neutral citrate or EDTA solutions. As the relative migration rates of the alkali metal ions remain nearly the same in all the solutions examined, the separation of potassium, rubidium, and cesium from one another is not feasible unless more selective migration conditions can be found. The separation of lithium, sodium, and potassium, by contrast, is very effective. Electrical migration should, therefore, provide practical analytical techniques for the separation of lithium, sodium, and potassium contained in materials such as minerals, waters, plants, and animals. The separation of calcium, strontium, and barium from one another is most effective in the presence of complexing agents. But even in the presence of diammonium citrate, which provides a good separation of the more concentrated regions of the zones
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Figure 3. Electrical migration of alkali metal and alkaline earth ions in paper moistened with 0.05M diammonium citrate solution
Figure 4. Electrical migration of alkali metal and alkaline earth ions in paper moistened with 0.05M triammonium citrate solution
5 volts per cm. for 3 hours, pH 5.2
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(Figure 3), there is still significant overlapping. For quantitative separations, longer migration distances should be more effective. Perhaps more selective complexing reagents may yet be found for use with two-way migrations and with p H gradients as well as nith one-way migrations. As the ions of each element form a single nearly symmetrical zone under the different migration conditions (Figures 1 to 4), all the ionic species of each element must be rapidly interconvertible. If this were not so, more than one zone would be expected, particularly in mixtures containing both negatively charged complexes and the uncomplexed cations (Figure 3). Decrease of the cation mobility with increasing pH of the background solution, as shown for calcium, barium, lithium, and cesium in Table 11, may be attributed, in large part, to variation of the ionic Concentration of the background electrolytic solution and to variation of complex formation. As the weakly dissociated organic acids are neutralized with ammonia, the formation of the strongly dissociated ammonium salts increases the ionic concentration of the medium. With this increase of ionic concentration, the mobility of all the ions usually decreases. This latter well-known effect probably accounts for some of the decrease of the mobility of the alkali metal ions. With the bivalent alkaline earth ions, formation of chelatelike complexes with negative charges accounts for the continued decrease in the mobility with eventual migration toward the anode. Decrease of the cation mobility with increasing pH of the acid solutions, as shown in Table 11, may be related in part to the kinds of ions and to the proportions of these ions migrating in the background electrolytic solution. In the acid solutions, for example, m o d of the current is carried by the hydrogen ions, so that there is very little migration of the anions. In these acid solutions, alkali metal and alkaline earth cations migrate through an ionic field in which migration of hydrogen ions predominates. As indicated by numerous early measurements of ionic mobilities, the migration of cations in acid solutions may be several per cent greater than the migration in neutral solutions of the same anionic concentration. In the partially neutralized, background solutions, however, the concentration of the anions is much higher than in the acid solutions, and the hydrogen ions are largely replaced by ammonium ions. Owing to the lon-er mobility of the ammonium ions and to the reduced concentration of the hydrogen ions, much of the current is carried by the anions. In these partially neutralized solutions, therefore, alkali metal and alkaline earth cations migrate through a more concentrated ionic field, in which the migration of hydrogen ions no longer predominates and in which the migration of anions has increased
many times. Under these circumstances, the resistance to the migration of the alkali metal and alkaline earth cations should be much greater than in the acid solutions. The migration rate of the alkali metal and alkaline earth cations in the partially neutralized solutions should, therefore, be much less than the rate in the acid solutions. Chromatographic sequences, which depend upon the condition of the ions in solution as well as upon their combination with the sorptive phase ( 7 ) , might be expected to yield clues regarding conditions favorable for the electrochromatographic separation of the alkali metal ions. In paper and in columns of ion exchange resins, the lighter alkali metal ions usually migrate faster than the heavier ones ( 2 , 3,5, 7 ) . One exception was observed when the hydroxides rather than the salts were sorbed on paper (1). Another exception a a s found when a solution of strong hydrochloric acid in methanol plus methyl propyl ketone was employed to form the chromatogram in paper (11). The relationships among the chromatographic sequences, the electrochromatographic sequences, and the ionic dimensions are obviously complex. LITERATURE CITED
(1) Burma, D. P., Analyst 77, 382 (1952). (2) Burstall, F. H., Davies, G. R., Linstead, R. P., Wells, R. A., J. Chem. SOC.1950, 516. (3) Chakrabarty, S., Burma, D. P., Sciencs a n d Culture 16, 485 (1951). (4) Engelke, J. L., Strain, H. H., Wood, S. E., ANAL.CHEM.26, 1864 (1954). (5) Harasawa, S , Sakamoto, T., J . Chem. SOC.Japan (Pure Chem. Sect.) 73,614 (1952). ( 6 ) Ibid., 74, 862 (1953). (7) Lederer, E., Lederer, hi., “Chromatography,” Elsevier, Amsterdam, Houston, London, New York, 1953. (8) Lederer, LI., “An Introduction t o Paper Electrophoresis and Re-
lated Methods,” Elsevier, Amsterdam, Houston, London, New York, 1955. (9) Lederer, M., Nature 176,462 (1955). (IO) McDonald, H. J., “Ionography, Electrophoresis in Stabilized Media,” Year Book Publishers, Chicago, 1955. (11) Miller, C. C., Magee, R. J., J . Chem. SOC.1951,3183. (12) Sato, T. R., Diamond, H., Norris, W. P., Strain, H. H., J . Am. Chem. SOC.74, 6154 (1952). (13) Sato, T. R., Norris, W. P., Strain, H. H., ~ ~ N A CHEM. L . 26, 267 (19.54)\ - - - - I
(14) I b i d . , 27, 521 (1955). (15) Strain, H. H., I b i d . , 24, 356 (1952). (16) Wood, S. E., Strain, H. H., Ibid., 26, 1869 (1954)
RECEIVED for review December 15, 1955. Accepted June 15, 1956. Baaed on work performed under the auspices of the U. S. Atomic Energy Commiasion.
